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ter 8: Visual Field Examination

from the expected normal. In reviewing the test results, the examiner must look at die reliability measures, die numeric plot, the probability maps, and the global indices.

The reliability measures are the proportion of fixation losses, falsepositive errors, and false-negative errors. Fixation loss is the proportion of times that the patient responded inappropriately, because of wandering fixation, to a stimulus at the presumed blind spot. The falsepositive error rate is die proportion of times that die patient responded when no stimulus was presented. The false-negative error rate is the proportion of times that the patient did not respond when a suprathreshold stimulus (ie, a luminance that had been seen earlier in the test) was presented. The printout flags these parameters with xx when low reliability is suspected. The visual field examination is considered unreliable if three or more of the following parameters occur:

•Total questions >400

•False-negative responses >3 3 %

•Short-term fluctuation >4.0 dB

The numeric plot gives the measured threshold values at each test point. Recall that a higher number means a dimmer light, and thus a more sensitive visual area. Repeated determinations are given in parentheses under the retested value. A < symbol indicates that the target was not seen or that the bulb in the perimeter needs to be replaced. The grayscale rendering interpolates the raw data in a graphic format that looks like a topographic map similar to one obtained by kinetic perimetry. The darker the area, the greater the loss of sensitivity for a given target size. Because data between test points are extrapolated, the grayscale may not be accurate.

Mathematical deviation results are displayed as integers, showing how the patient deviates above and below normal comparable subjects, and as probability maps illustrated in graphic format. A single visual field test will compare the patient's results on a point-by-point basis with results expected from a healthy population of comparable age. A series of tests by a patient are compared to a baseline visual field test. The total deviation shows all defects, whether localized or generalized. The pattern deviation adjusts for any diffuse change in visual field sensitivity and thus shows the localized defects. Loci more likelv to be abnormal are shown by darker symbols. In a reliable test, an abnormal field generally requires a cluster of at least three abnormal points, not at the edge, on the pattern deviation probability plot.

The global indices are MD (mean deviation from age-corrected normal), PSD (pattern standard deviation), SF (short-term fluctua-

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Remote from horizontal meridian

Straddles horizontal meridian

Figure 8.10 Interpretation tree for visual field defects. (From Trobe JD, Glaser JS.

The Visual Fields Manual:A Practical Guide to Testing and Interpretation. Gainesville,

FL:Triad;1983.)

Symmetry

Field defects that are similar between the two eyes are called congruous, and defects that are asymmetric or differently sized for each eye are noncongruous. Because corresponding fibers from the two retinas lie close together as they near the visual cortex, lesions of the posterior radiations tend to be congruous, while lesions of the anterior visual pathways are more frequently noncongruous.

Localizing Visual Field Defects

The physician needs to know the typical patterns obtained in perimetry to determine the probable location of a lesion. A decision-making approach based on knowledge of neuroanatomy helps the examiner make an accurate medical interpretation. Figure 8.10 depicts a variety of common perimetric defects and their likely anatomic origins. ,-

Figure 8.11 Progressive visual field loss as detected by repeat automated perimetry in a patient with chronic glaucoma.

Progression

Visual fields often must be tested on several occasions to get a reliable picture of the patient's status. Chronic diseases such as glaucoma can produce progressive visual field loss that may be detected before optic nerve or other changes are visible (Figure 8.11).

Pitfalls and Pointers

Make the patient comfortable witfi the examination. The results of a visual field examination will be most accurate if the examiner provides a quiet, nondisturbing environment and ensures that the patient fully understands the testing procedure. Some patients

Ch;apter 8: Visual Field Examination

approach perimetry as if taking a final exam and are anxious not to make a mistake. The examiner can help alleviate patient stress by avoiding any show of impatience or irritation and by beginning with a brief trial run before starting the actual test. During the test, patient fatigue can also lead to errors. A sympathetic examiner provides rest periods, minimizes a lengthy test as much as possible, and avoids presenting test objects too rapidly or too slowly.

•Scrupulously ensure that the patient maintains fixation. The examiner must concentrate on watching the patient's fixation. Patients with a central scotoma who have poor vision will have problems fixating accurately. If the patient cannot see a standard fixation target, use a larger one. People with very poor vision may find it easier to hold their finger on a point to help maintain steady gaze.

•Remember always to plot the blind spot. Identifying the blind spot helps ensure the reliability of perimetric testing and the patient's comprehension of the procedure. Baring of the blind spot (ie, a tested isopter passes through the inner side of the upper or lower aspect of the blind spot, thus laying it bare) may be a normal variant caused by reduced retinal sensitivity adjacent to the optic nerve head.

