Which visual system assessment technique provides a magnified view of the retina

Of all the organs of the body, the eye is most accessible to direct examination. Visual function can be quantified by simple subjective testing. The external anatomy of the eye is visible to inspection with the unaided eye and with fairly simple instruments. With more complicated instruments, the interior of the eye is visible through the clear cornea. The eye is the only part of the body where blood vessels and central nervous system tissue (retina and optic nerve) can be viewed directly. Important systemic effects of infectious, autoimmune, neoplastic, and vascular diseases may be identified from ocular examination.

The purpose of sections I and II of this chapter is to provide an overview of the ocular history and basic complete eye examination as performed by an ophthalmologist. In section III, more specialized examination techniques will be presented.

The chief complaint is characterized according to its duration, frequency, intermittency, and rapidity of onset. The location, the severity, and the circumstances surrounding onset are important, as well as any associated symptoms. Current eye medications being used and all other current and past ocular disorders are recorded, and a review of other pertinent ocular symptoms is performed.

The past medical history centers on the patient's general state of health and principal systemic illnesses, if any. Vascular disorders commonly associated with ocular manifestations—such as diabetes and hypertension—should be asked about specifically. Just as a general medical history should include ocular medications being used, the eye history should list the patient's systemic medications. This provides a general indication of health status and may include medications that affect ocular health, such as corticosteroids. Finally, any drug allergies should be recorded.

The family history is pertinent for ocular disorders, such as strabismus, amblyopia, glaucoma, or cataracts, and retinal problems, such as retinal detachment or macular degeneration. Medical diseases such as diabetes may be relevant as well.

Common Ocular Symptoms

A basic understanding of ocular symptomatology is necessary for performing a proper ophthalmic examination. Ocular symptoms can be divided into three basic categories: abnormalities of vision, abnormalities of ocular appearance, and abnormalities of ocular sensation—pain and discomfort.

Symptoms and complaints should always be fully characterized. Was the onset gradual, rapid, or asymptomatic? (For example, was blurred vision in one eye not discovered until the opposite eye was inadvertently covered?) Was the duration brief, or has the symptom continued until the present visit? If the symptom was intermittent, what was the frequency? Is the location focal or diffuse, and is involvement unilateral or bilateral? Finally, does the patient characterize the degree as mild, moderate, or severe?

One should also determine what therapeutic measures have been tried and to what extent they have helped. Has the patient identified circumstances that trigger or worsen the symptom? Have similar instances occurred before, and are there any other associated symptoms?

The following is a brief overview of ocular complaints. Representative examples of some causes are given here and discussed more fully elsewhere in this book.

Abnormalities of Vision

Visual Loss

Loss of visual acuity may be due to abnormalities anywhere along the optical and neurologic visual pathway. One must therefore consider refractive (focusing) error, lid ptosis, clouding or interference from the ocular media (eg, corneal edema, cataract, or hemorrhage in the vitreous or aqueous space), and malfunction of the retina (macula), optic nerve, or intracranial visual pathway.

A distinction should be made between decreased central acuity and peripheral vision. The latter may be focal, such as a scotoma, or more expansive, as with hemianopia. Abnormalities of the intracranial visual pathway usually disturb the visual field more than central visual acuity.

Transient loss of central or peripheral vision is frequently due to circulatory changes anywhere along the neurologic visual pathway from the retina to the occipital cortex, for example amaurosis fugax and migrainous scotoma.

The degree of visual impairment may vary under different circumstances. For example, uncorrected nearsighted refractive error may seem worse in dark environments. This is because pupillary dilation allows more misfocused rays to reach the retina, increasing the blur. A central focal cataract may seem worse in sunlight. In this case, pupillary constriction prevents more rays from entering and passing around the lens opacity. Blurred vision from corneal edema may improve as the day progresses owing to corneal dehydration from surface evaporation.

Visual Aberrations

Glare or halos may result from uncorrected refractive error, scratches on spectacle lenses, excessive pupillary dilation, and hazy ocular media, such as corneal edema or cataract. Visual distortion (apart from blurring) may be manifested as an irregular pattern of dimness, wavy or jagged lines, and image magnification or minification. Causes may include the aura of migraine, optical distortion from strong corrective lenses, or lesions involving the macula and optic nerve. Flashing or flickering lights may indicate retinal traction (if instantaneous) or migrainous scintillations that last for several seconds or minutes. Floating spots may represent normal vitreous strands due to vitreous “syneresis” or separation (see Chapter 9) or the pathologic presence of pigment, blood, or inflammatory cells. Oscillopsia is a shaking field of vision due to ocular instability.

It must be determined whether double vision is monocular or binocular (ie, disappears if one eye is covered). Monocular diplopia is often a split shadow or ghost image. Causes include uncorrected refractive error, such as astigmatism, or focal media abnormalities, such as cataracts or corneal irregularities (eg, scars, keratoconus). Binocular diplopia (see Chapters 12 and 14) can be vertical, horizontal, diagonal, or torsional. If the deviation occurs or increases in one gaze direction as opposed to others, it is called “incomitant.” Neuromuscular dysfunction or mechanical restriction of globe rotation is suspected. “Comitant” deviation is one that remains constant regardless of the direction of gaze. It is usually due to childhood or long-standing strabismus.

Abnormalities of Appearance

Complaints of “red eye” call for differentiation between redness of the lids and periocular area versus redness of the globe. The latter can be caused by subconjunctival hemorrhage or by vascular congestion of the conjunctiva, sclera, or episclera (connective tissue between the sclera and conjunctiva). Causes of such congestion may be either external surface inflammation, such as conjunctivitis and keratitis, or intraocular inflammation, such as iritis and acute glaucoma (see Differential Diagnosis of Common Causes of the Inflamed Eye). Color abnormalities other than redness may include jaundice and hyperpigmented spots on the iris or outer ocular surface.

Other changes in appearance of the globe that may be noticeable to the patient include focal lesions of the ocular surface, such as a pterygium, and asymmetry of pupil size (anisocoria). The lids and periocular tissues may be the source of visible signs, such as edema, redness, focal growths, and lesions, and abnormal position or contour, such as ptosis. Finally, the patient may notice bulging or displacement of the globe, such as with exophthalmos.

Pain & Discomfort

“Eye pain” may be periocular, ocular, retrobulbar (behind the globe), or poorly localized. Examples of periocular pain are tenderness of the lid, tear sac, sinuses, or temporal artery. Retrobulbar pain can be due to orbital inflammation of any kind. Certain locations of inflammation, such as optic neuritis or orbital myositis, may produce pain on eye movement. Many nonspecific complaints such, as “eyestrain,” “pulling,” “pressure,” “fullness,” and certain kinds of “headaches,” are poorly localized. Causes may include fatigue from ocular accommodation or binocular fusion or referred discomfort from nonocular muscle tension or fatigue.

Ocular pain itself may seem to emanate from the surface or from deeper within the globe. Corneal epithelial damage typically produces a superficial sharp pain or foreign body sensation exacerbated by blinking. Topical anesthesia will immediately relieve this pain. Deeper internal aching pain occurs with acute glaucoma, iritis, endophthalmitis, and scleritis. The globe is often tender to palpation in these situations. Reflex spasm of the ciliary muscle and iris sphincter can occur with iritis or keratitis, producing brow ache and painful “photophobia” (light sensitivity). This discomfort is markedly improved by instillation of cycloplegic/mydriatic agents (see Chapter 22).

Eye Irritation

Superficial ocular discomfort usually results from surface abnormalities. Itching, as a primary symptom, is often a sign of allergic sensitivity. Symptoms of dryness, burning, grittiness, and mild foreign body sensation can occur with dry eyes or other types of mild corneal irritation. Tearing may be of two general types. Sudden reflex tearing is usually due to irritation of the ocular surface. In contrast, chronic watering and “epiphora” (tears rolling down the cheek) may indicate abnormal lacrimal drainage (see Chapter 4).

Ocular secretions are often diagnostically nonspecific. Severe amounts of discharge that cause the lids to be glued shut upon awakening usually indicate viral or bacterial conjunctivitis. More scant amounts of mucoid discharge can also be seen with allergic and noninfectious irritations. Dried matter and crusts on the lashes may occur acutely with conjunctivitis or chronically with blepharitis (lid margin inflammation).

The purpose of the ophthalmologic physical examination is to evaluate both the function and the anatomy of the two eyes. Function includes vision and nonvisual functions, such as eye movements and alignment. Anatomically, ocular problems can be subdivided into three areas: those of the adnexa (lids and periocular tissue), the globe, and the orbit.

Vision

Just as assessment of vital signs is a part of every physical examination, any ocular examination must include assessment of vision, regardless of whether vision is mentioned as part of the chief complaint. Good vision results from a combination of an intact neurologic visual pathway, a structurally healthy eye, and proper focus of the eye. An analogy might be made to a video camera, requiring a functioning cable connection to the monitor, a mechanically intact camera body, and a proper focus setting. Vision can be divided broadly into central and peripheral, quantified by visual acuity and visual field testing, respectively. Clinical assessment of visual acuity and visual field is subjective rather than objective, since it requires responses on the part of the patient.

Visual Acuity Testing

Visual acuity can be tested either for distance or near, conventionally at 20 feet (6 meters) and 14 inches (33 cms) away, respectively, but distance acuity is the general standard for comparison. For diagnostic purposes visual acuity is always tested separately for each eye, whereas binocular visual acuity is useful for assessing functional vision (see Chapter 25), such as for assessing the eligibility to drive.

Visual acuity is measured with a display of different-sized optotypes shown at the appropriate distance from the eye. The familiar “Snellen chart” is composed of rows of progressively smaller letters, each row designated by a number corresponding to the distance in feet (or meters) from which a normal eye can read the letters of the row. For example, the letters in the “40” row are large enough for the normal eye to see from 40 feet away. Whereas wall-mounted illuminated charts, or projection systems are commonly used, wall-mounted LCD screens provide better standardization and calibration (Figure 2–1). Mainly for clinical trials but increasingly for specific clinical situations, LogMAR charts are being used (see Chapter 24).

Figure 2-1.

LCD screen displaying Snellen visual acuity chart with 20/40 letters at the top (Photo courtesy of M&S Technologies).

Visual acuity is scored as a fraction (eg, “20/40”). The first number represents the testing distance between the chart and the patient, and the second number represents the smallest row of letters that the patient's eye can read. Hence normal vision is 20/20 and 20/60 acuity indicates that the patient's eye can only read from 20 feet letters large enough for a normal eye to read from 60 feet.

