Visual Loss

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Visual Loss: Overview, Visual Field Testing, and Topical Diagnosis GRANT T. LIU, NICHOLAS J. VOLPE and STEVEN L. GALETTA

The afferent visual pathways encompass structures that perceive, relay, and process visual information: the eyes, optic nerves (cranial nerve II), chiasm, tracts, lateral geniculate nuclei, optic radiations, and striate cortex (Fig. 3.1). Lesions anterior to and including the chiasm may result in visual acuity (clarity) loss, color deficits, and visual field defects (abnormal central or peripheral vision). From a neuroophthalmic standpoint, unilateral retrochiasmal (posterior to the chiasm) disturbances can present primarily with homonymous (both eyes involved with the same laterality) visual field defects without acuity loss. Higher-order processing, instrumental in interpreting visual images, occurs in extrastriate association cortex. Abnormalities in these areas can cause, for instance, deficits in object recognition, color perception, motion detection, and visual attention (neglect of visual stimuli in left or right hemifields). This chapter provides an overview of these structures, details methods of visual field testing, and describes a framework for the localization and diagnosis of disorders affecting the afferent visual pathways. Determining where the lesion is first, then finding out what it is second is the advocated approach. Further details regarding these structures’ anatomy, blood supply, organization, and neuro-ophthalmic symptoms, as well as the differential diagnosis of lesions affecting them, are detailed in Chapters 4–12.

and converge to form the optic disc and optic nerve. Temporal to the fovea, the axons are strictly oriented above and below the horizontal raphae. For instance, ganglion cells above the raphae project their axons in an arcuate pattern to the top of the optic nerve (see Fig. 5.1). The optic disc represents the intraocular portion of the optic nerve anterior to the lamina cribrosa (see Fig. 2.38). The retina is normally transparent, and the orange-red color visible on fundus examination derives from the retinal pigment epithelium and choroidal circulation. The retina nasal to the macula receives visual information from the temporal field, and the temporal retina from the nasal field (Fig. 3.2). The superior and inferior halves of the retina have a similar crossed relationship with respect to lower and upper fields of vision. The ophthalmic artery, a branch of the internal carotid, provides most of the blood supply to the eye, although there are external carotid anastomoses (see Fig. 4.1). The first major branch of the ophthalmic artery, the central retinal artery, pierces the dura of the optic nerve behind the globe, then travels within the nerve to emerge at the optic disc to supply the inner two-thirds of the retina. The ophthalmic artery also gives rise to the posterior ciliary arteries, which supply the optic nerve head, choroid, and outer third of the retina.

Neuroanatomy of the Afferent Visual Pathway: Overview

OPTIC NERVE, CHIASM, AND TRACT

THE EYE AND RETINA The eyes are the primary sensory organs of the visual system. Before reaching the retina, light travels through the ocular media, consisting of the cornea, anterior chamber, lens, and vitreous. The size of the pupil, like the aperture of a camera, regulates the amount of light reaching the retina. The cornea and lens focus light rays to produce a clear image on the retina in the absence of refractive error, and the ciliary muscle can change the lens shape to adjust for objects at different distances (accommodation). Retinal photoreceptors hyperpolarize in response to light. Cone photoreceptors are more sensitive to color and are concentrated in the posterior pole of the retina, or macula, the center of which is the fovea. Rod photoreceptors, more important for night vision, predominate in the retinal periphery. Visual information is processed via horizontal, bipolar, and amacrine cells before reaching the ganglion cells, the axons of which make up the innermost portion of the retina

The optic nerve has four major portions: intraocular, intraorbital, intracanalicular, and intracranial. Posterior to the lamina cribrosa, optic nerve axons are myelinated by oligodendrocytes similar to those in white matter tracts in the brain and spinal cord. Axons from the two optic nerves join at the optic chiasm, which lies in the suprasellar region, superior to the diaphragma sellae and inferior to the third ventricle and hypothalamus. At the chiasm, fibers from the nasal retina cross, and the most ventral axons from the inferior nasal retina bend into the most proximal aspect of the contralateral optic nerve (Wilbrand’s knee; see the discussion in Chapter 7), whereas the fibers from the temporal retina remain ipsilateral in the lateral portion of the chiasm (see Fig. 3.2). The ratio of crossed to uncrossed fibers is 53 : 47.1 Ipsilateral temporal fibers and contralateral nasal fibers join to form the optic tracts.

