Ординатура / Офтальмология / Английские материалы / Clinical Neuro-ophthalmology A Practical Guide_Schiefer, Wilhelm , Hart_2007

Perimetric testing should be considered appropriate when the following problems, findings, or factors are present:

■The presence of a relative afferent pupillary defect (RAPD)

■Monitoring of visual field defects already known to exist

■Signs or symptoms of damage to the afferent visual pathway

■Visual disturbances of unknown cause (e.g., for nyctalopia, loss of brightness perception, and disturbances of reading or of visual orientation)

■Abnormalities of ocular fundus (optic disc and retina)

■Certification of visual function for driving or occupational tasks

Additional Historical Information

It is useful to ask patients specifically whether they have been aware of visual field defects (called positive scotomas) or changes in their field of view, such as metamorphopsia. One should also briefly inquire whether there have been episodic periods of visual loss that might indicate a problem with transient ischemic episodes (see Chap. 14).

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Temporary blurring of vision or passing bouts of darkening can be manifestations of the obscurations of vision associated with papilledema, or may suggest a problem with intermittent optic nerve compression.

Not uncommonly, such symptoms begin as monocular, affecting one eye more heavily than the other, can be provoked with only minimal head movements, and indicate a disturbance of axoplasmic flow (see Chap. 3, ■ Fig. 3.3, and Chaps. 9 and 12).

Fig. 4.3. Topologically diagnostic classification of scotomas (schematic diagram). Visual field defects have been divided into seven primary types (left side of diagram); in the right half of the diagram are commonly occurring subtypes. The numerical labels in the left half of the diagram refer to the corresponding numbers of the Compendium of Visual Field Defects. DLS Differential luminance sensitivity

Perimetry

Poster 4.2

Poster 4.1

Although the swinging flashlight test (see Chap. 2) is not a visual field test in the narrower sense, it should be used routinely as a quick and simple means of detecting afferent visual problems prior to a more thorough examination of the visual field. It can detect monocular or significantly asymmetric, bilateral disturbances of vision in the pregeniculate (anterior) portions of the afferent pathway, and is an objective method that does not depend on the patient’s cooperation. The swinging flashlight test is indicated by any unexplained disturbance of vision, as well as by any suspicion of a lesion in the afferent visual pathway.

As an initial step in the examination, the hands or fingers of the examiner can be held (under steady control of fixation) in either or both halves of the visual field, but moved in only one hemifield at a time (■ Fig. 4.4 b).

Alternatively, when there is a suspicion of a hemianopic loss of field, the examiner can present both hands – held motionless – to either side of the vertical meridian, and ask the patient to report the total number of fingers being held out. This “finger perimetry” method can detect most widespread losses of the peripheral visual field, including hemianopsias and quadrantanopsias (■ Fig. 4.4 c).

a

b

c

Fig. 4.4. Confrontation testing of the visual field a Maintenance of fixation (e.g., on the bridge of examiner’s nose) is essential. b Bimanual testing of the visual field: The examiner closes the eye that is directly opposite to the eye the patient has closed and uses the field of his or her open eye’s monocular perception as a basis for comparison to the patient’s visual field, while presenting visual stimuli (e.g., moving one or both hands) in the opposing portion of the patient’s visual field. This serves to reduce problems with fixation. c Counting of fingers held to either side of the vertical meridian tests the central visual field with relatively smaller“test objects”, as compared with the bimanual tests

When examining small children, the examiner can maintain control of fixation while an assistant positioned behind the child introduces objects of visual interest into the various quadrants of the peripheral visual field. The child’s head movements on detection of these stimuli allow a judgment of the intactness of the visual field.

Confrontation testing should be used routinely, when examining small children or uncooperative patients, whenever there is a suspicion of an advanced degree of visual loss. This preliminary method can also be put to use when circumstances do not allow use of conventional perimetric testing, such as at bedside consultations or in intensive care units. It also serves as a quick check of the plausibility of a patient’s responses.

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A very effective modification of confrontation testing, useful in cases of homonymous visual field loss, has the examiner face the patient at a distance of about 30 in., while the patient fixes on the center of the examiner’s face. The examiner then asks the patient whether the entire face is simultaneously visible. Missing portions of the face give an indication as to the extent of a homonymous defect. The closer a defect is to the vertical meridian, and the smaller is the remaining, congruous portion of the paracentral visual field (■ Fig. 4.5), the more likely there is to be a loss of reading fluency (see below).