Charts containing numerals can be used for patients not familiar with the English alphabet. The “illiterate E” chart is used to test small children or if there is a language barrier. “E” figures are randomly rotated in each of four different orientations throughout the chart. For each target, the patient is asked to point in the same direction as the three “bars” of the E (Figure 2–2). Most children can be tested in this manner beginning at about age 3½ years.

Figure 2-2.

“Illiterate E” chart.

Uncorrected visual acuity is measured without glasses or contact lenses. Corrected acuity means that these aids were worn. Since poor uncorrected distance acuity may simply be due to refractive error, corrected visual acuity is a more relevant assessment of ocular health.

Pinhole Test

If the patient needs glasses or if his or her glasses are unavailable, the corrected acuity can be estimated by testing vision through a “pinhole.” Refractive blur (eg, myopia, hyperopia, astigmatism) is caused by multiple misfocused rays entering through the pupil and reaching the retina. This prevents formation of a sharply focused image.

Viewing the Snellen chart through a placard of multiple tiny pinhole-sized openings prevents most of the misfocused rays from entering the eye. Only a few centrally aligned focused rays will reach the retina, resulting in a sharper image. In this manner, the patient may be able to read within one or two lines of what would be possible if proper corrective glasses were being used.

Refraction

The unaided distant focal point of the eye varies among normal individuals depending on the shape of the globe and the cornea (Figure 2–3). An emmetropic eye is naturally in optimal focus for distance vision. An ametropic eye (ie, one with myopia, hyperopia, or astigmatism) needs corrective lenses to be in proper focus for distance. This optical abnormality is called refractive error.

Figure 2-3.

Common imperfections of the optical system of the eye (refractive errors). Ideally, light rays from a distant target should automatically arrive in focus on the retina if the retina is situated precisely at the eye's natural focal point. Such an eye is called emmetropic. In hyperopia (“farsightedness”), the light rays from a distant target instead come to a focus behind the retina, causing the retinal image to be blurred. A biconvex (+) lens corrects this by increasing the refractive power of the eye and shifting the focal point forward. In myopia (“nearsightedness”), the light rays come to a focus in front of the retina, as though the eyeball is too long. Placing a biconcave (–) lens in front of the eye diverges the incoming light rays; this effectively weakens the optical power of the eye enough so that the focus is shifted backward and onto the retina. (Modified and reproduced, with permission, from Ganong WF: Review of Medical Physiology, 15th ed. McGraw-Hill, 1991.)

Refraction is the procedure by which any refractive error is characterized and quantified (Figure 2–4) (see Chapter 21), allowing the best measure of corrected visual acuity. In addition, it is the most reliable means to distinguish between blurred vision caused by refractive error or by other abnormalities of the visual system. Thus, in addition to being the basis for prescription of corrective glasses or contact lenses, refraction serves a crucial diagnostic function.

Figure 2-4.

Refraction being performed using a “phoropter.” This device contains the complete range of corrective lens powers, which can quickly be changed back and forth, allowing the patient to subjectively compare various combinations while viewing the eye chart at a distance. (Photo by M Narahara.)

Testing Poor Vision

The patient unable to read the largest (“20/200”) letter on a Snellen chart should be moved closer to the chart until that letter can be read. The distance from the chart is then recorded as the first number. Visual acuity of “5/200” means that the patient can identify correctly the largest letter from a distance of 5 feet but not further away. An eye unable to read any letters is tested by the ability to count fingers. “CF at 2 ft” indicates that the eye was able to count fingers held 2 feet away but not farther away. If counting fingers is not possible, the eye may be able to detect a hand moving vertically or horizontally (“HM,” or “hand motions” vision). The next lower level of vision would be the ability to perceive light (“LP,” or “light perception”). An eye that is totally blind is recorded as having no light perception (“NLP”).

Visual Field Testing

Visual field testing should be included in every complete ophthalmologic examination because even dense visual field abnormalities may not be apparent to the patient. Since the visual fields of the two eyes overlap, for diagnostic purposes each eye must be tested separately. Binocular visual field testing is useful in assessment of functional vision (see Chapter 25).

Assessment of visual fields can be quickly achieved using confrontation testing. The patient is seated facing the examiner with one eye covered while the examiner closes the opposite eye (eg, the patient's left eye is covered and the examiner's right eye is closed so that the patient's right eye looks into the examiner's left eye). Presentation of targets at a distance halfway between the patient and the examiner allows direct comparison of the field of vision of each eye of the patient and the examiner. Since the patient and examiner are staring eye to eye, any loss of fixation by the patient will be noticed.

For gross assessment, the examiner briefly shows a number of fingers of one hand (usually one, two, or four fingers) peripherally in each of the four quadrants. The patient must identify the number of fingers flashed while maintaining straight-ahead fixation. The upper and lower temporal and the upper and lower nasal quadrants are all tested in this fashion for each eye.

A 5-mm-diameter red sphere or disk attached to a handle as the target allows detection and quantification of more subtle visual field defects, particularly if areas of abnormal reduction in color (desaturation) are sought.

In disease of the right cerebral hemisphere, particularly involving the parietal lobe, there may be visual neglect (visual inattention) in which there is no comparable visual field loss on testing of each eye separately, but objects are not identified in the left hemifield of either eye if objects are simultaneously presented in the right hemifield. The patient functions as if there is a left homonymous hemianopia. Visual neglect is detected by simultaneous confrontation testing. The examiner holds both hands out peripherally, one on each side. The patient, with both eyes open, is asked to signify on which side (right, left, or both) the examiner is intermittently wiggling his or her fingers. The patient will still be able to detect the fingers in the left hemifield when wiggled alone but not when the fingers in the right hemifield are wiggled simultaneously.

More sophisticated means of visual field testing, important for detection of subtle visual field loss, such as in the diagnosis of early glaucoma and for quantification of any visual field defect, are discussed later in this chapter.

Pupils

Basic Examination

The pupils should be symmetric, and each one should be examined for size, shape (circular or irregular), and reactivity to both light and accommodation. Pupillary abnormalities may be due to (1) neurologic disease, (2) intraocular inflammation causing either spasm of the pupillary sphincter or adhesions of the iris to the lens (posterior synechiae), (3) markedly elevated intraocular pressure causing atony of the pupillary sphincter, (4) prior surgical alteration, (5) the effect of systemic or eye medications, and (6) benign variations of normal.

To avoid accommodation, the patient is asked to fixate on a distant target as a penlight is directed toward each eye. Dim lighting conditions help to accentuate the pupillary response and may best demonstrate an abnormally small pupil. Likewise, an abnormally large pupil may be more apparent in brighter background illumination. The direct response to light refers to constriction of the illuminated pupil. The reaction may be graded as either brisk or sluggish. The consensual response is the normal simultaneous constriction of the opposite nonilluminated pupil. The neuroanatomy of the pupillary pathway is discussed in Chapter 14.

Swinging Penlight Test for Marcus Gunn Pupil

As a light is swung back and forth in front of the two pupils, one can compare the reactions to stimulation of each eye, which should be equal. If the neural response to stimulation of the left eye is impaired, the pupil response in both eyes will be reduced on stimulation of the left eye compared to stimulation of the right eye. As the light is swung from the right to the left eye, both pupils will begin to dilate normally as the light is moved away from the right eye and then not constrict or paradoxically widen as the light is shone into the left eye (since the direct response in the left eye and the consensual response in the right eye are reduced compared to the consensual response in the left eye and direct response in the right eye from stimulation of the right eye). When the light is swung back to the right eye, both pupils will begin to dilate as the light is moved away from the left eye and then constrict normally as the light is shone into the right eye. This phenomenon is called a relative afferent pupillary defect (RAPD). It is usually a sign of optic nerve disease but may occur in retinal disease. Importantly, it does not occur in media opacities such as corneal disease, cataract, and vitreous hemorrhage. Because the pupils are normal in size and may appear to react normally when each is stimulated alone, the swinging flashlight test is the only means of demonstrating a relative afferent pupillary defect. Also, because the pupils react equally, detection of a relative afferent pupillary defect requires inspection of only one pupil and can still be achieved when one pupil is structurally damaged or cannot be visualized, as in dense corneal opacity. Relative afferent pupillary defect is further discussed and illustrated in Chapter 14.

Ocular Motility

The objective of ocular motility testing is to evaluate the alignment of the eyes and their movements, both individually (“ductions”) and in tandem (“versions”). A more complete discussion of ocular motility testing and eye movement abnormalities is presented in Chapters 12 and 14.

Testing Alignment

Normal patients have binocular vision. Since each eye generates a visual image separate from and independent of that of the other eye, the brain must be able to fuse the two images in order to avoid “double vision.” This is achieved by having each eye positioned so that both foveas are simultaneously fixating on the object of regard.

A simple test of binocular alignment is performed by having the patient look toward a penlight held several feet away. A pinpoint light reflection, or “reflex,” should appear on each cornea and should be centered over each pupil if the two eyes are straight in their alignment. If the eye positions are convergent, such that one eye points inward (“esotropia”), the light reflex will appear temporal to the pupil in that eye. If the eyes are divergent, such that one eye points outward (“exotropia”), the light reflex will be located more nasally in that eye. This test can be used with infants.

The cover test (see Chapter 12) is a more accurate method of verifying normal ocular alignment. The test requires good vision in both eyes. The patient is asked to gaze at a distant target with both eyes open. If both eyes are fixating together on the target, covering one eye should not affect the position or continued fixation of the other eye.

To perform the test, the examiner suddenly covers one eye and carefully watches to see that the second eye does not move (indicating that it was fixating on the same target already). If the second eye was not identically aligned but was instead turned abnormally inward or outward, it could not have been simultaneously fixating on the target. Thus, it will have to quickly move to find the target once the previously fixating eye is covered. Fixation of each eye is tested in turn.

An abnormal cover test is expected in patients with diplopia. However, diplopia is not always present in many patients with long-standing ocular misalignment. When the test is abnormal, prism lenses of different power can be used to neutralize the refixation movement of the misaligned eye (prism cover test). In this way, the amount of eye deviation can be quantified based on the amount of prism power needed.

Testing Extraocular Movements

The patient is asked to follow a target with both eyes as it is moved in each of the four cardinal directions of gaze. The examiner notes the speed, smoothness, range, and symmetry of movements and observes for unsteadiness of fixation (eg, nystagmus).

Impairment of eye movements can be due to neurologic problems (eg, cranial nerve palsy), primary extraocular muscular weakness (eg, myasthenia gravis), or mechanical constraints within the orbit limiting rotation of the globe (eg, orbital floor fracture with entrapment of the inferior rectus muscle). Deviation of ocular alignment that is the same amount in all directions of gaze is called “comitant.” It is “incomitant” if the amount of deviation varies with the direction of gaze.