GENICULOCALCARINE PATHWAY At the lateral geniculate nucleus, a part of the thalamus located above the ambient cistern, the ganglion cell axons 39

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Calcarine fissure

Calcarine cortex

A

Calcarine cortex

Optic radiations Meyer’s loop

Optic chiasm

Optic nerve

B

Optic nerve Chiasm

Lateral geniculate body

Optic radiations

Optic nerves

Optic chiasm

Meyer’s loop

Lateral geniculate body

Optic tract Lateral geniculate body

Optic radiations

C

Optic radiations

Calcarine cortex

in the optic tract synapse with neurons destined to become the optic radiations. This latter structure is divided functionally and anatomically. Fibers coursing through the temporal lobe, termed Meyer’s loop, subserve visual information from the lower retina and connect to the inferior bank of the calcarine cortex. The parietal portion of the optic radiations relays information from the upper retina to the superior bank of the calcarine cortex. Most of the optic radiations derive their blood supply from the middle cerebral artery. The medial temporal section is supplied in part by branches of the posterior cerebral artery.

STRIATE CORTEX Brodmann area 17 (or V1, primary or striate cortex) is the end organ of the afferent visual system and is located within the calcarine cortex in the occipital lobe. Most of the striate cortex, especially the portion situated posteriorly, is devoted to macular vision. Superior and inferior banks of calcarine cortex are separated by the calcarine fissure and subserve contralateral inferior and superior quadrants, respectively. The majority of the occipital lobe is supplied by the posterior

Figure 3.1.  Afferent visual pathways. Major structures as viewed from (A) the lateral side, (B) the medial side, and (C) the underside of the brain. (Redrawn from Cushing H. The field defects produced by temporal lobe lesions. Trans Am Neurol Assoc 1921;47:374–423.)

cerebral artery, with distinct branches serving the superior and inferior calcarine cortex banks along with a contribution from the middle cerebral artery in the occipital pole region.

VISUAL ASSOCIATION AREAS Higher processing of visual information occurs, for example, in the lingual and fusiform gyri bordering the inferior calcarine bank in structures believed to be equivalent to monkey area V4, which is responsible for color vision. In an oversimplification, temporal lobe structures govern visual recognition and memory, whereas parietal lobe areas are responsible for motion and spatial analysis.

Visual Field Testing In patients with visual loss, the pattern of the visual field deficit can be highly localizing. Confrontation field testing, the techniques of which are detailed in Chapter 2, often provides extremely useful information. In general, the technique is specific, because field loss detected by confrontation

3  •  Visual Loss: Overview, Visual Field Testing, and Topical Diagnosis

Left visual field

Right visual field

Nasal retina

Nasal retina RE

LE

Temporal retina

Left visual cortex

Temporal retina

Right visual cortex

Figure 3.2.  Separation of pathways for temporal and nasal visual fields.55 Visual information from the temporal visual field projects to the nasal retina, then via ganglion cell axons in the optic nerve crosses in the chiasm to reach the contralateral optic radiations and striate cortex anteriorly. In contrast, visual information from the nasal visual field projects to the temporal retina and the ipsilateral optic radiations and posterior striate cortex. Note that visual information from the left visual field (dotted lines) projects to the right cerebral hemisphere, and visual information from the right visual field (solid lines) projects to the left cerebral hemisphere. LE, left eye; RE, right eye.

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is usually real.2,3 However, confrontation is insensitive, because more subtle defects may be missed.3,4 More sensitive, reproducible, and precise visual field testing may be achieved by automated or computerized threshold perimetry or kinetic testing with a Goldmann perimeter or tangent screen.5 Threshold computerized perimetry of the central 24 or 30 degrees of vision, although relatively lengthy and tedious, in many instances is a more objective and more reproducible test for patients with optic neuropathies and chiasmal disturbances and those requiring serial testing. The kinetic techniques, because they are shorter and allow interaction with the examiner, may be more appropriate for screening and for patients with significant neurologic impairment. Manual kinetic perimetry also allows the knowledgeable examiner to “search” for suspected field defects. Computerized perimetry, because of its wide availability and ease of administration, is currently the most popular test method. Table 3.1 summarizes the advantages, disadvantages, and most appropriate neuro-ophthalmic uses of each modality. The examiner should always keep in mind that all modalities for visual field evaluation are inherently subjective and depend on the patient’s level of alertness, cooperation, ability to fixate centrally, and response rapidity. In addition, astute patients feigning visual loss can voluntarily alter their visual fields during perimetric testing (see Chapter 11). In most cases, the visual field is tested for each eye separately. Except in patients using miotic eye drops for glaucoma, for instance, field testing should take place before pharmacologic dilation of the pupils, which tends to worsen performance even if accommodative dysfunction has been corrected with lenses.6,7 Visual fields are recorded so that the field of the right eye is on the right and the field of the left eye is on the left (see Fig. 2.11). The blind spot, caused by the absence of photoreceptors overlying the optic nerve, is located approximately 15 degrees temporal to and slightly below fixation and is drawn as an area without vision. As previously stated, homonymous defects are those present in both eyes with the same laterality. A hemianopia refers to loss of half of the visual field, respecting the vertical (usually) or horizontal meridian. Congruity refers to the symmetry of the field defect in both eyes.

