Ординатура / Офтальмология / Английские материалы / Practical Ophthalmology A Manual for the Beginning Ophthalmology Residents 4th edition_Wilson_1996
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146 Chapter 8: Visual Field Kxamination
Screening Tests
Visual field screening is routinely done at a patient's initial eve examination. The confrontation fields test can screen lor an unsuspected visual field defect caused by a lesion of the central nervous system, but confrontation testing is often unreliable for detecting subtle visual field loss, as in glaucoma. The Arnsler grid, used when the patient has symptoms of central distortion or loss, helps evaluate macular [unction. Both of these screening tests are discussed below.
Confrontation Fields Testing
Confrontation testing of a patient's visual fields is done in a tace-to- face position at a distance of about 1 meter (3 feet). By convention, the right eye is tested first, although if there is a marked difference in visual acuitv it is advisable to begin with whichever is the better e\e. The eye not being tested must be completely occluded, either bv usino a handheld or press-on occluder, by putting a folded facial tissue under an elastic eye occluder, or by asking the patient to cover the eve with the palm of the hand. When the patient's left eye is covered, the examiner's right eye should be closed, and vice versa, to permit comparison. Then present fingers midway between yourself and the patient, testing all four quadrants (Figure 8.2). . • ; |
B
Figure 8.2 The confrontation fields test. (A) Correct presentation of fingers, side by side frontally. (B) Incorrect presentation, so that one finger hides the
other. (Reprirr |
Walsh TJ, ed: Visual Fields: Examination and |
Interpretation. |
Monograph 3. San Francisco, American |
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Screening Tests |
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To assess the patient's visual field, compare the patient's responses to your own normal visual field. Testing the edges of the central visual field is more rewarding than testing the extreme periphery, because the hand is often about the size of the 20/400 optotype, which is the visual level 30° from the fovea. (All examiners should know how their hand compares in size to the Snellen chart.) To outline the visual field determined by the confrontation screening method, slowly bring your hand inward from different directions, testing each of the patient's meridians. Instructions for performing the confrontation fie ds test are provided in Clinical Protocol 8.1.
Special Situations
Confrontation testing to screen for visual field defects may not be possible in infants, obtunded patients, and patients with optic nerve dis-
ease. Alternative screening methods for such patients are described below. ' ' * -; -
Reflex eye movement test for infants
Test the visual field of infants and toddlers by making use of their involuntary fixational reflexes. First get the child's attention in a frontal gaze. While the child is watching your face, silently bring an interesting toy or other object from the periphery to elicit fixational head and eye movements.
Blink reflex test for obtunded patients
Quickly flicking your hand toward a sighted patient's open eye normally elicits a blink reflex. This test can help find a dense hemianopia or quadrantanopia.
Comparison testing for patients with optic nerve disease
The blind spot usually cannot be adequately evaluated by confrontation fields testing. If you suspect optic neuropathy, ask the patient to subjectively compare the brightness of a light between the two eyes. To do this, shine a penlight directly into the patient's open eye, first into one and then into the other. By assigning a 100% score to the brighter eye, have the patient estimate the relative reduction in light intensity perceived by the dimmer eye. For example, ask the patient, "If the right eye is one dollar, how much is the left eye worth?"
The color desaturation test requires the use of a bright red object to compare the two eyes. Neutral density filters can be used to help quantify the dillerence in brightness.
148 Chapter 8: Visual Field Examination
Amsler Grid Test
The Amsler grid is used to test the central 10° to 20° of each eye's visual field. It helps test for suspected macular disease that produces a central scotoma. The Amsler grid is a white or red pattern of lines withi a central spot printed on a black background (Figure 8.3A). When viewed at a distance of 30 cm (12 to 14 inches) using near correction, the lines are 1° apart. A patient with no abnormalities perceives the lines as straight and complete. Patients with abnormalities report distorted or missing lines, which they can record themselves on a preprinted black-on-white version of the grid (Figure 8.3B). Other grids are available but not commonly used. Instructions for Amsler grid testing are given in Clinical Protocol 8.2.