External Examination

Before studying the eye under magnification, a general external examination of the ocular adnexa (eyelids and periocular area) is performed. Skin lesions, growths, and inflammatory signs such as swelling, erythema, warmth, and tenderness are evaluated by gross inspection and palpation.

The positions of the eyelids are checked for abnormalities, such as ptosis or lid retraction. Asymmetry can be quantified by measuring the central width (in millimeters) of the “palpebral fissure”—the space between the upper and lower lid margins. Abnormal motor function of the lids, such as impairment of upper lid elevation or forceful lid closure, may be due to either neurologic or primary muscular abnormalities.

Malposition of the globe, such as proptosis, may occur in orbital disease. Palpation of the bony orbital rim and periocular soft tissue should always be done in instances of suspected orbital trauma, infection, or neoplasm. The general facial examination may contribute other pertinent information as well. Depending on the circumstances, checking for enlarged preauricular lymph nodes, sinus tenderness, temporal artery prominence, or skin or mucous membrane abnormalities may be diagnostically relevant.

Slitlamp Examination

Basic Slitlamp Biomicroscopy

The slitlamp (Figure 2–5) is a table-mounted binocular microscope with a special adjustable illumination source attached. A linear slit beam of incandescent light is projected onto the globe, illuminating an optical cross section of the eye (Figure 2–6). The angle of illumination can be varied along with the width, length, and intensity of the light beam. The magnification can be adjusted as well (normally 10× to 16× power). Since the slitlamp is a binocular microscope, the view is “stereoscopic,” or three-dimensional.

Figure 2-5.

Slitlamp examination. (Photo by M Narahara. Courtesy of the American Academy of Ophthalmology.)

Figure 2-6.

Slitlamp photograph of a normal right eye. The curved slit of light to the right is reflected off of the cornea (C), while the slit to the left is reflected off of the iris (I). As the latter slit passes through the pupil, the anterior lens (L) is faintly illuminated in cross section. (Photo by M Narahara.)

The patient is seated while being examined, and the head is stabilized by an adjustable chin rest and forehead strap. Using the slitlamp alone, the anterior half of the globe—the “anterior segment”—can be visualized. Details of the lid margins and lashes, the palpebral and bulbar conjunctival surfaces, the tear film and cornea, the iris, and the aqueous can be studied. Through a dilated pupil, the crystalline lens and the anterior vitreous can be examined as well.

Because the slit beam of light provides an optical cross section of the eye, the precise anteroposterior location of abnormalities can be determined within each of the clear ocular structures (eg, cornea, lens, vitreous body). The highest magnification setting is sufficient to show the abnormal presence of cells within the aqueous, such as red or white blood cells or pigment granules. Aqueous turbidity, called “flare,” resulting from increased protein concentration can be detected in the presence of intraocular inflammation. Normal aqueous is optically clear, without cells or flare.

Adjunctive Slitlamp Techniques

The eye examination with the slitlamp is supplemented by the use of various techniques. Tonometry is discussed separately in a subsequent section.

Lid Eversion

Lid eversion, to examine the undersurface of the upper lid, can be performed either at the slitlamp or without the aid of that instrument. It should always be done if the presence of a superficial foreign body is suspected but not already identified (see Chapter 19). A semirigid plate of cartilage called the tarsus gives each lid its contour and shape. In the upper lid, the superior edge of the tarsus lies centrally about 8–9 mm above the lashes. On the undersurface of the lid, it is covered by the tarsal palpebral conjunctiva.

Following topical anesthesia, the patient is positioned at the slitlamp and instructed to look down. The examiner gently grasps the upper lashes with the thumb and index finger of one hand while using the other hand to position an applicator handle just above the superior edge of the tarsus (Figure 2–7). The lid is everted by applying slight downward pressure with the applicator as the lash margin is simultaneously lifted. The patient continues to look down, and the lashes are held pinned to the skin overlying the superior orbital rim as the applicator is withdrawn. The tarsal conjunctiva is then examined under magnification. To undo eversion, the lid margin is gently stroked downward as the patient looks up.

Figure 2-7.

Technique of lid eversion. A: With the patient looking down, the upper lashes are grasped with one hand as an applicator stick is positioned at the superior edge of the upper tarsus (at the upper lid crease). B and C: As the lashes are lifted, slight downward pressure is simultaneously applied with the applicator stick. D: The thumb pins the lashes against the superior orbital rim, allowing examination of the undersurface of the tarsus. (Photos by M Narahara.)

Fluorescein is a specialized dye that stains the cornea and highlights any irregularities of its epithelial surface. Sterile paper strips containing fluorescein are wetted with sterile saline or local anesthetic and touched against the inner surface of the lower lid, instilling the yellowish dye into the tear film. The illuminating light of the slitlamp is made blue with a filter, causing the dye to fluoresce.

A uniform film of dye should cover the normal cornea. If the corneal surface is abnormal, excessive amounts of dye will absorb into or collect within the affected area. Abnormalities can range from tiny punctate dots, such as those resulting from excessive dryness or ultraviolet light damage, to large geographic defects in the epithelium, such as those seen in corneal abrasions or infectious ulcers.

Special Lenses

Special examining lenses can expand and further magnify the slitlamp examination of the eye's interior. A goniolens (Figure 2–8) provides visualization of the anterior chamber “angle” formed by the iridocorneal junction. Other lenses placed on or in front of the eye allow slitlamp evaluation of the posterior half of the globe's interior—the “posterior segment.” Since the slitlamp is a binocular microscope, these lenses provide a magnified three-dimensional view of the posterior vitreous, the fundus, and the disk. Examples are the Goldmann-style three-mirror lens (Figure 2–8) and the Volk-style range of lenses.

Figure 2-8.

Three types of goniolenses. Left: Goldmann three-mirror lens. Besides the goniomirror, there are also two peripheral retinal mirrors and a central fourth mirror for examining the central retina. Center: Koeppe lens. Right: Posner/Zeiss-type lens. (Photo by M Narahara.)

Special Attachments

Special attachments to the slitlamp allow it to be used with several techniques requiring microscopic visualization. Special camera bodies can be attached for photographic documentation and for special applications such as corneal endothelial cell studies. Special instruments for the study of visual potential require attachment to the slitlamp. Finally, laser sources are attached to a slitlamp to allow microscopic visualization and control of eye treatment.

Tonometry

The globe can be thought of as an enclosed compartment through which there is a constant circulation of aqueous humor. This fluid maintains the shape and a relatively uniform pressure within the globe. Tonometry is the method of measuring intraocular pressure using calibrated instruments. The normal range is 10 to 21 mm Hg.

In applanation tonometry, intraocular pressure is determined by the force required to flatten the cornea by a standard amount. The force required increases with intraocular pressures. The Schiotz tonometer, now rarely used, measures the amount of corneal indentation produced by preset weights. Less corneal indentation is produced as intraocular pressure rises. Since both methods employ devices that touch the patient's cornea, they require topical anesthetic and disinfection of the instrument tip prior to use. (Tonometer disinfection techniques are discussed in Chapter 21.) With any method of tonometry, care must be taken to avoid pressing on the globe and artificially increasing its pressure.

Applanation Tonometry

The Goldmann applanation tonometer (Figure 2–9) is attached to the slitlamp and measures the amount of force required to flatten the corneal apex by a standard amount. The higher the intraocular pressure, the greater the force required. Since Goldmann applanation tonometer is a more accurate method than Schiotz tonometry, it is preferred by ophthalmologists.

Figure 2-9.

Applanation tonometry, using the Goldmann tonometer attached to the slit lamp. (Photo by M Narahara. Courtesy of the American Academy of Ophthalmology.)

Following topical anesthesia and instillation of fluorescein, the patient is positioned at the slitlamp and the tonometer is swung into place. To visualize the fluorescein, the cobalt blue filter is used with the brightest illumination setting. After grossly aligning the tonometer in front of the cornea, the examiner looks through the slitlamp ocular just as the tip contacts the cornea. A manually controlled counterbalanced spring varies the force applied by the tonometer tip.

Upon contact, the tonometer tip flattens the central cornea and produces a thin circular outline of fluorescein. A prism in the tip visually splits this circle into two semicircles that appear green while viewed through the slitlamp oculars. The tonometer force is adjusted manually until the two semicircles just overlap, as shown in Figure 2–10. This visual end point indicates that the cornea has been flattened by the set standard amount. The amount of force required to do this is translated by the scale into a pressure reading in millimeters of mercury.

Figure 2-10.

Appearance of fluorescein semicircles, or “mires,” through the slitlamp ocular, showing the end point for applanation tonometry.

Accuracy of intraocular pressure measurement is affected by central corneal thickness. The thinner the cornea, the more easily it is indented, but the calibration of tonometers generally assumes a cornea of standard thickness. If the cornea is relatively thin, the actual intraocular pressure is higher than the measured value, and if the cornea is relatively thick, the actual intraocular pressure is lower than the measured value. Thus ultrasonic measurement of corneal thickness (pachymetry) may be helpful in assessment of intraocular pressure. The Pascal dynamic contour tonometer, a contact but non-applanating technique, measures intraocular pressure independent of corneal thickness.

Other applanation tonometers are the Perkins tonometer, a portable mechanical device with a mechanism similar to the Goldmann tonometer, the Tono-Pen, a portable electronic applanation tonometer that is reasonably accurate but requires daily recalibration, and the pneumatotonometer, which is particularly useful when the cornea has an irregular surface. The Perkins tonometer and Tono-Pen are commonly used when examination at the slitlamp is not feasible, for example, in emergency rooms in cases of orbital trauma with retrobulbar hemorrhage and in operating rooms during examinations under anesthesia.

Schiotz Tonometry

The advantage of this method is that it is simple, requiring only a relatively inexpensive, easily portable hand-held instrument. It can be used in any clinic or emergency room setting, at the hospital bedside, or in the operating room, but it requires greater expertise and has generally been superseded by applanation tonometers.

Noncontact Tonometry

The noncontact (“air-puff”) tonometer is not as accurate as applanation tonometers. A small puff of air is blown against the cornea. The air rebounding from the corneal surface hits a pressure-sensing membrane in the instrument. This method does not require anesthetic drops, since no instrument touches the eye. Thus, it can be more easily used by optometrists or technicians and is useful in screening programs.