Table 3.1  Advantages, Disadvantages, and Most Appropriate Neuro-Ophthalmic Uses of Computerized Threshold, Goldmann Kinetic, and Tangent Screen Kinetic Perimetry Advantages

Disadvantages

Best Neuro-ophthalmic Uses

Computerized threshold

Reproducible More objective More standardized Less reliance on a technician Intertechnician variability less important

Lengthy Tedious

Optic neuropathy Papilledema Chiasmal disorders Repeated follow-up

Goldmann kinetic

Short Driven by technician or doctor; skilled perimetrist or physician can focus attention to suspected defect areas

More subjective Depends on the skills of the perimetrist

Retrochiasmal disorders Neurologically impaired patients Patients who are unable to perform a computerized field test Severe visual loss Functional vision loss

Tangent screen kinetic

Short Can be performed in the examination room

Central 30 degrees only

Central field defects Functional visual loss

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Visual field testing in children. Some studies have suggested that computerized visual field testing, usually requiring several minutes per eye, can be performed reliably in young children.8,9 However, in our experience most children younger than 10 years of age have difficulty with the monotony and length of formal visual field testing, leading to high numbers of errors. Kinetic (Goldmann) visual field testing is easier for young, less-cooperative children,10 but there is still great test–retest variability in this age group. Therefore, clinical decision-making based upon unreliable visual fields and small changes during serial visual field testing in children is problematic.

perimetry plots the visual field (in the x,y-plane) for a stimulus at a given sensitivity (z-axis) level. The plot of a kinetic field can be considered to be a two-dimensional representation of the hill of vision (Fig. 3.4). When visual field defects occur, the corresponding part of the island is lost (Fig. 3.5). Generalized visual field constriction can be conceptualized as the island of vision sinking into the sea of blindness. In these cases, the central peak occurs at a lower sensitivity level, and the field of vision at any particular sensitivity level is smaller.

THE HILL OF VISION CONCEPT

There are many types of computerized threshold perimetry in wide use, including Humphrey (Carl Zeiss), Meditec, and Octopus. The major advantages of computerized perimetry over other forms are that it permits more standardized testing procedures, it requires less technician skill, and it is affected

Although the visual field is plotted on a piece of paper in two dimensions, it can be conceptualized three dimensionally as an “island or hill of vision in a sea of darkness” (Fig. 3.3).11,12 The z-axis value indicates visual sensitivity, while the location within the field of vision is plotted in the x,y-plane. Foveal vision has the highest sensitivity but extends nasally and temporally only a few degrees. Thus, with increasing sensitivity (up on the z-axis), the field of vision decreases in size, and the hill peaks at fixation (x = 0, y = 0). In contrast, at low sensitivities (lower on the z-axis), the field of vision is much larger. The blind spot is depicted as an opening in the island temporal to the central peak,13 and the opening extends all the way to the bottom of the island. Sensitivity falls more rapidly nasally than temporally. Outside of the x- and y-coordinates delimiting the bottom of the island, nothing is seen. The major difference between threshold (static) and kinetic perimetry can be described using the hill of vision concept. Threshold perimetry determines the visual sensitivity (z-axis value) at any particular x,y point. On the other hand, kinetic

COMPUTERIZED THRESHOLD PERIMETRY

z-axis Foveolar vision

Blind spot y-axis

x-axis

Nasal visual field

Temporal visual field

Figure 3.3.  “Island of vision in a sea of blindness.” This three-dimensional representation of the visual field plots visual sensitivity along the z-axis versus location within the x,y-plane.

Figure 3.4.  The island of vision (left) contains the information produced by kinetic perimetry (right). Kinetic perimetry plots the visual field (in the x,y-plane) for a stimulus at a given sensitivity (z-axis) level. Thus each isopter (I4e plot, for instance) on the kinetic perimetry can be translated from the island of vision.