Manual Perimetry
A perimeter is an instrument that measures the visual field by the patient's subjective responses. A perimeter is used to quantify or confirm a visual field defect discovered or suspected through screening, to detect subtle field defects such as those associated with glaucoma, and to monitor changes in a previously known condition. The tangent screen and Goldmann perimeter are two forms of kinetic perimetry that are performed manually.
A
Figure 8.3 The Amsler grid. (A) The test pattern has white or red lines on a black background. (B) The patient draws the central field defect on the preprinted pad that has black lines on a white background.
Manual Perimetry |
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The tangent screen (at a distance of 1 m and with a white test stimulus or target that is 1 to 2 mm in diameter) tests the central 30° of the visual field. Tangent screen testing is principally used to assess the size of the blind spot, to detect a central or cecocentral scotoma, and to evaluate functional or factitious defects. The Goldmann perimeter tests the entire visual field, using different target sizes and intensities. The patient's corrective lenses are used when testing the central 30°; testing outside this zone is often done without correction. The tangent screen and the Goldmann perimeter plot the visual field in degrees of arc.
Exploring the visual field with different target sizes is known as quantitative perimetry. The use of colored targets is called qualitative perimetry and can help distinguish a retinal disturbance, in which there is a relatively greater loss for blue, from a visual pathway defect, which has a greater loss for red.
Before beginning manual perimetry, describe the test to the patient, explaining how the test object is going to be presented and how to respond. Display the test object and then tell the patient to fixate on the central target. Before mapping the blind spot, relieve the patient's anxiety by explaining that it is normal to be unable to see the test object in certain areas. Throughout the entire test, watch the patient vigilantly to ensure continued fixation. Further details about tangent screen testing and Goldmann perimetry appear below.
Tangent Screen Testing
The tangent screen is a square black cloth, usually made of felt, that hangs on a wall (Figure 8.4). Some tangent screens have concentric circles and radiating meridians sewn into the fabric. The concentric circles
Figure 8.4 Tangent screen testing.
~r*-
ter 8: Visual Field Examination
represent the angle in degrees from the center; the radiating meridians represent degrees of the circle, at 30° intervals, around the fixation point. The tangent screen test examines the central 30° of the visual field, no matter how far the patient is from the tangent screen.
Four blind spots are faintly indicated on the screen. The smaller pair of right and left ovals is for testing at 1 m (the usual testing distance), and the larger pair is for a 2 m testing distance. A white disk is positioned at the center of the screen as a fixation target. For a patient with decreased vision, this fixation target can be larger, or a cross made of two pieces of white adhesive tape can be used.
The examiner presents the test object against the screen. The choice of test objects depends on the patient's visual acuity and mental alertness. The central field is normally tested by holding a 2 mm white disk or sphere on a black wand against the tangent screen. For patients who are familiar with the test, a 1 mm white test object may be more effective for revealing a subtle defect.
Tangent screen testing generally consists of plotting the blind spots and checking for scotomas. Patients with suspected nonorganic visual field loss may be tested at both 1 m and 2 m. Test results are recorded by outlining the limits of the patient's visual fields on a standard preprinted record form (Figure 8.5).
For a bedridden patient who cannot come to the office, a type of tangent screen perimetry might be possible by using a laser pointer to test the visual fields on a hospital wall or ceiling. Instructions for standard tangent screen testing are provided in Clinical Protocol 8.3.
Figure 8.5 Tangent screen test record form.
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Goldmann Perimetry
The Goldmann perimeter is a hemispheric dome with a white background that is illuminated near the lower limit of photopic vision. A movable pantographic device permits a light target to be projected within the dome, continuously or intermittently, at various sizes and brightness levels (Figure 8.6). The examiner directly observes the patient's visual fixation from behind the dome through a telescope, and the patient responds to stimuli with a buzzer.
The instrument must be calibrated to give a standard target stimulus and background illumination. Corrective lenses are not needed for testing the intermediate or peripheral zones. For central visual field testing, use the optimal lens correction (spherical equivalent) in a trial lens holder for each eye at a working distance of 33 cm (13 inches). Each eye is tested separately while the other eye is occluded. Allow 2 minutes for the patient to adapt to the illuminated perimeter before beginning the test.