Diagnostic Medications

Topical Anesthetics

Eye drops such as proparacaine, tetracaine, and benoxinate provide rapid onset, short-acting topical anesthesia of the cornea, and conjunctiva. They are used prior to ocular contact with diagnostic lenses and instruments such as the tonometer. Other diagnostic manipulations utilizing topical anesthetics will be discussed later. These include corneal and conjunctival scrapings, lacrimal canalicular and punctal probing, and scleral depression.

Mydriatic (Dilating) Drops

The pupil can be pharmacologically dilated by either stimulating the iris dilator muscle with a sympathomimetic agent (eg, 2.5% phenylephrine) or by inhibiting the sphincter muscle with an anticholinergic eye drop (eg, 0.5% or 1% tropicamide) (see Chapter 22). Anticholinergic medications also inhibit accommodation (cycloplegic). This may aid the process of refraction but causes further inconvenience for the patient. Therefore, drops with the shortest duration of action (usually several hours) are used for diagnostic applications. Combining drops from both pharmacologic classes produces the fastest onset (15–20 minutes) and widest dilation.

Because dilation can cause a small rise in intraocular pressure, tonometry should always be performed before these drops are instilled. There is also a small risk of precipitating an attack of acute angle-closure glaucoma if the patient has preexisting narrow anterior chamber angles (between the iris and cornea). Such an eye can be identified using the technique illustrated in Figure 11–4. Finally, excessive instillation of these drops should be avoided because of the systemic absorption that can occur through the nasopharyngeal mucous membranes following lacrimal drainage.

A more complete discussion of diagnostic drops is found in Chapter 22.

Direct Ophthalmoscopy

Instrumentation

The hand-held direct ophthalmoscope provides a monocular image, including a 15× magnified view of the fundus. Because of its portability and the detailed view of the disk and retinal vasculature it provides, direct ophthalmoscopy is a standard part of the general medical examination. The intensity, color, and spot size of the illuminating light can be adjusted, as well as the ophthalmoscope's point of focus. The latter is changed using a wheel of progressively higher-power lenses that the examiner dials into place. These lenses are sequentially arranged and numbered according to their power in diopters. Usually the (+) converging lenses are designated by black numbers and the (–) divergent lenses are designated by red numbers.

Anterior Segment Examination

Using the high plus lenses, the direct ophthalmoscope can be focused to provide a magnified view of the conjunctiva, cornea, and iris. The slitlamp allows a far superior and more magnified examination of these areas, but it is not portable and may be unavailable.

Red Reflex Examination

If the illuminating light is aligned directly along the visual axis, more obviously when the pupil is dilated, the pupillary aperture normally is filled by a homogeneous bright reddish-orange color. This red reflex, equivalent to the “red eye” effect of flash photography, is formed by reflection of the illuminating light by the fundus through the clear ocular media—the vitreous, lens, aqueous, and cornea. It is best observed by holding the ophthalmoscope at arm's length from the patient as he looks toward the illuminating light and dialing the lens wheel to focus the ophthalmoscope in the plane of the pupil.

Any opacity located along the central optical pathway will block all or part of the red reflex and appear as a dark spot or shadow. If a focal opacity is seen, have the patient look momentarily away and then back toward the light. If the opacity is still moving or floating, it is located within the vitreous (eg, small hemorrhage). If it is stationary, it is probably in the lens (eg, focal cataract) or on the cornea (eg, scar).

Fundus Examination

The primary value of the direct ophthalmoscope is in examination of the fundus (Figure 2–11). The view may be impaired by cloudy ocular media, such as a cataract, or by a small pupil. Darkening the room usually causes enough natural pupillary dilation to allow evaluation of the central fundus, including the disk, the macula, and the proximal retinal vasculature. Pharmacologically dilating the pupil greatly enhances the view and permits a more extensive examination of the peripheral retina.

Figure 2-11.

Photo and corresponding diagram of a normal fundus. Note that the retinal vessels all stop short of and do not cross the fovea. (Photo by Diane Beeston.)

Examination of the fundus is also optimized by holding the ophthalmoscope as close to the patient's pupil as possible (approximately 1–2 inches), just as one can see more through a keyhole by getting as close to it as possible. This requires using the examiner's right eye and hand to examine the patient's right eye and the left eye and hand to examine the patient's left eye (Figure 2–12).

Figure 2-12.

Direct ophthalmoscopy. The examiner uses the left eye to evaluate the patient's left eye. (Photo by M Narahara. Courtesy of the American Academy of Ophthalmology.)

The spot size and color of the illuminating light can be varied. If the pupil is well dilated, the large spot size of light affords the widest area of illumination. With an undilated pupil, however, much of this light would be reflected back toward the examiner's eye by the patient's iris, interfering with the view, and the pupil will constrict. For this reason, the smaller spot size of light is usually better for undilated pupils.

The refractive error of the patient's and the examiner's eyes will determine the lens power needed to bring the fundus into optimal focus. If the examiner wears spectacles, they can be left either on or off. The patient's spectacles are usually left off, but it may be helpful to leave them on if there is high refractive error.

As the patient fixates on a distant target with the opposite eye, the examiner first brings retinal details into sharp focus. Since the retinal vessels all arise from the disk, the latter is located by following any major vascular branch back to this common origin. At this point, the ophthalmoscope beam will be aimed slightly nasal to the patient's line of vision, or “visual axis.” One should study the shape, size, and color of the disk, the distinctness of its margins, and the size of the pale central “physiologic cup.” The ratio of cup size to disk size is of diagnostic importance in glaucoma (Figures 2–13 and 2–14).

Figure 2-13.

Diagram of a moderately cupped disk viewed on end and in profile, with an accompanying sketch for the patient's record. The width of the central cup divided by the width of the disk is the “cup-to-disk ratio.” The cup-to-disk ratio of this disk is approximately 0.5.

Figure 2-14.

Cup-to-disk ratio of 0.9 in a patient with end-stage glaucoma. The normal disk tissue is compressed into a peripheral thin rim surrounding a huge pale cup.

The macular area (Figure 2–11) is located approximately two “disk diameters” temporal to the edge of the disk. A small pinpoint white reflection or “reflex” marks the central fovea. This is surrounded by a more darkly pigmented and poorly circumscribed area called the macula. The retinal vascular branches approach from all sides but stop short of the fovea. Thus, its location can be confirmed by the focal absence of retinal vessels or by asking the patient to stare directly into the light.

The major retinal vessels are then examined and followed as far distally as possible in each of the four quadrants (superior, inferior, temporal, and nasal). The veins are darker and wider than their paired arteries. The vessels are examined for color, tortuosity, and caliber, as well as for associated abnormalities, such as aneurysms, hemorrhages, or exudates. Sizes and distances within the fundus are often measured in “disk diameters” (DD). (The typical optic disk is generally 1.5–2 mm in diameter.) Thus, one might describe a “1 DD area of hemorrhage located 2.5 DD inferotemporal to the fovea.” The green “red-free” filter assists in the examination of the retinal vasculature and the subtle striations of the nerve fiber layer as they course toward the disk (see Chapter 14).

To examine the retinal periphery, which is greatly enhanced by dilating the pupil, the patient is asked to look in the direction of the quadrant to be examined. Thus, the temporal retina of the right eye is seen when the patient looks to the right, while the superior retina is seen when the patient looks up. When the globe rotates, the retina and the cornea move in opposite directions. As the patient looks up, the superior retina rotates downward into the examiner's line of vision.

Indirect Ophthalmoscopy

Instrumentation

The binocular indirect ophthalmoscope (Figure 2–15) complements and supplements the direct ophthalmoscopic examination. Since it requires wide pupillary dilation and is difficult to learn, this technique is used primarily by ophthalmologists. The patient can be examined while seated, but the supine position is preferable.

Figure 2-15.

Examination with head-mounted binocular indirect ophthalmoscope. A 20-diopter hand-held condensing lens is used. (Photo by M Narahara.)

The indirect ophthalmoscope is worn on the examiner's head and allows binocular viewing through a set of lenses of fixed power. A bright adjustable light source attached to the headband is directed toward the patient's eye. As with direct ophthalmoscopy, the patient is told to look in the direction of the quadrant being examined. A convex lens is hand-held several inches from the patient's eye in precise orientation so as to simultaneously focus light onto the retina and an image of the retina in midair between the patient and the examiner. Using the preset head-mounted ophthalmoscope lenses, the examiner can then “focus on” and visualize this midair image of the retina.

Comparison of Indirect & Direct Ophthalmoscopy

Indirect ophthalmoscopy is so called because one is viewing an “image” of the retina formed by a hand-held “condensing lens.” In contrast, direct ophthalmoscopy allows one to focus on the retina itself. Compared with the direct ophthalmoscope (15× magnification), indirect ophthalmoscopy provides a much wider field of view (Figure 2–16) with less overall magnification (approximately 3.5× using a standard 20-diopter hand-held condensing lens). Thus, it presents a wide panoramic fundus view from which specific areas can be selectively studied under higher magnification using either the direct ophthalmoscope or the slitlamp with special auxiliary lenses.

Figure 2-16.

Comparison of view within the same fundus using the indirect ophthalmoscope (A) and the direct ophthalmoscope (B). The field of view with the latter is approximately 10°, compared with approximately 37° using the indirect ophthalmoscope. In this patient with diabetic retinopathy, an important overview is first seen with the indirect ophthalmoscope. The direct ophthalmoscope can then provide magnified details of a specific area. (Photos by M Narahara.)

Indirect ophthalmoscopy has three distinct advantages over direct ophthalmoscopy. One is the brighter light source that permits much better visualization through cloudy media. A second advantage is that by using both eyes, the examiner enjoys a stereoscopic view, allowing visualization of elevated masses or retinal detachment in three dimensions. Finally, indirect ophthalmoscopy can be used to examine the entire retina, even out to its extreme periphery, the ora serrata. This is possible for two reasons. Optical distortions caused by looking through the peripheral lens and cornea interfere very little with the indirect ophthalmoscopic examination compared with the direct ophthalmoscope. In addition, the adjunct technique of scleral depression can be used.

Scleral depression (Figure 2–17) is performed as the peripheral retina is being examined with the indirect ophthalmoscope. A smooth, thin metal probe is used to gently indent the globe externally through the lids at a point just behind the corneoscleral junction (limbus). As this is done, the ora serrata and peripheral retina are pushed internally into the examiner's line of view. By depressing around the entire circumference, the peripheral retina can be viewed in its entirety.

Figure 2-17.

Diagrammatic representation of indirect ophthalmoscopy with scleral depression to examine the far peripheral retina. Indentation of the sclera through the lids brings the peripheral edge of the retina into visual alignment with the dilated pupil, the hand-held condensing lens, and the head-mounted ophthalmoscope.