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A

B Figure 3.5.  Dicon computerized threshold fields and corresponding hill of vision plots. A. Normal visual field of a left eye. B. Temporal field defect, with depression of the hill corresponding to the defective visual field. (Courtesy of Lawrence Gray, OD.)

less by intertechnician variability. Some studies have also demonstrated that automated computerized perimetry may be more sensitive to subtle field loss than Goldmann perimetry.14 The discussion in this section will highlight the testing features of the Humphrey Field Analyzer (Fig. 3.6), which is the most popular. In the Humphrey threshold 24–2 or 30–2 test, the computer presents white light stimuli against a white background within the central 24 or 30 degrees of vision of each eye, respectively. Lens correction for near is provided, and the patient looks at a central target and hits a button when he or she sees the light. The stimulus size is kept the same, but the stimulus intensity is varied, and the computer records the intensity of the dimmest stimulus the patient saw at various points in the visual field. This threshold intensity is recorded in decibels, and the higher the number, the dimmer

the stimulus and the higher the sensitivity. The computer also determines the location of the blind spot. The test can be laborious and sometimes soporific, even for the most cooperative individuals. In the full threshold evaluation, it is not unusual for each eye to be tested with more than 450 points over approximately 15 minutes. Swedish Interactive Threshold Algorithm (SITA) software programs may save up to 50–70% of test time for a Humphrey field.15,16 Largely because the shorter test time vastly improves patient cooperation, sensitivity and reproducibility are enhanced in neuro-ophthalmic patients with these programs.17 Thus, SITA-Standard and Fast programs have become vastly preferred over standard full threshold tests in clinical practice.18,19 Patient reliability during a Humphrey field test is reflected in the number and proportion of fixation losses and

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PART 2  •  Visual Loss and Other Disorders of the Afferent Visual Pathway

Figure 3.6.  Humphrey visual field analyzer.

false-positive and false-negative responses (Fig. 3.7). When the patient responds to a stimulus presented in the originally plotted blind spot, a fixation loss is considered to have occurred. A false-positive response happens when the patient hits the buzzer but no light stimulus was presented. If a patient does not hit the buzzer when a stimulus of identical location and greater intensity to one that was previously detected is presented, this is considered a false-negative response. Either a fixation loss rate of 20% or a false-positive or a false-negative rate of 33% indicates an unreliable visual field.20 Severe and nonorganic (see Chapter 11) visual field loss may be associated with abnormally high false-negative rates. The printout displays the visual field data in several ways (see Fig. 3.7),20 including some with statistical analysis. The threshold intensities are presented in raw data form. The gray scale provides a graphical representation of the threshold intensities and offers the best information for a “quick-glance” interpretation of the threshold visual field. However, the gray scale summary can be misleading, and important defects may not be evident. The total deviation map plots the difference between each measured threshold value and those of age-matched normal values for each point in the visual field. The pattern deviation map accents local visual field defects in the total deviation map by correcting for the overall height of the hill of vision. This is the most accurate representation of the visual field and should be reviewed in all patients and used in comparisons for changes in examinations over time. Diffuse field loss, as might occur with a cataract, is factored out this way. Probability displays are provided for the total deviation and pattern deviation maps. Of the global visual field indices, the mean deviation (MD) is the most important clinically in neuro-ophthalmic patients because it gives a “numerical equivalent” of the visual field. This figure is an average of the numbers in the total deviation plot, with each value weighted according to the normal range at that point. As a patient’s visual field worsens, because of an enlarging scotoma for instance, the mean deviation becomes more

negative. Serial visual fields can be compared by following the mean deviation. The foveal threshold is another important number reported during testing, because it correlates directly with visual acuity in many cases. For instance, a low foveal threshold may be the only indication of a subtle central field defect (especially from maculopathy). A high threshold, on the other hand, might indicate that a patient who had decreased visual acuity may have nonorganic visual loss.21,22 Several factors may influence the patient’s performance on this test. Instructions should be provided clearly to the patient.23 The learning curve is steep, and individuals undergoing threshold perimetry have a natural tendency to provide more reliable visual fields on subsequent testing.24 Mild ptosis may be associated with depression of the superior visual field,25 and cataracts, pupillary miosis, and myopia26 may cause diffusely decreased sensitivity. Increasing age may associated with a decrease in threshold sensitivity as well.27,28 There may also be some variability in performance over time in different disease states.29 During screening full-field examinations, such as 120-point ones, the computer presents static light stimuli of fixed size and intensity, and the patient presses the button if he or she sees them. They are not typically threshold tests, and the plot depicts merely whether the light stimulus was seen or not. A “quantify defects” modification can be used to threshold test missed points. The full-field screens are helpful in detecting large field defects such as hemianopias or altitudinal field loss. However, because the points can be spread far apart, often the information provided is so vague that the test becomes uninterpretable. A computerized binocular Esterman visual field protocol, which tests 130 degrees of the visual field along the horizontal meridian, can be used to assess whether the patient’s visual field is sufficient for driving. In many states 120 degrees of binocular field is required to operate an automobile.