•Limit the testing to vital detail. Visual field testing is exhausting. For Goldmann perimetry, there is time to test only two or three isopters before the patient burns out. Do not waste time testing peripheral isopters, because most visual field defects can be found in the central zone. Concentrate on finding the defect rather than on drawing a pretty picture. If patients tire or lose concentration during perimetry, allow them to rest for a minute or two, then give encouragement to continue.

•Ensure that the ambient light is controlled. To compare fields obtained from a patient on different days, the background illumination needs to be the same. A generally accepted standard is 7 footcandles, which can be tested with a light meter. Perimeters must be regularly calibrated.

•Check the pupil. Changes in the pupil's size can affect the appearance of the visual field. The pupillary diameter should be measured

and recorded at every visual field examination. When it is less than 3 mm, the patient's pupil should be dilated. If this is not possible, each examination should be repeated at the same pupillary size.

• Remember to correct a refractive error. An inaccurate refractive correction can cause a significant decrease in the measured central visual threshold. Presbyopic correction should be used when needed. \phakic patients are best tested wearing contact lenses. Patients with aphakia and those with high degrees of hyperopia, mvopia, or

astigmatism must be tested with the appropriate near correction. A correcting lens should be placed as close to the eye as possible without touching the eyelashes when die patient blinks. The examiner should ask the patient to confirm that the test object can be seen clearly with the chosen lens before starting the test procedure.

Be aware of normal artifacts. A drooping upper eyelid, a prominent brow or nose, and any media opacity, such as a cataract, should be noted before the field is interpreted. In patients with ptosis, the eyelid may need to be held up with tape to get an accurate record of the superior visual field. Remember to remove a correcting lens when testing outside 30°, otherwise the field will seem to be contracted or to have a ring scotoma.

Avoid overdiagnosis. Visual field testing is a subjective, psychophysical examination. Confrontation fields testing, especially double simultaneous testing, often leads to a range of acceptable answers. The examiner must beware of confusing normal field contraction with a hemianopic defect or a nasal step. Atypical defects or contraction can be caused by delayed patient responses, uncorrected refractive error, ptosis, or malingering.

Learn to interpret the threshold and probability plots. Because the darkness of the points in a computerized grayscale visual field printout depends on the amount of ink in the printer, be cautious when using the grayscale rendering to compare visual fields done on two different visits.

Test and retest. Most patients have a learning curve for visual field testing, especially for automated testing. At least two visual fields are usually necessary to obtain an accurate baseline. A change between visual fields may require another visual field test for confirmation.

Avoid misdiagnosis of a defect in a patient with functional visual loss. Tubular field contraction may be a sign of hysteria or malingering. The visual field extends as a cone outward from the eye, but an individual with nonorganic visual field constriction responds as if the field were a cylinder. A tubular field has the same dimensions at all distances of testing. Other tests for functional visual loss make use of involuntary movements, such as testing for optokinetic nystagmus or putting a base-out prism in front of the affected eye to induce a fusional movement.

Remember always to record the interpretation. Visual field testingis often delegated to a technician, but the visual field results require interpretation by the ophthalmologist. This assessment must be charted in the patient record.

164 Chapter 8: Visual Field Examination

Suggested Resources

Anderson DR. Automated Static Perimetry•. St. Louis: Mosbv-Year

Book; 1992.

Automated Perimetry [Ophthalmic Procedures Assessment]. San

Francisco: American Academy of Ophthalmology; 1995.

Chaplin NT. Visual Field Testing With the Humphrey Field Analyzer.

Thorofare, NJ: Slack; 1995.

Drake MV. A primer on automated perimetry. Focal Points: Clinical Modides for Ophthalmologists, Vol IX, Module 8. San Francisco: American Academy of Ophthalmology; 1993.

Henson DB. Visual Fields. New York: Oxford University Press; 1993.

Lieberman MF Glaucoma and automated perimetry. Focal Points: Clinical Modules for Ophthalmologists, Vol XI, Module 9. San Francisco: American Academy of Ophthalmology; 1993.

Newman SA. Automated perimetry in neuro-ophthalmology. Focal Points: Clinical Modules for Ophthalmologists, Vol XI, Module 6. San Francisco: American Academy of Ophthalmology; 1995.

Walsh TJ, ed. Visual Fields: Examination and Interpretation.

Ophthalmology Monograph 3. San Francisco: American Academy of Ophthalmology; 1990.

Clinical Protocol 8.1

Performing the Confrontation Fields Test

Test Setup

1.Seat the patient and make sure the eye not being tested is occluded.

2.Seat yourself facing the patient at a distance of about 1 m. Close your *••• eye that is directly opposite the patient's occluded eye.

3.Ask the patient to fixate on your nose or on your open eye.

4. Finger counting. Hold vour hands stationary midway between yourself and the patient in opposite quadrants about 30° from central fixation