Because of all of these advantages, indirect ophthalmoscopy is used preoperatively and intraoperatively in the evaluation and surgical repair of retinal detachments. A disadvantage of indirect ophthalmoscopy, which also applies to the Volk-style of lenses for examination of the posterior segment with a slitlamp, is that it provides an inverted image of the fundus, which requires a mental adjustment on the examiner's part. Its brighter light source can also be more uncomfortable for the patient.

Eye Examination by the Nonophthalmologist

The preceding sequence of tests would comprise a complete routine or diagnostic ophthalmologic evaluation. A general medical examination would often include many of these same testing techniques.

Assessment of pupils, extraocular movements, and confrontation visual fields is part of any complete neurologic assessment. Direct ophthalmoscopy should always be performed to assess the appearance of the disk and retinal vessels. Separately testing the visual acuity of each eye (particularly with children) may uncover either a refractive or a medical cause of decreased vision. The three most common preventable causes of permanent visual loss in developed nations are amblyopia, diabetic retinopathy, and glaucoma. All can remain asymptomatic while the opportunity for preventive measures is gradually lost. During this time, the pediatrician or general medical practitioner may be the only physician the patient visits.

By testing children for visual acuity in each eye, examining and referring diabetics for regular dilated fundus ophthalmoscopy, and referring patients with suspicious discs to the ophthalmologist, the nonophthalmologist may indeed be the one who truly “saves” that patient's eyesight. This represents both an important opportunity and responsibility for every primary care physician.

This section will discuss ophthalmologic examination techniques with more specific indications that would not be performed on a routine basis. They will be grouped according to the function or anatomic area of primary interest.

Diagnosis of Visual Abnormalities

Perimetry

Perimetry is used to examine the central and peripheral visual fields. Usually performed separately for each eye, it assesses the combined function of the retina, the optic nerve, and the intracranial visual pathway. It is used clinically to detect or monitor field loss due to disease at any of these locations. Damage to specific parts of the neurologic visual pathway may produce characteristic patterns of change on serial field examinations.

The visual field of the eye is measured and plotted in degrees of arc. Measurement of degrees of arc remains constant regardless of the distance from the eye that the field is checked. The sensitivity of vision is greatest in the center of the field (corresponding to the fovea) and least in the periphery. Perimetry relies on subjective patient responses, and the results will depend on the patient's psychomotor as well as visual status. Perimetry must always be performed and interpreted with this in mind.

The Principles of Testing

Although perimetry is subjective, the methods discussed below have been standardized to maximize reproducibility and permit subsequent comparison. Perimetry requires (1) steady fixation and attention by the patient; (2) a set distance from the eye to the screen or testing device; (3) a uniform, standard amount of background illumination and contrast; (4) test targets of standard size and brightness; and (5) a universal protocol for administration of the test by examiners.

As the patient's eye fixates on a central target, test objects are randomly presented at different locations throughout the field. If they are seen, the patient responds either verbally or with a hand-held signaling device. Varying the target's size or brightness permits quantification of visual sensitivity of different areas in the field. The smaller or dimmer the target seen, the higher the sensitivity of that location.

There are two basic methods of target presentation—static and kinetic—that can be used alone or in combination during an examination. In static perimetry, different locations throughout the field are tested one at a time. A dim stimulus, usually a white light, is first presented at a particular location. If it is not seen, the size or intensity is incrementally increased until it is just large enough or bright enough to be detected. This is called the “threshold” sensitivity level of that location. This sequence is repeated at a series of other locations, so that the sensitivity of multiple points in the field can be evaluated and combined to form a profile of the visual field.

In kinetic perimetry, the sensitivity of the entire field to one single test object (of fixed size and brightness) is first tested. The object is slowly moved toward the center from a peripheral area until it is first spotted. By moving the same object inward from multiple directions, a boundary called an “isopter” can be mapped out that is specific for that target. The isopter outlines the area within which the target can be seen and beyond which it cannot be seen. Thus, the larger the isopter, the better the visual field of that eye. The boundaries of the isopter are measured and plotted in degrees of arc. By repeating the test using objects of different size or brightness, multiple isopters can then be plotted for a given eye. The smaller or dimmer test objects will produce smaller isopters.

Methods of Perimetry

The tangent screen is the simplest apparatus for standardized perimetry. It utilizes different-sized pins on a black wand presented against a black screen and is used primarily to test the central 30° of visual field. The advantages of this method are its simplicity and rapidity, the possibility of changing the subject's distance from the screen, and the option of using any assortment of fixation and test objects, including different colors.

The more sophisticated Goldmann perimeter (Figure 2–18) is a hollow white spherical bowl positioned a set distance in front of the patient. A light of variable size and intensity can be presented by the examiner (seated behind the perimeter) in either static or kinetic fashion. This method can test the full limit of peripheral vision and was for years the primary method for plotting fields in glaucoma patients.

Figure 2-18.

Goldmann perimeter. (Photo by M Narahara.)

Computerized automated perimeters (Figure 2–19) now constitute the most sophisticated and sensitive equipment available for visual field testing. Using a bowl similar to the Goldmann perimeter, these instruments display test lights of varying brightness and size but use a quantitative static threshold testing format that is more precise and comprehensive than other methods. Numerical scores (Figure 2–20) corresponding to the threshold sensitivity of each test location can be stored in the computer memory and compared statistically with results from previous examinations or from other normal patients. The higher the numerical score, the better the visual sensitivity of that location in the field. Another important advantage is that the test presentation is programmed and automated, eliminating any variability on the part of the examiner. Analysis of the results provides information on whether visual field loss is diffuse or focal and on the patient's ability to perform the test reliably.

Figure 2-19.

Computerized automated perimeter.

Figure 2-20.

A: Numerical printout of threshold sensitivity scores derived by using the static method of computerized perimetry. This is the 30° field of a patient's right eye with glaucoma. The higher the numbers, the better the visual sensitivity. The computer retests many of the points (bracketed numbers) to assess consistency of the patient's responses. B: Diagrammatic “gray scale” display of these same numerical scores. The darker the area, the poorer the visual sensitivity at that location.

Amsler Grid

The Amsler grid is used to test the central 20° of the visual field. The grid (Figure 2–21) is viewed by each eye separately at normal reading distance and with reading glasses on if the patient uses them. It is most commonly used to test macular function.

Figure 2-21.

Amsler grid.

While fixating on the central dot, the patient checks to see that the lines are all straight, without distortion, and that no spots or portions of the grid are missing. One eye is compared with the other. A scotoma or blank area—either central or paracentral—can indicate disease of the macula or optic nerve. Wavy distortion of the lines (metamorphopsia) can indicate macular edema or submacular fluid.

The grid can be used by patients at home to test their own central vision. For example, patients with age-related macular degeneration (see Chapter 10) can use the grid to monitor for sudden metamorphopsia. This often is the earliest symptom of acute fluid accumulation beneath the macula arising from leaking sub-retinal neovascularization. Since these abnormal vessels may respond to prompt treatment, early detection is important.

Brightness Acuity Testing

The visual abilities of patients with media opacities may vary depending on conditions of lighting. For example, when dim illumination makes the pupil larger, one may be able to “see around” a central focal cataract, whereas bright illumination causing pupillary constriction would have the contrary effect. Bright lights may also cause disabling glare in patients with corneal edema or diffuse clouding of the crystalline lens due to light scattering.

Because the darkened examining room may not accurately elicit the patient's functional difficulties in real life, instruments have been developed to test the effect of varying levels of brightness or glare on visual acuity. Distance acuity with the Snellen chart is usually tested under standard levels of incrementally increasing illumination, and the information may be helpful in making therapeutic or surgical decisions. Asking cataract patients specific questions about how their vision is affected by various lighting conditions is even more important.

Color Vision Testing

Normal color vision requires healthy function of the macula and optic nerve. The most common abnormality is red-green “color blindness,” which is present in approximately 8% of the male population. This is due to an X-linked congenital deficiency of one specific type of retinal photoreceptor. Depressed color vision may also be a sensitive indicator of certain kinds of acquired macular or optic nerve disease. For example, in optic neuritis or optic nerve compression (eg, by a mass), abnormal color vision is often an earlier indication of disease than visual acuity, which may still be 20/20.

The most common testing technique utilizes a series of polychromatic plates, such as those of Ishihara or Hardy-Rand-Rittler (Figure 2–22). The plates are made up of dots of the primary colors printed on a background mosaic of similar dots in a confusing variety of secondary colors. The primary dots are arranged in simple patterns (numbers or geometric shapes) that cannot be recognized by patients with deficient color perception.

Figure 2-22.

Hardy-Rand-Rittler (H-R-R) pseudoisochromatic plates for testing color vision.

Contrast-Sensitivity Testing

Contrast sensitivity is the ability of the eye to discern subtle degrees of contrast. Retinal and optic nerve disease and clouding of the ocular media (eg, cataracts) can impair this ability. Like color vision, contrast sensitivity may become depressed before Snellen visual acuity is affected in many situations.

Contrast sensitivity is best tested by using standard preprinted charts with a series of test targets (Figure 2–23). Since illumination greatly affects contrast, it must be standardized and checked with a light meter. Each separate target consists of a series of dark parallel lines in one of three different orientations. They are displayed against a lighter, contrasting gray background. As the contrast between the lines and their background is progressively reduced from one target to the next, it becomes more difficult for the patient to judge the orientation of the lines. The patient can be scored according to the lowest level of contrast at which the pattern of lines can still be discerned.

Figure 2-23.

Contrast-sensitivity test chart. (Courtesy of Vistech Consultants, Inc.)

Assessing Potential Vision

When opacities of the cornea or lens coexist with disease of the macula or optic nerve, the visual potential of the eye is often in doubt. The benefit of corneal transplantation or cataract extraction will depend on the severity of coexisting retinal or optic nerve impairment. Several methods are available for assessing central visual potential under these circumstances.

Even with a totally opaque cataract that completely prevents a view of the fundus, the patient should still be able to identify the direction of a light directed into the eye from different quadrants. When a red lens is held in front of the light, the patient should be able to differentiate between white and red light. The presence of a relative afferent pupillary defect indicates significant disease of the retina or optic nerve, and thus a poor visual prognosis.

A gross test of macular function involves the patient's ability to perceive so-called entoptic phenomena. For example, as the eyeball is massaged with a rapidly moving penlight through the closed lids, the patient should be able to visualize an image of the paramacular vascular branches if the macula is healthy. These may be described as looking like “the veins of a leaf.” Because this test is highly subjective and subject to interpretation, it is only helpful if the patient is able to recognize the vascular pattern in at least one eye. Absence of the pattern in the opposite eye then suggests macular impairment.