Six different target sizes are available, each four times the size of the previous target: 0 (0.0625 mm2), I (0.25 mm2), II (1 mm2), III (4 mm2), IV (16 mm2), and V (64 mm2). Target brightness (intensity) is measured in decibels (dB). Gray filters allow the brightness of the target to be reduced in 0.5 dB steps from 4 to 1 and in 0.1 dB steps from e to a. Perimetry almost always begins with target size I and intensity 4e (this isopter line would be labeled I4e on the diagram). Larger targets (II to V) are chosen if this isopter is revealed to be constricted. The choice of targets is usually limited to two or three of the following: 12,14, 114, IV4, and V4. Clinical Protocol 8.4 lists the steps required to perform Goldmann perimetry.
Figure 8.6 Goldmann perimetry. (A) From the patient's side. (B) From the
examiner's side. |
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1:>2 Chapter 8: Visual Held Kxamination
Automated Perimetry
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A static perimeter is an instrument that tests for visual field defects |
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using fixed light locations. Static perimetry can often detect smaller or |
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shallower defects than kinetic perimetry. Because the testing is tedious |
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and time consuming, most modern static perimeters are computerized, |
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new automated instrumentation continues to be developed. |
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Automation also has other advantages, including the ability to compare |
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results statistically with normal individuals of the same age group and |
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with |
previous tests for the same patient. Also, automated perimetry |
does not require highly skilled operators, and it eliminates certain operator errors. Some patients, such as young children and individuals who need vigorous encouragement to maintain fixation, may not be good candidates for automated perimetry.
Test Targets and Strategies
A projection perimeter has the best sensitivity and specificity and offers the most flexibility of test strategies and target presentation patterns. The standard target size for automated perimetry is equivalent to a Goldmann size III (4 mm-) white target. Brightness is indicated in decibels. For many instruments, a decibel is defined as 10 x log (10,000/asb), where an apostilb (asb) is a unit of brightness per unit area (and is defined as K candela/irr). In contrast to kinetic perimetry, the higher numbers indicate a logarithmic reduction in test object brightness, and hence greater sensitivity of vision.
Iwo basic testing strategies are used in automated static perimetry. Suprathreshold testing, a screening procedure to detect gross defects, uses targets that are well above the brightness that the patient should be able to see. Threshold testing places a target of a given size in the visual field and gradually increases its brightness until the patient just sees it. Threshold testing provides more precise results than suprathreshold testing and is generally preferred, although it takes more time and the equipment often costs more. Because a threshold is the sensitivity of a retinal location at which a stimulus intensity is perceived 50% of the time, many patients feel frustrated when they do not see targets during much of the test.
Patient Preparation
Most automated perimeters use a bowl setup similar to the Goldmann perimeter with a white background luminance of 31.5 asb. Adjust the
Automated Perimetry |
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table height and chin rest so that the patient's forehead rests comfortably against the band. Align die patient's eye in the center of the monitor, and measure the patient's pupils to the nearest 0.5 mm. If the pupil is less than 3 mm in diameter, consider dilation with a 2.5% phenylephrine eyedrop. Occlude the eye not being tested, and use tape to lift a drooping brow or upper eyelid. Reduce the room lighting to a moderate level. Allow 3 minutes for the patient to adapt to the perimeter's brightness.
The patient's vision must be correctly refracted before testing, and the patient's fixation must be constantly monitored during the test. Assuming a 33 cm test distance (ie, the radius of the perimeter bowl), the proper lens correction is the distance correction (including astigmatic errors of 1.00 D or more) with an appropriate near add rounding up to the nearest 0.25 D (Table 8.1). Contact lenses are preferred if the spherical equivalent needed for the test is more than ±6.00 D. When inserting and adjusting the perimeter's trial lens holder, align the pupil in the center of the lens, and put the lens as close as comfortably possible to the patient's eye without touching the eyelashes.