Amsler Grid Testing

The Amsler grid is a visual field test that encompasses the central visual field to an eccentricity of 10°. It allows for the subjective detection and ongoing monitoring of retinal, primarily macular, diseases. The patient views an orthogonal grid of vertical and horizontal straight lines at a reading distance of about 17 in (40 cm). The grid has a central fixation point and is shown on a featureless background. It is a very effective test for the detection of metamorphopsia (e.g., caused by retinal edema or other disturbances of photoreceptor alignment), and its use can be delegated to the patient as a solo screening test.

The Amsler grid test is particularly useful when evaluating subjective symptoms of distorted images, reading problems, or signs of abnormal fundus appearance in the macular region. A small, printed figure of the grid can be given to a patient for self-monitoring of visual changes at home.

7.5°

4°

1°

75 cm

Reading Ability

Tests of reading ability, with an age-appropriate correction for near, are an effective method for examining a portion of the central visual field. Fluency of reading requires the normal function of a region at least 2° to the left and right and 1° above and below the point of central fixation (see Chap. 24). When a reading problem is found, a careful ophthalmoscopic examination of the macula will often reveal

ocular fundus is unremarkable in aplikely to be a homonymous hemianopic central visual field, inthe perifoveal field on the is a suspicion of a homthere are any ophthalmocomplaints of reading reading ability are a nec-

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saturation is best done best) object, such as object should be shown to alternating monocular neuropathy causing an seen by the affected eye perception, usually deor “faded” color, paran area of relative visual the same colored obbe done, while monitorthe object in various

Particularly noteworthy rechange in the color’s moved into or out of the can sometimes be quite the vertical meridian

in the visual field.

In cases where optic atrophy is seen or an optic neuropathy is likely (as in patients with relative afferent pupillary defects), testing of color saturation by these simple methods is often very helpful (see Chap. 6).

Conventional Methods of Perimetric Testing

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When correlating visual field defects with lesions in the afferent visual pathway, it is helpful to keep in mind that the optical inversion of images in the eye causes an inversion of the visual field with respect to the afferent pathway. Thus, objects in the superior visual field are imaged in the inferior half of the retina, and the encoded data for that image remain in the fibers of the inferior half the pathway, at least for the optic nerves, the chiasm, and the retrogeniculate path. In addition, the temporal half of the visual field is imaged in the nasal fundus, etc. (see Chap. 3, ■ Figs. 3.4 and 3.7, and ■ Fig. 4.2).

There are two types of perimetry, kinetic and static. In kinetic perimetry, moving test objects are presented, while in static perimetry test objects are held in stationary positions when presented (■ Fig. 4.6). The common goal of these methods is to determine, as precisely as possible, the sensitivity of visual perception as a function of location in the visual field. The correct physiologic term for sensitivity is differential luminance sensitivity ([DLS] also incremental luminance sensitivity). These terms arise from the use of a background light to maintain uniform dark/light adaptation in all visual field areas, while test objects (small spots of light) are projected onto the background, thus adding their luminance to that of the adapting surround. The stimulus strength is expressed as a function of the incremental addition of the stimulus light to the adapting surround on which it is projected. Definitions and descriptions of related physiologic terms in perimetry are listed in ■ Tables 4.1

and 4.2. Three-dimensional reconstructions of the spatial distribution of visual sensitivity produce a conceptual “hill of vision” (■ Fig. 4.6). Radial lines extending from the visual field center are termed meridians (■ Fig. 4.6 c). Their angles of meridional orientation are analogous to those of the geographic meridians of the globe.

The currently accepted definition of the differential luminance sensitivity is related to the perimeter-specific maximum stimulus luminance capacity, complicating any comparison of results between differing instruments (■ Fig. 4.7). It made more sense to adopt a standard of reference in which the adapting background (surround) luminance level was agreed upon and held constant for all instruments. The level of 10 cd/m2 was accepted, which is a low photopic level of retinal adaptation.