In addition to these gross methods, sophisticated quantitative instruments have been developed for more direct determination of visual potential in eyes with media opacities. These instruments project a narrow beam of light containing a pattern of images through any relatively clear portion of the media (eg, through a less-dense region of a cataract) and onto the retina. The patient's vision is then graded according to the size of the smallest patterns that can be seen.

Two different types of patterns are used. Laser interferometry employs laser light to generate interference fringes or gratings, which the patient sees as a series of parallel lines. Progressively narrowing the width and spacing of the lines causes an end point to be reached where the patient can no longer discern the orientation of the lines. The narrowest image width the patient can resolve is then correlated with a Snellen acuity measurement to determine the visual potential of that eye. The potential acuity meter projects a standard Snellen acuity chart onto the retina. The patient is then graded in the usual fashion, according to the smallest line of letters read.

Although both instruments appear useful in measuring potential visual acuity, false-positive and false-negative results do occur, with a frequency dependent on the type of disease present. Thus, these methods are helpful but not completely reliable in determining the visual prognosis of eyes with cloudy media.

Tests for Functional Visual Loss

The measurement of vision is subjective, requiring responses on the part of the patient. The validity of the test may therefore be limited by the alertness or cooperation of the patient. “Functional” visual loss is a subjective complaint of impaired vision without any demonstrated organic or objective basis. Examples include hysterical blindness and malingering.

Recognition of functional visual loss or malingering depends on the use of testing variations in order to elicit inconsistent or contradictory responses. An example would be eliciting “tunnel” visual fields using the tangent screen. A patient claiming “poor vision” and tested at the standard distance of 1 meter may map out a narrow central zone of intact vision beyond which even large objects—such as a hand—allegedly cannot be seen. The borders (“isopter”) of this apparently small area are then marked. The patient is then moved back to a position 2 meters from the tangent screen. From this position, the field should be twice as large as the area plotted from 1 meter away. If the patient outlines an area of the same size from both testing distances, this raises a strong suspicion of functional visual loss, but a number of conditions, such as advanced glaucoma, severe retinitis pigmentosa, and cortical blindness, would need to be excluded.

A variety of other different tests can be chosen to assess the validity of different degrees of visual loss that may be in question.

Diagnosis of Ocular Abnormalities

Microbiology & Cytology

Like any mucous membrane, the conjunctiva can be cultured with swabs for the identification of bacterial infection. Specimens for cytologic examination are obtained by lightly scraping the palpebral conjunctiva (ie, lining the inner aspect of the lid), such as with a small platinum spatula, following topical anesthesia. For the cytologic evaluation of conjunctivitis, Giemsa's stain is used to identify the types of inflammatory cells present, while Gram's stain may demonstrate the presence (and type) of bacteria. These applications are discussed at length in Chapter 5.

The cornea is normally sterile. The base of any suspected infectious corneal ulcer should be scraped with the platinum spatula or other device for Gram staining and culture. This procedure is performed at the slitlamp. Because in many cases only trace quantities of bacteria are recoverable, the scrapings should be transferred directly onto culture plates without the intervening use of transport media. Any amount of culture growth, no matter how scant, is considered significant, but many cases of infection may still be “culture-negative.”

Culture of intraocular fluids is the standard method of diagnosing or ruling out bacterial endophthalmitis. Aqueous can be tapped by inserting a short 25-gauge needle on a tuberculin syringe through the limbus parallel to the iris. Care must be taken not to traumatize the lens. The diagnostic yield is better if vitreous is cultured. Vitreous specimens can be obtained by a needle tap through the pars plana or by doing a surgical vitrectomy. Polymerase chain reaction of vitreous samples has become the standard method of diagnosing viral retinitis. In the evaluation of noninfectious intraocular inflammation, cytology specimens are occasionally obtained using similar techniques.

Techniques for Corneal Examination

Several additional techniques are available for more specialized evaluation of the cornea. The keratometer is a calibrated instrument that measures the radius of curvature of the cornea in two meridians 90° apart. If the cornea is not perfectly spherical, the two radii will be different. This results in corneal astigmatism, which is quantified by the difference between the two radii of curvature. Keratometer measurements are used in contact lens fitting and for intraocular lens power calculations prior to cataract surgery.

Many corneal diseases result in distortion of the otherwise smooth surface of the cornea, which impairs its optical quality. The photokeratoscope is an instrument that assesses the uniformity and evenness of the surface by reflecting a pattern of concentric circles onto it. This pattern, which can be visualized and photographed through the instrument, should normally appear perfectly regular and uniform. Focal corneal irregularities will instead distort the circular patterns reflected from that particular area.

Computerized corneal topography is an advanced technique of mapping the anterior corneal surface. Whereas keratometry provides only a single corneal curvature measurement and photokeratoscopy provides only qualitative information, these computer systems combine and improve on the features of both. A real-time video camera records the concentric keratoscopic rings reflected from the cornea. A computer digitizes the data from thousands of locations across the corneal surface and displays the measurements in a color-coded map (Figure 2–24). This enables one to quantify and analyze minute changes in shape and refractive power across the entire cornea induced by disease or surgery. Wavefront aberrometry measures the quality of the eye's optics, and may be combined together with corneal topography in a single instrument (Figure 2–24). By recording the path of diagnostic laser beams bouncing off of the retina, these devices can diagnose optical distortions called higher-order aberrations that are caused either by the cornea or the lens. Higher-order aberrations can result in blurred vision, halos, glare, and starbursts that are most symptomatic at night due to larger pupil size. These optical distortions are not corrected by eyeglasses.

Figure 2-24.

A: Computerized corneal topography and wavefront aberrometry system. B: Color-coded corneal topographic display of curvature across the entire corneal surface, combined with quantitative measurements of higher order aberrations from the total eye (top right), lens (top left), and cornea (bottom left). (Photos courtesy of Tracey Technologies, Inc.)

The endothelium is a monolayer of cells lining the posterior corneal surface, which function as fluid pumps and are responsible for keeping the cornea thin and dehydrated, thereby maintaining its optical clarity. If these cells become impaired or depleted, corneal edema and thickening result, ultimately decreasing vision. The endothelial cells themselves can be photographed with a special slitlamp camera, enabling one to study cell morphology and perform cell counts. Central corneal thickness can be accurately measured with an ultrasonic pachymeter. These measurements are useful for monitoring increasing corneal thickness due to edema caused by progressive endothelial cell loss and, as discussed earlier, in determining the validity of intraocular pressure measurements obtained by applanation tonometry.

Gonioscopy

The anterior chamber—the space between the iris and the cornea—is filled with liquid aqueous humor, which is produced behind the iris by the ciliary body and exits the eye through the sieve-like drainage system of the trabecular meshwork. The meshwork is arranged as a thin circumferential band of tissue just anterior to the base of the iris and within the angle formed by the iridocorneal junction (Figure 11–3). This angle recess can vary in its anatomy, pigmentation, and width of opening—all of which may affect aqueous drainage and be of diagnostic relevance for glaucoma.

Gonioscopy is the method of examination of the anterior chamber angle anatomy using binocular magnification and a special goniolens. The Goldmann and Posner–Zeiss types of goniolenses (Figure 2–8) have special mirrors angled so as to provide a line of view parallel with the iris surface and directed peripherally toward the angle recess, the anterior chamber angle not being amenable to direct visualization (see Chapter 21). After topical anesthesia, the patient is seated at the slitlamp and the goniolens is placed on the eye (Figure 2–25). Magnified details of the anterior chamber angle are viewed stereoscopically. By rotating the mirror, the entire 360° circumference of the angle can be examined. The same lens can be used to direct laser treatment toward the angle as therapy for glaucoma.

Figure 2-25.

Gonioscopy with slitlamp and Goldmann-type lens. (Photo by M Narahara.)

A third type of goniolens, the Koeppe lens, requires a special illuminator and a separate hand-held binocular microscope. It is used with the patient lying supine and can thus be used either in the office or in the operating room (either diagnostically or for surgery).

Goldmann Three-Mirror Lens

The Goldmann lens is a versatile adjunct to the slitlamp examination (Figure 2–8). Three separate mirrors, all with different angles of orientation, allow the examiner's line of sight to be directed peripherally at three different angles while using the standard slitlamp. The most anterior and acute angle of view is achieved with the goniolens, discussed above.

Through a dilated pupil, the other two mirrored lenses angle the examiner's view toward the retinal mid periphery and far periphery, respectively. As with gonioscopy, each lens can be rotated 360° circumferentially and can be used to aim laser treatment. A fourth central lens (no mirror) is used to examine the posterior vitreous and the central-most area of the retina. The stereoscopic magnification of this method provides the greatest three-dimensional detail of the macula and disk.

The patient's side of the lens has a concavity designed to fit directly over the topically anesthetized cornea. A clear, viscous solution of methylcellulose is placed in the concavity of the lens prior to insertion onto the patient's eye. This eliminates interference from optical interfaces, such as bubbles, and provides mild adhesion of the lens to the eye for stabilization.

Fundus Photography

Special retinal cameras are used to document details of the fundus for study and future comparison. In the past, standard film was used for 35-mm color slides. Digital photography is now more common. As with any form of ophthalmoscopy, a dilated pupil and clear ocular media provide the most optimal view. All of the fundus photographs in this textbook were taken with such a camera.

One of the most common applications is disk photography, used in the evaluation for glaucoma. Since the slow progression of glaucomatous optic nerve damage may be evident only by subtle alteration of the disk's appearance over time (see Chapter 11), precise documentation of its morphology is needed. By slightly shifting the camera angle on two consecutive shots, a “stereo” pair of slides can be produced that will provide a three-dimensional image when studied through a stereoscopic slide viewer. Stereo disk photography thus provides the most sensitive means of detecting increases in glaucomatous cupping.

Fluorescein Angiography

The capabilities of fundus photographic imaging can be tremendously enhanced by fluorescein, a dye whose molecules emit green light when stimulated by blue light. When photographed, the dye highlights vascular and anatomic details of the fundus. Fluorescein angiography is invaluable in the diagnosis and evaluation of many retinal conditions. Because it can so precisely delineate areas of abnormality, it is an essential guide for planning laser treatment of retinal vascular disease.