OTHER TYPES OF COMPUTERIZED STATIC PERIMETRY Frequency doubling technology (FDT) perimetry, essentially an evaluation of contrast sensitivity throughout the central visual field, tests the patient’s ability to detect sinusoidal gratings at 17 or 19 positions (Fig. 3.8).15,30 Because testing takes less than 1 minute per eye, several authors have suggested frequency doubling perimetry may be better in some instances than SITA algorithms for screening in neurologic patients31–34 and young children.35,36 In addition the instrument is portable and relatively inexpensive. Although a reasonably effective screening tool, a small but significant rate of false-positive testing on FDT perimetry does occur. Shortwavelength automated perimetry (SWAP), which uses a blue stimulus on a yellow background, has been used primarily in glaucoma screening but has little use in neuro-ophthalmic patients.18,37,38 Long testing time is one of the disadvantages of SWAP.

KINETIC PERIMETRY Goldmann Kinetic Perimetry. As the patient fixates on the central target in an illuminated bowl (Fig. 3.9), the perimetrist displays white dots of varying size and luminance. The stimuli are presented both centripetally and statically,

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Figure 3.7.  Normal Humphrey computerized 30–2 threshold visual field of the right eye. Note (A) the tabulation of the fixation losses and false-positive and false-negative errors; (B) the raw data, recording the luminance, given in decibels (dB), of the dimmest stimulus the subject saw at that position in the visual field; (C) the gray scale, containing a conversion of the raw data using the key at the bottom of the readout; (D) the total deviation; (E) the pattern deviation; and (F) the statistical analysis, including the mean deviation.

and the patient hits a buzzer when he or she detects the stimulus. The perimetrist monitors the patient’s fixation through a telescope, constantly encouraging the patient and reminding him or her to look straight ahead. This perimetrist– patient interaction makes the test ideal if the patient is young, has poor vision, or is neurologically impaired. Furthermore, Goldmann perimetry rarely requires more than 5–7 minutes per eye, and short breaks can be taken at the examiner’s

discretion. The test also can be tailored to the clinical situation, such as screening for chiasmal or hemianopic defects by paying particular attention to asymmetries along the vertical meridian,39,40 arcuate field defects in glaucoma, and infranasal defects in papilledema.41 Since the whole visual field is tested, Goldmann perimetry will detect the rare peripheral visual field defects, due to anterior calcarine, temporal lobe, or early chiasmal compressive lesions, for instance,

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30

30 30

Test duration: Fixation target:

0:59 Central

Fixation errors: 0/3 (0%) False pos. errors 0/3 (0%)

30

P ≥ 5% P < 5% P < 2% P < 1%

Test duration: Fixation target:

0:33 Central

Fixation errors: 0/3 (0%) False pos. errors 0/3 (0%)

Figure 3.8.  Frequency doubling technology (FDT) perimetry in a patient with a superior temporal field defect in the left eye from a chiasmal lesion. Relative sensitivity, portability, and short test duration make FDT an effective screening tool.

Figure 3.9.  Goldmann kinetic perimetry. Note the patient (left) has his head in a chin-rest in the middle of the spherical bowl and presses a button in his right hand when he sees the presented stimulus. The tester administers the examination from the other side of the bowl.

which may have been missed by computerized perimetry of the central 30 degrees.42,43 Goldmann perimetry is also particularly helpful in patients with functional vision loss, in whom typical findings of spiraling, criss-crossing isopters, and nonphysiologic constriction (see Chapter 11) can often be demonstrated. However, perimetrist bias can be a disadvantage, especially when Goldmann perimetry is used in serial follow-up. Another drawback to the technique is that the perimetrist needs a