Table 8.2 presents an example of possible test parameters. The test parameters and the performance steps differ for each computerized instrument and are explained in the manufacturer's instruction book. A
Table 8.1 Suggested Near Addition for Manual or Automated Perimetry
Age (Years) |
Near Correction (Diopters) |
40-44 |
+1.50 |
45-49 |
+2.00 |
50-54 |
+2.50 |
>55 |
+3.00 |
Table 8.2 Examples of Testing Parameters for Automated Perimetry
Threshold strategy: |
Full threshold |
Fixation target: |
Central |
Stimulus size: |
III |
Stimulus color: |
White |
Test speed: |
Normal |
Foveal threshold: |
On. |
Fluctuation: |
On |
FASTPAC: |
Off |
Chapter 8: Visual Field Examination
demonstration program is run for a patient new to automated perimetry, and the patient is instructed about what to expect and what to do, as in this example: "Always look straight ahead at the steady fixation light. Other lights will flash one at a time at other positions around the center light. Some may be bright, and others will be dim. Press the button whenever you see one of these flashes. You are not expected to see all of them. The best time to blink is just as you press the button."
Test Selection
Number and distribution of points
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The number of points tested determines test time. Since automated sta- |
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tic perimetry is fatiguing for die patient, you should limit the number |
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of test points as much as possible. The most commonly used tests |
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explore 50 to 120 test points. The most typical grid of points is an array |
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of 76 points that are 6° apart and blanket the central 30° of the field. |
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Different software programs test different areas of the visual field, |
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depending on the specific disorder known or suspected. Program |
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selection depends on whether the visual field examination is done for |
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diagnostic testing of a suspected defect or for followup of a progressive |
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condition. For example, a glaucoma test includes extra points to detect |
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such common glaucomatous visual field defects as a nasal step or an |
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arcuate defect, whereas neurologic visual field testing emphasizes |
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points along the vertical meridian and within the central field, where |
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many neurologic visual field defects may be found. |
Size of the field to be tested
For most patients with glaucoma or a neuro-ophthalmologic condition, a 30° or a 24° field is appropriate. The Central 30-2 test is an example of a program that evaluates the central 30° with 76 points. It is commonly used for monitoring glaucoma patients and for detecting neurologic visual field defects. The 24-2 test provides a 24° field from fixation with an extension of the nasal field to 30°, using a 6° grid. The 2 in the designation 24-2 test indicates a grid that straddles the horizontal and vertical meridians. The 24-2 test is generally preferred for testing most patients (instead of a 30-1 or 24-1 test, which align the grid onto the vertical and horizontal meridians).
Deficits less than 6° in diameter or located in the far periphery may be missed by the 30-2 and 24-2 tests. For patients with central visual field loss, the Central 10-2 test may be appropriate tor sequential testing, because it tests the central 10° field with a 2° grid. For patients with peripheral visual field defects, Goldmann perimetry may be useful.
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' • • ( |
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A u t o m a t e d P e r i m e t r y |
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Testing strategy
Most tests begin by testing the blind spot and then present one point in each quadrant. Subsequent point brightness is determined by the threshold levels at adjacent points. Missed points can be retested by using different thresholds, but this takes more time than simple screening. The foveal threshold must be requested separately but can be done as a separate portion of the testing procedure.
Most clinicians prefer a threshold, rather than a suprathreshold, testing strategy. A full threshold test is appropriate for a patient's first test, because it crosses the threshold twice (first with a 4 dB increment, then with a 2 dB increment). Accurately determining threshold values makes subsequent tests easier because it allows the perimeter to begin with the previous threshold values for determining future data points. FASTPAC is a more rapid testing strategy where the threshold is only crossed once (in 3 dB increments), but this strategy is often not appropriate for diagnosis or followup.
Interpretation of a Computerized Printout
Printed test results show basic patient information such as age and pupillary diameter. The raw data from automated static perimetry are shown as reliability measures and as the numeric plot, the actual sensitivity values at each tested point (Figure 8.7). The computerized printout also presents several statistical calculations, showing how the patient varies
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Figure 8.7
Computerized printout record of automated static perimetry. (Courtesy California Pacific Medical Center.)
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