Technique

The patient is seated in front of the retinal camera following pupillary dilation. After a small amount of fluorescein is injected into a vein in the arm, it circulates throughout the body before eventually being excreted by the kidneys. As the dye passes through the retinal and choroidal circulation, it can be visualized and photographed because of its properties of fluorescence. Two special filters within the camera produce this effect. A blue “excitatory” filter bombards the fluorescein molecules with blue light from the camera flash, causing them to emit a green light. The “barrier” filter allows only this emitted green light to reach the photographic film, blocking out all other wavelengths of light. A digital black and white photograph results, in which only the fluorescein image is seen.

Because the fluorescein molecules do not diffuse out of normal retinal vessels, the latter are highlighted photographically by the dye (Figure 2–26). The diffuse, background “ground glass” appearance results from fluorescein filling of the separate underlying choroidal circulation. The choroidal and retinal circulations are anatomically separated by a thin, homogeneous monolayer of pigmented cells—the “retinal pigment epithelium.” Denser pigmentation located in the macula obscures more of this background choroidal fluorescence (Figure 2–26), causing the darker central zone on the photograph. In contrast, focal atrophy of the pigment epithelium causes an abnormal increase in visibility of the background fluorescence (Figure 2–27).

Figure 2-26.

Normal angiogram of the central retina. The photo has been taken after the dye (appearing white) has already sequentially filled the choroidal circulation (seen as a diffuse, mottled whitish background), the arterioles, and the veins. The macula appears dark due to heavier pigmentation, which obscures the underlying choroidal fluorescence that is visible everywhere else. (Photo courtesy of R Griffith and T King.)

Figure 2-27.

Abnormal angiogram in which dye-stained fluid originating from the choroid has pooled beneath the macula. This is one type of abnormality associated with age-related macular degeneration (see Chapter 10). Secondary atrophy of the overlying retinal pigment epithelium in this area causes heightened, un-obscured visibility of this increased fluorescence. (Photo courtesy of R Griffith and T King.)

Applications

A high-speed motorized frame advance allows for rapid sequence photography of the dye's transit through the retinal and choroidal circulations over time. A fluorescein study or “angiogram” therefore consists of multiple sequential black and white photos of the fundi taken at different times following dye injection (Figure 2–28). Early-phase photos document the dye's initial rapid, sequential perfusion of the choroid, the retinal arteries, and the retinal veins. Later-phase photos may, for example, demonstrate the gradual, delayed leakage of dye from abnormal vessels. This extravascular dye-stained edema fluid will persist long after the intravascular fluorescein has exited the eye.

Figure 2-28.

Fluorescein angiographic study of an eye with proliferative diabetic retinopathy demonstrating variations in the dye pattern over several minutes' time. A: Fundus photograph of left eye (before fluorescein) showing neovascularization (abnormal new vessels) on the disk and inferior to the macula (arrows). This latter area has bled, producing the arcuate preretinal hemorrhage at the bottom of the photo (open arrow). B: Early-phase angiogram of the same eye, in which fluorescein has initially filled the arterioles and highlighted the area of the disk neovascularization. C: Midphase angiogram of the same eye, in which dye has begun to leak out of the hyperpermeable areas of neovascularization. In addition to the irregular venous caliber and the microaneurysms (white dots), extensive areas of ischemia are apparent by virtue of the gross absence of vessels (and therefore dye) in many areas (see arrows). D: Late-phase photo demonstrating increasing amounts of dye leakage over time. Although the preretinal hemorrhage does not stain with dye, it is detectable as a solid black area since it obscures all underlying fluorescence (arrows). (Photos courtesy of University of California, San Francisco.)

Figure 2–28 illustrates several of the retinal vascular abnormalities that are well demonstrated by fluorescein angiography. The dye delineates structural vascular alterations, such as aneurysms or neovascularization. Changes in blood flow such as ischemia and vascular occlusion are seen as an interruption of the normal perfusion pattern. Abnormal vascular permeability is seen as a leaking cloud of dye-stained edema fluid increasing over time. Hemorrhage does not stain with dye but rather appears as a dark, sharply demarcated void. This is due to blockage and obscuration of the underlying background fluorescence.

Indocyanine Green Angiography

The principal use for fluorescein angiography in age-related macular degeneration (Chapter 10) is in locating subretinal choroidal neovascularization for possible laser photocoagulation. The angiogram may show a well-demarcated neovascular membrane. Frequently, however, the area of choroidal neovascularization is poorly defined (“occult”) because of surrounding or overlying blood, exudate, or serous fluid.

Indocyanine green angiography is a separate technique that is superior for imaging the choroidal circulation. Fluorescein diffuses out of the choriocapillaris, creating a diffuse background fluorescence. As opposed to fluorescein, indocyanine green is a larger molecule that binds completely to plasma proteins, causing it to remain in the choroidal vessels. Thus, larger choroidal vessels can be imaged. Unique photochemical properties allow the dye to be transmitted better through melanin (eg, in the retinal pigment epithelium), blood, exudate, and serous fluid. This technique therefore serves as an important adjunct to fluorescein angiography for imaging occult choroidal neovascularization and other choroidal vascular abnormalities.

Following dye injection, angiography is performed using special digital video cameras. The digital images can be further enhanced and analyzed by computer.

Optical Coherence Tomography

Optical Coherence Tomography (OCT) is a computerized, cross sectional tomographic imaging modality used to examine and measure intraocular structures in three dimensions. The operational principle of OCT is analogous to ultrasound, except that it uses 840-nm-wavelength light instead of sound. Because the speed of light is nearly one million times faster than the speed of sound, OCT can image and measure structures on a 5-μm scale, compared to the 100-μm image resolution for ultrasound. OCT can be performed through an undilated pupil and, unlike ultrasound, does not require contact with the tissue examined. The instrumentation is similar to a fundus camera and is used in the office.

The OCT interferometer measures the echo delay time of light that is projected from a superluminescent diode and then reflected from different structures within the eye. Posterior segment OCT enables detailed analysis of the optic disk, retinal nerve fiber layer, and macula. Microscopic changes in the macula, such as edema (Figure 2–29), can be imaged and measured. For the anterior segment, a different OCT instrument projecting a longer-wavelength infrared light beam (1300 nm) is used. This can provide high-resolution images and measurements of the cornea, iris, and intraocular devices and lenses.

Figure 2-29.

Optical coherence tomography cross section image of a normal macula (A) and a macula with pigment epithelial detachment showing fluid beneath the retinal pigment epithelium (B). (Images taken with Cirrus Spectral Domain OCT, Carl Zeiss Meditec, Inc.)

Laser Imaging Technologies (for Disk & Retina)

In early glaucoma, morphologic changes of the disk and retinal nerve fiber layer (RNFL) usually precede the appearance of visual field abnormalities. Newer technologies such as scanning laser polarimetry, scanning laser tomography (SLT), and OCT are able to image and quantify the microscopic details of the optic disk and the surrounding RNFL.

In confocal SLT reflections from a scanning laser beam are recorded at different tissue depths so as to provide a series of 64 tomographic coronal sections perpendicular to the optical axis—like a series of computed tomography (CT) scans. Software programs display this data as three-dimensional topographic images (Figure 2–30), similar analyses also being obtainable from OCT images (Figure 2-31). Comparing the thickness of the RNFL and the volume of the cup to data from normal individuals and repeated examinations facilitates early detection and monitoring of glaucoma.

Figure 2-30.

Confocal scanning laser topographic image generated by the Heidelberg Retinal Tomograph II. Upper left image color codes areas according to height, the central area being the depression of the cup. Upper right image statistically analyzes cup-disk proportions in six sectors. “X” indicates abnormal sectors. Bottom graph plots retinal nerve fiber layer thickness. (Photo courtesy of Heidelberg Engineering.)

Figure 2-31.

OCT-derived color-coded maps of retinal nerve fiber layer thickness, with disc and cup masked (A) and indicating deviation from normal with cup and disc edges outlined (B).

Electrophysiologic Testing

Physiologically, “vision” results from a series of electrical signals initiated in the retina and ending in the occipital cortex. Electroretinography, electro-oculography, and visual evoked response testing are methods of evaluating the integrity to the neural circuitry.

Electroretinography & Electro-Oculography

Electroretinography (ERG) measures the electrical response of the retina to flashes of light, the flash electroretinogram, or to a reversing checkerboard stimulus, the pattern ERG (PERG). The recording electrode is placed on the surface of the eye, and a reference electrode is placed on the skin of the face. The amplitude of the electrical signal is less than 1 mV, and amplification of the signal and computer averaging of the response to repeated trials are thus necessary to achieve reliable results.

The flash ERG has two major components: the “a wave” and the “b wave.” An early receptor potential preceding the “a wave” and oscillatory potentials superimposed on the “b wave” may be recorded under certain circumstances. The early part of the flash ERG reflects photoreceptor function, whereas the later response particularly reflects the function of the Müller cells, which are glial cells within the retina. Varying the intensity, wavelength, and frequency of the light stimulus and recording under conditions of light or dark adaptation modulates the waveform of the flash ERG and allows examination of rod and cone photoreceptor function. The flash ERG is a diffuse response from the whole retina and is thus sensitive only to widespread, generalized diseases of the retina, eg, inherited retinal degenerations (retinitis pigmentosa), in which flash ERG abnormalities precede visual loss; congenital retinal dystrophies, in which flash ERG abnormalities may precede ophthalmoscopic abnormalities; and toxic retinopathies from drugs or chemicals (eg, iron intraocular foreign bodies). It is not sensitive to focal retinal disease, even when the macula is affected, and is not sensitive to abnormalities of the retinal ganglion cell layer, such as in optic nerve disease.

The PERG also has two major components: a positive wave at about 50 ms (P50) and a negative wave at about 95 ms (N95) from the time of the pattern reversal. The P50 reflects macular retinal function, whereas the N95 appears to reflect ganglion cell function. Thus, the PERG is useful in distinguishing retinal and optic nerve dysfunction and in diagnosing macular disease.

Electro-oculography (EOG) measures the standing corneoretinal potential. Electrodes are placed at the medial and lateral canthi to record the changes in electrical potential while the patient performs horizontal eye movements. The amplitude of the corneoretinal potential is least in the dark and maximal in the light. The ratio of the maximum potential in the light to the minimum in the dark is known as the Arden index. Abnormalities of the EOG principally occur in diseases diffusely affecting the retinal pigment epithelium and the photoreceptors and often parallel abnormalities of the flash ERG. Certain diseases, such as Best's vitelli-form dystrophy, produce a normal ERG but a characteristically abnormal EOG. EOG is also used to record eye movements.