working knowledge of afferent visual pathway anatomy and related patterns of field loss. The size of the stimulus (Fig. 3.10) is indicated by Roman numerals ranging from O to V, corresponding to dots measuring 0.062–64 mm2, respectively. Stimulus luminance is designated by an Arabic numeral from 1 to 4, in order of increasing brightness, together with a small letter from a to e, by convention usually held at e. The smallest, dimmest stimulus used in practice is labeled Ile, and the largest, brightest stimulus is designated by V4e. Usually the dots are white, but colored stimuli can be used in some situations, such as in the evaluation of hereditary optic neuropathies. If needed, lens correction for near is given when targets are presented within the central 30 degrees. The results are displayed from the patient’s point of view (i.e., what he or she sees). The area in which the stimulus was seen is called an isopter, which is labeled according to the stimulus size and luminance (see Fig. 3.10). The normal temporal V4e field can extend to at least 90 degrees, while the normal nasal V4e field can reach at least 60 degrees. In normal individuals, it is sometimes sufficient to plot only the 14e isopter, which may be suprathreshold for the entire peripheral field. The field using a test target that is either smaller or dimmer is always smaller than the field produced with a larger or brighter stimulus. Scotomas, circumscribed areas of visual field loss, are indicated by zones that are shaded in. The blind spot, the result of the absence of photoreceptors overlying the optic disc, is technically a scotoma and is temporal to fixation in all normal individuals. Automated kinetic perimetry. Automated and computerized combinations of static and kinetic perimetry (Fig. 3.11) performed on the Octopus perimeters can be used to produce results similar to Goldmann perimetry, but without the need for an experienced examiner.44,45 Automated kinetic perimetry has been shown to be accurate and reliable in both neuro-ophthalmic disease45 and glaucoma.46 This technique may become increasingly more important in the future

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Figure 3.10.  Normal Goldmann kinetic field of the right eye. The labels 14e, 13e, and 12e indicate the stimuli size and luminance (detailed in the key on the bottom left-hand side of the figure). The stimuli are presented in the periphery then directed radially to the center. The dark lines indicate the point at which the subject noticed the stimulus. Note the temporal location of the blind spot and that the temporal field is larger than the right.

as advantages of automation are tested (reproducibility and quantification),47 paradigms searching for specific defects (e.g., “chiasm program”) are designed, and the prevalence of manual perimeters, well-trained perimetrists, and physicians experienced with the older methods diminishes.

TANGENT SCREEN VISUAL FIELD TESTING This test can be performed quickly in any examination room equipped with a tangent screen hung on a wall.48 The perimetrist moves round white or colored discs or spheres over a black or gray flat felt background (Fig. 3.12A), and, as in Goldmann perimetry, the targets are generally moved centripetally. Wearing spectacle correction, the patient usually sits 1 meter away from the screen and indicates verbally when he or she sees the visual stimulus. At this distance, on a flat surface only the central 30 degrees can be tested. The perimetrist can outline with chalk where the patient saw the target, and the results can be recorded similar to the way that confrontation fields are drawn (see Chapter 2). The isopters are designated as a fraction, such as 3w/1000, which indicates that a 3-mm white target was used at a distance of 1000 mm (1 meter). Small central and paracentral defects that may be missed by Goldmann perimetry or computerized field testing may be more evident with the tangent screen method. Subtle hemianopic deficits can be detected by holding two equivalent

targets on both sides of fixation (Fig. 3.12B) and asking the patient whether one appears different from the other. Perhaps the most common use of the tangent screen test is in the demonstration of nonphysiologic tubular visual fields in patients with functional visual loss (see Chapter 11). A laser pointer with a round red or green target can also be used to present the visual stimulus during tangent screen testing in the office (Fig. 3.13, Video 3.1).49 The examiner should stand behind the patient, thereby avoiding any hints regarding the direction of the target presentation. The examiner can also use the laser pointer during inpatient consultations for rough assessment of visual fields, substituting the tangent screen with a light-colored wall as the background.

Topical Diagnosis (“Where” Then “What”) First, the examiner should decide where the lesion is neuroanatomically. Based on the history, examination, and visual field testing, the examiner should be able to localize the process to the retina, optic nerve, chiasm, tract, radiations, occipital lobe, or higher cortical area. Then the examiner should generate a differential diagnosis and attempt to determine what the lesion is. Historical features often guide the differential diagnosis, and neuroimaging combined with other ancillary tests typically narrows the list of possible causes.

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PART 2  •  Visual Loss and Other Disorders of the Afferent Visual Pathway

Figure 3.11.  Combined static and automated Goldmann perimetry in a patient with an inferior nasal quadrantic defect in the left eye. The Octopus 101 test consists of a short TOP (tendency-oriented perimetry) strategy 32 static examination followed by a series of preprogrammed kinetic vectors (arrows) using III4E (brown larger isopter) and I4E (black small isopter) stimuli. Darker areas in the static portion of the test correlated to denser defects. The vectors (arrows) used in the automated kinetic portion of the test were preprogrammed to detect defects along the vertical and horizontal meridians, and the computer corrects for reaction time.