Kinetic Perimetry

When using kinetic perimetry, the examiner presents stimuli that move with a constant angular velocity and a constant brightness and size – the corresponding physiologic terms being luminance intensity and angular subtense (for additional help and definitions, see ■ Tables 4.1 and 4.2). The stimulus is projected onto a matte surface of uniform light intensity that serves as an adapting background. The test objects (the projected light stimuli) are moved from nonseeing to seeing areas of the visual field, as perpendicular as is possible to the expected peripheral boundaries of perception (■ Fig. 4.6 a).

An angular velocity of 4°/s is regarded as an optimal compromise between spatial resolution on the one hand and examination duration on the other.

Multiple presentations with a constant stimulus value detect contours of isosensitivity, comparable to the isobars ofmeteorologicalmaps,andarecalledisopters(■ Fig. 4.6 a). Each isopter is tested within at least eight evenly spaced meridians, with stimuli moving from the periphery toward the center of the visual field. By choosing appropriate stimulus sizes and luminous intensities, the hill of vision’s (conceptional) three-dimensional shape and size can be documented by plotting the various isopters. Kinetic perimetry is suited to the documentation of visual function for certification of visual performance, e.g., as necessary for safe operation of a motor vehicle. It is particularly suited to the examination of patients capable of only poor cooperation, permitting a high level of efficiency when examining large visual field defects.

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of about 26¢ and a luminance of about 320 cd/m2. This stimulus should be used in all kinetic perimetric tests so that the results of the various examinations can be more easily compared to one another.

When testing the physiologic blind spot, the usual test object is the I/4e, having a size of 6.5¢ and a luminance of 320 cd/m2. For diagnostic testing, the most discriminating (and therefore most significant) test objects are those of low luminance and small size. These are best suited to the detection of subtle, relative central and paracentral scotomas.

The dimensions of the test objects in the Goldmann perimeter are listed in ■ Table 4.1, and their luminance levels are given in ■ Fig. 4.7.

Static Profile Perimetry

For static profile perimetry, test objects are held in stationary positions, and their luminance levels are varied. The various locations are arrayed along straight lines that pass through the field center, and the choices of lines are determined by the regions of interest in the central and paracentral parts of the visual field (usually within 30° of eccentricity). Profiles of the island (or “hill”) of vision (■ Fig. 4.6 b) are produced. This method of testing is no longer in general use, but remains a particularly good technique for

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Hart .W Schiller, .J Schiefer, .U 4 Chapter

defining the locations and depths of small scotomas, and can be used for sequential exams to follow the course of the profile along the changing boundaries of scotomas or even at the foveal projection at the visual field’s center.

Automated Static Perimetry

Automated static perimetry was developed as a logical extension of static profile perimetry. The profile technique was actually done manually by examiners that were skilled in the control of stimulus values, manually moving the controlling levers in conventional perimeters then being used for kinetic testing. The advent of microprocessors was the enabling technology for further development of the static methods. The manual technique was too complex for use in patterns of more than a linear sequence, but with the automated algorithms of the microprocessor, patterns of arbitrary complexity could be exploited. Quadratic grids of

test locations evolved as the most frequently applied patterns. At each location within the grid of sites being tested, the automated methods determine the threshold of differential luminance sensitivity (■ Fig. 4.6 c). This approach yielded immediate improvements by allowing for a complex randomization of stimulus presentations. This means that several locations can be measured simultaneously, rather than determining threshold at one location before moving on to the next.

Strategies of Perimetric Testing

Two primary strategies for automated static perimetry have been developed (■ Fig. 4.8). In threshold static perimetry (■ Fig. 4.8, left side), stimulus intensity is varied for each location in the testing grid, as a function of the patient’s responses. Using this type of strategy allows for a rather precise determination of the depths of a defect, although at the cost of multiple stimulus presentations and/or an increased density of stimulus locations.