Visual Evoked Response

Like electroretinography, the visual evoked response (VER) measures the electrical potential resulting from a visual stimulus. However, because it is measured by scalp electrodes placed over the occipital cortex, the entire visual pathway from retina to cortex must be intact in order to produce a normal electrical waveform reading. Like the ERG wave, the VER pattern is plotted on a scale displaying both amplitude and latency (Figure 2–32).

Figure 2-32.

Top: Normal VER generated by stimulating the left eye (OS) is contrasted with the absent response from the right eye (OD), which has a severe optic nerve lesion. LH and RH signify recordings from electrodes over the left and right hemispheres of the occipital lobe. Bottom: VER with right homonymous hemianopia. No response is recorded from over the left hemisphere. (Courtesy of M Feinsod.)

Interruption of neuronal conduction by a lesion will result in reduced amplitude of the VER. Reduced speed of conduction, such as with demyelination, abnormally prolongs the latency of the VER. Unilateral prechiasmatic (retinal or optic nerve) disease can be diagnosed by stimulating each eye separately and comparing the responses. Postchiasmatic disease (eg, homonymous hemianopia) can be identified by comparing the electrode responses measured separately over each hemisphere.

Proportionately, the majority of the occipital lobe area is devoted to the macula. This large cortical area representing the macula is also in close proximity to the scalp electrode, so that the clinically measured VER is primarily a response generated by the macula and optic nerve. Thus the VER can be used to assess visual acuity, making it a valuable objective test in situations where subjective testing is unreliable, such as in infants, unresponsive individuals, and suspected malingerers.

Dark Adaptation

In going from conditions of bright light to darkness, a certain period of time must pass before the retina regains its maximal sensitivity to low amounts of light. This phenomenon is called dark adaptation. It can be quantified by measuring the recovery of retinal sensitivity to low-light levels over time following a standard period of bright-light exposure. Dark adaptation is often abnormal in retinal diseases characterized by rod photoreceptor dysfunction and impaired night vision.

Diagnosis of Extraocular Abnormalities

Lacrimal System Evaluation

Evaluation of Tear Production

Tears and their components are produced by the lacrimal gland and accessory glands in the lid and conjunctiva (see Chapter 5). The Schirmer test is a simple method for assessing gross tear production. Schirmer strips are disposable 35-mm-length dry strips of filter paper. The tip of one end is folded at the preexisting notch so that it can drape over the lower lid margin just lateral to the cornea.

Tears in the conjunctival sac will cause progressive wetting of the paper strip. The distance between the leading edge of wetness and the initial fold can be measured after 5 minutes using a millimeter ruler. The ranges of normal measurements vary depending on whether topical anesthetic is used. Without anesthesia, irritation from the Schirmer strip itself will cause reflex tearing, thereby increasing the measurement. With anesthesia, less than 5 mm of wetting after 5 minutes is considered abnormal.

Significant degrees of chronic dryness cause surface changes in the exposed areas of the cornea and conjunctiva. Fluorescein will stain punctate areas of epithelial loss on the cornea. Another dye, rose bengal, is able to stain devitalized cells of the conjunctiva and cornea before they actually degenerate and drop off.

Evaluation of Lacrimal Drainage

The anatomy of the lacrimal drainage system is discussed in Chapters 1 and 4. The pumping action of the lids draws tears nasally into the upper and lower canalicular channels through the medially located “punctal” openings in each lid margin. After collecting in the lacrimal sac, the tears then drain into the nasopharynx via the nasolacrimal duct. Symptoms of watering are frequently due to increased tear production as a reflex response to some type of ocular irritation. However, the patency and function of the lacrimal drainage system must be checked in the evaluation of otherwise unexplained tearing.

The Jones I test evaluates whether the entire drainage system as a whole is functioning. Concentrated fluorescein dye is instilled into the conjunctival sac on the side of the suspected obstruction. After 5 minutes, a cotton Calgiswab is used to attempt to recover dye from beneath the inferior nasal turbinate. Alternatively, the patient blows his or her nose into a tissue, which is checked for the presence of dye. Recovery of any dye indicates that the drainage system is functioning.

The Jones II test is performed if no dye is recovered, indicating some abnormality of the system. Following topical anesthesia, a smooth-tipped metal probe is used to gently dilate one of the puncta (usually lower). A 3-mL syringe with sterile water or saline is prepared and attached to a special lacrimal irrigating cannula. This blunt-tipped cannula is used to gently intubate the lower canaliculus, and fluid is injected as the patient leans forward. With a patent drainage system, fluid should easily flow into the patient's nasopharynx without resistance.

If fluorescein can now be recovered from the nose following irrigation, a partial obstruction might have been present. Recovery of clear fluid without fluorescein, however, may indicate inability of the lids to initially pump dye into the lacrimal sac with an otherwise patent drainage apparatus. If no fluid can be irrigated through to the nasopharynx using the syringe, total occlusion is present. Finally, some drainage problems may be due to stenosis of the punctal lid opening, in which case the preparatory dilation may be therapeutic.

Methods of Orbital Evaluation

Exophthalmometry

A method is needed to measure the anteroposterior location of the globe with respect to the bony orbital rim. The lateral orbital rim is a discrete, easily palpable landmark and is used as the reference point.

The exophthalmometer (Figure 2–33) is a hand-held instrument with two identical measuring devices (one for each eye), connected by a horizontal bar. The distance between the two devices can be varied by sliding one toward or away from the other, and each has a notch that fits over the edge of the corresponding lateral orbital rim. When properly aligned, an attached set of mirrors reflects a side image of each eye profiled alongside a measuring scale, calibrated in millimeters. The tip of the corneal image aligns with a scale reading representing its distance from the orbital rim.

Figure 2-33.

Hertel exophthalmometer. (Photo by M Narahara.)

The patient is seated facing the examiner. The distance between the two measuring devices is adjusted so that each aligns with and abuts against its corresponding orbital rim. To allow reproducibility for repeat measurements in the future, the distance between the two devices is recorded from an additional scale on the horizontal bar. Using the first mirror scale, the patient's right eye position is measured as it fixates on the examiner's left eye. The patient's left eye is measured while fixating on the examiner's right eye.

The distance from the cornea to the orbital rim typically ranges from 12 to 20 mm, and the two eye measurements are normally within 2 mm of each other. A greater distance is seen in exophthalmos, which can be unilateral or bilateral. This abnormal forward protrusion of the eye can be produced by any significant increase in orbital mass, because of the fixed size of the bony orbital cavity. Causes might include orbital hemorrhage, neoplasm, inflammation, or edema.

Ultrasonography

Ultrasonography utilizes the principle of sonar to study structures that may not be directly visible. It can be used to evaluate either the globe or the orbit. High-frequency sound waves are emitted from a special transmitter toward the target tissue. As the sound waves bounce back off the various tissue components, they are collected by a receiver that amplifies and displays them on an oscilloscope screen.

A single probe that contains both the transmitter and receiver is placed against the eye and used to aim the beam of sound (Figure 2–34). Various structures in its path will reflect separate echoes (which arrive at different times) back toward the probe. Those derived from the most distal structures arrive last, having traveled the farthest.

Figure 2-34.

Ultrasonography using B-scan probe. The image will appear on the oscilloscope screen, visible in the background. (Photo by M Narahara.)

There are two methods of clinical ultrasonography: A scan and B scan. In A scan ultrasonography, the sound beam is aimed in a straight line. Each returning echo is displayed as a spike whose amplitude is dependent on the density of the reflecting tissue. The spikes are arranged in temporal sequence, with the latency of each signal's arrival correlating with that structure's distance from the probe (Figure 2–35). If the same probe is now swept across the eye, a continuous series of individual A scans is obtained. From spatial summation of these multiple linear scans, a two-dimensional image, or B scan, can be constructed.

Figure 2-35.

A scan (left) and B scan (right) of an intraocular tumor (melanoma). C = cornea; I = iris; L = posterior lens surface; O = optic nerve; R = retina; T = tumor. (Courtesy of RD Stone).

Both A and B scans can be used to image and differentiate orbital disease or intraocular anatomy concealed by opaque media. In addition to defining the size and location of intraocular and orbital masses, A and B scans can provide clues to the tissue characteristics of a lesion (eg, solid, cystic, vascular, calcified).

For purposes of measurement, the A scan is the most accurate method. Sound echoes reflected from two separate locations will reach the probe at different times. This temporal separation can be used to calculate the distance between the points, based on the speed of sound in the tissue medium. The most commonly used ocular measurement is the axial length (cornea to retina). This is important in cataract surgery in order to calculate the power for an intraocular lens implant. A scan can also be used to quantify tumor size and monitor growth over time.

The application of pulsed ultrasound and spectral Doppler techniques to orbital ultrasonography provides information on the orbital vasculature. It is certainly possible to determine the direction of flow in the ophthalmic artery and the ophthalmic veins and reversal of flow in these vessels occurring in internal carotid artery occlusion and carotid-cavernous fistula, respectively. As yet, the value of measuring flow velocities in various vessels, including the posterior ciliary arteries, without being able to measure blood vessel diameter is not fully established.

Ophthalmic Radiology (X-Ray, CT Scan)

Plain x-rays and CT scans (Figures 13–1 and 13–2) are useful in the evaluation of orbital and intracranial conditions. CT scan in particular has become the most widely used method for localizing and characterizing structural disease in the extraocular visual pathway. Common orbital abnormalities demonstrated by CT scan include neoplasms, inflammatory masses, fractures, and extraocular muscle enlargement associated with Graves' disease (Figure 13–4).

The intraocular applications of radiology are primarily in the detection of foreign bodies following trauma and the demonstration of intraocular calcium in tumors such as retinoblastoma. CT scan is useful for foreign body localization because of its multidimensional reformatting capabilities and its ability to image the ocular walls.

Magnetic Resonance Imaging

The technique of magnetic resonance imaging (MRI) has many applications in orbital and intracranial diagnosis. Improvements such as surface receiver coils and thin section techniques have improved the anatomic resolution in the eye and orbit.

Unlike CT, the MRI technique does not expose the patient to ionizing radiation. Since MRI might cause movement of metal, it should not be used if a metallic foreign body is suspected.

Because it can better differentiate between tissues of different water content, MRI is superior to CT in its ability to image edema, areas of demyelination, and vascular lesions. Bone generates a weak MRI signal, allowing improved resolution of intraosseous disease and a clearer view of the intracranial posterior fossa. Examples of MRI scans are presented in Chapters 13 and 14.

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Which visual assessment technique provides a magnified view of the retina and optic nerve head?

Which visual system assessment technique provides a magnified view of the retina quizlet?

Which of the following would increase visual acuity?

When performing an assessment on a patient's eyes what might the nurse use the ophthalmoscope for?