HISTORY Common complaints encountered with visual loss include so-called negative phenomena such as “blurry vision” or “gray vision.” Patients with higher cortical disorders may have nonspecific complaints such as “I’m having trouble seeing” or “Focusing is difficult.” Patients with lesions of the afferent visual pathway may also complain of positive phenomena, such as flashing or colored lights (phosphenes or photopsias), jagged lines, or formed visual hallucinations (a false perception that a stimulus is present). The complexity of positive phenomena does not specify localization.

The temporal profile of the visual loss will suggest possible diagnoses, and its monocularity or binocularity will help in localization. As a general rule, acute or subacute visual deficits result from ischemic or inflammatory injury to the optic nerve. Vitreous hemorrhage and retinal detachment are other important considerations. Chronic or progressive visual loss, in turn, may result from a compressive, infiltrative, or degenerative process. Cataracts, refractive error, open-angle glaucoma, and retinal disorders such as age-related macular degeneration or diabetic retinopathy also need to be considered when visual symptoms are insidious.

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A Figure 3.13.  Laser pointer visual field test. The examiner stands behind the subject and tests the visual field by pointing a laser light stimulus at a wall or black tangent screen in front of the subject, who verbalizes when he or she sees the stimulus.

EXAMINATION

B Figure 3.12.  Tangent screen visual field test. Each eye is tested separately. A. The examiner moves a white or colored object in front of a black felt screen, and the patient points or verbalizes when he or she sees the target. The results can be recorded in white chalk on the felt, then transferred into the patient’s record. B. Two equivalent targets can be presented while the patient fixates centrally to test for subtle central visual field defects respecting the vertical meridian.

If a patient complains of monocular visual loss, a process in one eye or optic nerve should be considered. Painless transient visual loss characterized by a “gray shade” that encroaches on vision superiorly then resolves after seconds or minutes is typical of amaurosis fugax related to carotid disease. Painful monocular visual loss occurring over days is characteristic of an inflammatory or demyelinating optic neuropathy. With binocular visual loss, a lesion of both eyes or optic nerves, or of the chiasm, tract, radiations, or occipital lobe, should be investigated. Further details regarding these specific complaints and their localizing value are provided in the respective chapters. Associated neurologic deficits, such as motor or sensory abnormalities, will also assist in localization and often indicate a hemispheric abnormality. Medical conditions should always be investigated in the review of systems. Hypertension, diabetes, and smoking, for instance, predispose the patient to vascular disease, and a history of coronary artery disease should alert the examiner to the possibility of carotid artery insufficiency as well. Visual loss accompanied by endocrine symptoms, such as those consistent with hypopituitarism (amenorrhea, decreased libido, or impotence, for example) or pituitary hypersecretion (galactorrhea or acromegaly, for example), suggests a chiasmal disorder.

Particular attention should be paid to assessment of acuity, color vision, confrontation visual fields, pupillary reactivity, and fundus appearance. Neuro-ophthalmic examination techniques are discussed in Chapter 2. Monocular acuity loss, deficits in color vision, a central scotoma, a relative afferent pupillary defect, and optic disc swelling or pallor suggest an optic neuropathy. In an acute retrobulbar optic neuropathy, the optic disc may have a normal appearance. Monocular visual loss with a central scotoma, metamorphopsia, preserved color vision, and no afferent pupillary defect makes a maculopathy more likely. Bilateral loss of acuity suggests bilateral macular, optic nerve, chiasm, or bilateral retrochiasmal lesions. Homonymous hemianopic field loss with normal acuity, pupillary reactivity, and fundi are normally associated with a retrogeniculate process. A hemianopia accompanied by an ipsilateral hemiparesis or sensory loss is likely to be the result of a parietal process, while an isolated hemianopia is more likely the result of an occipital lobe lesion. When suspected, ocular causes of visual loss such as corneal or lens opacities, retinal detachments, or glaucoma should be excluded by an ophthalmologist. In general, patients with cataracts complain of blurry vision with glare, especially with automobile headlights, and those with glaucoma have peripheral visual field loss but preserved central acuity; the visual loss associated with both of these problems is insidious. Retinal detachments may present acutely with flashes of light, floaters, or peripheral field loss.