Fig. 4.8. Testing strategies used with automated static perimetry. Left The principle of threshold static perimetry, in which the threshold for perception of a stimulus is determined by sequentially presenting test objects with stimulus values that lie just above or below a putative value at a given location, and threshold is defined as a probability of perception of 0.5. Threshold determination requires a large number of stimulus presentations at each location and costs a comparatively great deal of time, which patients find very tiring. Right The principle of slightly supraliminal static perimetry, in which stimuli are presented with differential luminance

values that are just above expected threshold (based on agecorrected data). If the patient detects a stimulus presentation, no additional stimuli are used at that location, but if the stimulus is not seen, a stimulus of maximal intensity is presented at the location. One of three potential levels of response is thus detected at each site: normal, a relative deficit, or a maximum luminance defect. This method uses a much shorter length of testing time, making it much easier for the patient. Subtle depressions of the local DLS are more likely to go undetected by this method

Supraliminal static perimetry, the second primary strategy (■ Fig. 4.8, right half), differs by presenting test objects that are intentionally above the expected threshold at each location. If the patient responds to the presentation of a test object, it is assumed that visual function is intact, and that location is not tested again. If the patient fails to respond at a given location, a test of the location is repeated with maximal stimulus intensity. If the patient fails to respond to the maximal intensity stimulus, an absolute scotoma is inferred, while a response to the brighter stimulus suggests a relative scotoma at that location. This method is relatively quick and places a minimal burden on the patient’s ability to perform, but will also miss detection of shallow relative scotomas that may be of clinical importance. On the other hand, the supraliminal strategy allows for a comparatively greater number of test locations and a correspondingly higher degree of spatial resolution. This latter property is particularly important for topographic localization of defects in neuro-ophthalmology. It permits a more precise determination of the location and size of scotomas and well as their positions relative to important structures of reference, such as the vertical and horizontal meridians and/or the physiologic blind spot.

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A particularly appropriate pattern for testing patients with neuro-ophthalmic disorders is a grid of stimulus locations that increases in spatial density (becomes more crowded) as the center of the field is approached. This matches the visual field’s physiologic property of maximal spatial resolution at the foveal projection with steadily decreasing resolution as a function of eccentricity. In addition, an increased number of test sites that straddle the horizontal and vertical meridians permits a more sensitive detection of defects that “respect” these boundaries. Such defects are common in the presence of chiasmal and/or retrochiasmal disease.

Absolute and Relative Visual Field Defects

:Definition

An absolute visual field defect is present at any given location, if the I/4e test object (10¢ of angular size and 320 cd/m2) or any higher stimulus value is not seen.

Another definition judges an absolute defect of the increment luminance sensitivity as one that is reduced by 20 dB (i.e., 2 log units), relative to the age-corrected norm at the same location.

A relative scotoma is one in which the standard test object (I/4e) is seen, but the expected level of light increment sensitivity is not fully achieved.

Sources of Error and Important Prerequisites for Perimetric Testing

As in all forms of psychophysical testing, the visual field examination should not be interrupted by disturbances in or around the room in which the examination is being done. Noise or audible background interference and intrusion by stray sources of light in the instrument cupola are just as important as the patient’s comfort during the examination.

The patient must be carefully instructed as to how he/ she is to cooperate in the examination. The task of divided attention (which is not familiar to inexperienced patients), the probable sequence of events, and the expected duration of the test should be reviewed. The presence of the examiner is required throughout the examination, so that signs of fatigue – such as gradual sinking of the upper lid, pupillary miosis, or poor fixation maintenance – can be detected. Intervention to motivate the patient, even with frequent interruptions of testing, is preferable to the risk of accepting unreliable responses.

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Examination of the visual field periphery (for all eccentricities greater than 30°) can be done with no optical correction. Within the central 30° of the field, however, an appropriate correction is required. Suitable lenses (with very narrow frames) should be used. The lenses should include an age-corrected addition for the distance between eye and cupola surface and a correction for any astigmatic error of one diopter or more.

To keep the spherical lens as thin as possible, cylinders of both plus and minus series should be available. Transposition of spherocylindrical notation between equivalent corrections then allows selection of the lesser spherical power, e.g.:

+2.0 sphere –3.0 cyl | 70° corresponds to

–1.0 sphere +3.0 cyl | 160°.

Given the relatively poor stimulus for accommodation inside the instrument cupola, an age-appropriate near correction for the distance between the eye and the hemispheric surface of the cupola should be used, as summarized in ■ Table 4.3.

Fine-tuning of the near correction can profit from the patient’s input, and the power of sphere and cylinder used for the test should be recorded.