Pattern of Visual Field Loss Fig. 3.14 illustrates the visual field deficits characteristic of various lesions within the afferent visual pathway. As alluded to earlier, monocular visual field defects most commonly localize to the retina or optic nerve, and the patterns of field loss are typically altitudinal, central, cecocentral, or arcuate. Monocular field defects emanating from the blind spot, as in an arcuate scotoma, for instance, are almost always related

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PART 2  •  Visual Loss and Other Disorders of the Afferent Visual Pathway

Figure 3.14.  Visual pathways: correlation of lesion site and field defect, view of underside of the brain. Homonymous refers to a defect present in both eyes with the same laterality, while a hemianopia refers to visual loss respecting the vertical meridian. Congruous fields are symmetric in both eyes. Note that lesions of upper or lower occipital banks produce quadrantic defects, while lesions within temporal and parietal lobes cause field defects that tend not to respect the horizontal meridian.

to an optic nerve lesion or a retinal vascular occlusion. A bitemporal hemianopia is very specific to the chiasm. Homonymous hemianopic deficits suggest a retrochiasmal lesion. In general, incongruous hemianopias tend to be more anterior, while congruous hemianopias, with or without macular sparing, are more characteristic of occipital disturbances. One possible explanation for this pattern is the closer grouping of retrochiasmal fibers from each eye as they proceed posteriorly. If the homonymous hemianopia is complete, congruity cannot be assessed, so the injury may be anywhere along the retrochiasmal pathway. The particular patterns of visual field loss are discussed in greater detail in Chapters 4–8.

ANCILLARY VISUAL TESTING Electrophysiologic testing such as a visual evoked potential (VEP) or electroretinogram (ERG) can confirm the localization to optic nerve or retina, but these tests should never replace the clinical examination. VEPs measure the cortical activity in response to flash or patterned stimuli and are abnormal in the presence of a lesion in the afferent visual pathway. A normal VEP in the setting of decreased vision and an otherwise normal examination suggests functional visual loss (see Chapter 11). Multifocal VEPs (mfVEPs) assess the response simultaneously from multiple regions throughout the visual field—the distribution is similar to areas on a

3  •  Visual Loss: Overview, Visual Field Testing, and Topical Diagnosis

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Table 3.2  Topical Diagnosis: Various Causes of Visual Loss According to Lesion Site Within the Afferent Visual Pathway Lesion Site

Common Causes

Examination Findings

Ocular

Refractive error, media opacities

Vision may improve with pinhole; normal pupillary reactivity

Retina

Macular degeneration, epiretinal membrane, central retinal artery occlusion, central retinal vein occlusion

Visible retinal abnormality on ophthalmoscopy

Optic nerve

Inflammatory lesions (idiopathic optic neuritis, sarcoid); ischemia (atherosclerotic, vasculitic); infiltrative/infectious (neoplastic, syphilis)

Afferent pupillary defect present if unilateral; central, centrocecal, or arcuate field defect; disc swelling may be visualized if optic nerve head involved

Chiasm

Sellar mass (pituitary adenoma, craniopharyngioma, meningioma, aneurysm)

Bitemporal hemianopia; optic atrophy if chronic

Optic tract

Sellar mass

Afferent pupillary defect variable; classically incongruous hemianopia; “bow-tie” disc atrophy if long-standing

Lateral geniculate

Stroke

Incongruous hemianopia, optic atrophy late; horizontal sectoranopia or quadruple quadrantanopia suggestive of infarction

Optic radiations (parietal)

Stroke, neoplasm

Inferior contralateral quadrantanopia; normal pupillary reactivity; defective optokinetic response with targets drawn towards the lesion

Optic radiations (temporal)

Stroke, neoplasm

Superior contralateral quadrantanopia; normal pupillary reactivity

Occipital

Stroke, neoplasm

Congruous contralateral hemianopia with or without macular sparing; normal pupillary reactivity; normal optokinetic response

dartboard.50 Some authors have used mfVEPs as objective perimetry in the detection and follow-up of optic neuropathies and other central visual pathway disorders.51–54 ERGs and multifocal ERGs, which measure rod and cone photoreceptor function, are particularly helpful in sorting out retinal dystrophies and degenerations. ERG testing is discussed in more detail in Chapter 4.

DIFFERENTIAL DIAGNOSIS Table 3.2 highlights examination findings, visual field abnormalities, and common causes to consider for various localizations within the afferent visual pathway. Acquired optic neuropathies in young adults are typically inflammatory, while in older adults they are more commonly associated with vascular disease. The most common cause of a chiasmal syndrome is a compressive sellar mass. Strokes and neoplasms are the most common causes of retrochiasmal visual loss. The reader is referred to Chapters 4–8 for further details regarding the differential diagnoses and discussion of the various responsible causes.

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