Visual Field Measurement PDF

VISUAL FIELD MEASUREMENT Visual field losses are more common in older patients. Field defects may come from glaucoma, chorioretinal disease, optic atrophy, and visual pathway disorders. Also, visual sensitivity is reduced with age and, for a given visual stimulus strength, the measured visual BOX 7...

VISUAL FIELD MEASUREMENT Visual field losses are more common in older patients. Field defects may come from glaucoma, chorioretinal disease, optic atrophy, and visual pathway disorders. Also, visual sensitivity is reduced with age and, for a given visual stimulus strength, the measured visual BOX 7-1 Purposes of Testing To screen for otherwise unsuspected pathological conditions To seek evidence that will confirm the presence of an already suspected pathological condition To monitor the progress of a previously identified field defect To evaluate the impact that the field loss already known to be present will have on the person’s ability to function field becomes reduced in size for older patients.8,31 When measuring visual fields, the clinician should be conscious of the purpose of conducting the test (Box 7-1). The stimulus parameters and testing strategies will vary accordingly. Today most visual field testing is done with automated perimetry instruments that present lights at various luminosities at selected locations in the visual field following computer controlled sequences. The presence of risk factors and symptoms associated with glaucoma and other neurological and retinal diseases guides the clinician’s choice of which field tests and what testing strategies should be used. The Humphrey Visual Field Analyzer (Carl Zeiss Meditec AG, Jena, Germany) and the Octopus (Bio- Rad, Cambridge, Mass.) are the two most widely used automated perimeter instruments. They both offer a wide range of options for selecting different sets of stimulus locations by using different strategies for determining thresholds in given locations. They can also check the consistency of the patient’s responses and their maintenance of fixation. The visual field testing programs for screening are of relatively short duration (often 2 or 3 minutes), with fewer points tested and fewer presentations at each location, whereas more extensive and longer routines ranging from 7 to 20 minutes per eye may be used for monitoring and for more thorough diagnostic purposes. The results for a given eye can be analyzed in different ways. Summary statistics, such as the mean deviation, provide an index of the average reduction in visual sensitivity compared with value to normal visual field for the age-matched population. Graphic display printouts typically show the regional variations in visual sensitivity with gray scales to indicate the severity of reductions in visual sensitivity. Such plots may show absolute threshold values, reductions relative to the thresholds for an agematched normal population (commonly called “total deviation”), and regional reductions relative to the individual’s overall sensitivity level (“pattern deviation”). Most of the automated perimetry devices concentrate on testing the central 25 to 30 degrees of the visual field. The characteristics of field loss in glaucoma have influenced many of the testing and analysis strategies. The glaucoma hemi-field test analyses threshold data and compares the relative sensitivity of selected regions in the superior and inferior visual fields, looking for patterns characteristic of glaucomatous field loss. Display technology and advances in visual science have lead to the development of new visual field tests that specify properties of visual processing of information originating in different regions of the visual field. They attempt to identify deficits in specific neural processing mechanism that may be especially vulnerable to certain diseases. Again, the central visual fields receive the most attention. The frequency-doubling perimetry test is a central visual field test in which a large area (5 degrees square) of flickering grating is presented in one of 17 different locations. The grating has a spatial frequency of less than 1 cycle per degree, and when presented in rapid counterphase flicker (more than 15 Hz), there is an illusion that twice as many stripes are in the target pattern.20 This frequency-doubling illusion depends on the sparse large-diameter magnocellular nerve fibers, which are believed to be especially vulnerable to damage from glaucoma. The visual system is more sensitive to contrast at the higher spatial frequency. The test measures contrast thresholds for detecting the grating in each of the selected locations. This test is reported to be sensitive for the early detection of glaucoma. The visibility of the stimulus is relatively unaffected by optical defocus, ambient illumination, pupil diameter, or media clarity. Another test that shows good sensitivity to early losses in glaucoma is the short wavelength automated perimetry (SWAP), in which the a large blue target is presented on a bright yellow background.19 Some automated perimetry procedures use temporal variations and involve detection of flickering stimuli or detection of motion or displacements. High-pass resolution perimetry is performed on a display screen, and the patient’s task is to see rings that have a light central region with a dark region on either side. The average luminance of the dark and light regions is equal to the luminance of the background. The diameter of the rings and the widths of the dark and light components are gradually increased until the ring can be seen in different regions of the visual field. Automated perimeters do include some programs that present stimuli beyond the central 30 degrees, typically testing out to 60 degrees. Testing in these more peripheral regions is obviously important in some diseases (e.g., retinitis pigmentosa, retinal detachment, visual pathway disorders). The automated visual field testing procedures have special advantages that come mainly from consistent control of the test stimuli, the test procedures, and the computerized analysis of the results. They are not as useful for mapping out the detailed shape of scotomas or for testing the more peripheral parts of the visual field. Detailed information about the shape and location of scotomas across the entire visual field can be very important for predicting functional abilities. The classic perimetric techniques of tangent screen and bowl perimeter examination for fields are less commonly used today, but they remain the best methods for determining the shape and location of scotomas. The targets may be spots of various sizes or lights of various luminosity and sizes at the end on a handheld wand, or they may be projected spots of light, usually with variable luminance and size. Illumination conditions such as luminance of the illuminated perimeter bowl, or the ambient illumination on the tangent screen, need to be controlled and a record should be kept of the test conditions. Usually most of such testing is done with kinetic perimetry, in which the clinician moves the chosen target across the visual field. Sometimes static perimetry techniques are used; a target location is chosen and the clinician turns the target on and off while the patient reports when the target is seen. The test parameters (target size and luminance as well as background luminance) and strategies for presentation will vary according to the purpose for conducting the test. For screening, test spots should be just comfortably detectable, and a systematic broad search should be made of the whole visual field. For confirming tentative diagnoses, the test targets should be just detectable—and only just detectable in the region where the field defect is most likely to occur. The test target presentation should be confined largely to this region of the visual field, and the motion of dynamic targets should be such that the direction is approximately at right angles to the probable border of the scotoma. Results are more reliable and the scotoma shape becomes better defined if the direction of motion goes from nonseeing to seeing. To monitor the progression of visual field defects with tangent screen or bowl perimetry techniques, the stimulus conditions should be identical (or as similar as possible) to those previously used. Again, any target motion should be orthogonal to the known border of the scotoma. For functional evaluation of the visual fields, binocular observation may be more relevant then the monocular fields.23 Relatively easy to see targets (large or bright) should be used. Special problems may be associated with visual field measurement in low vision patients. If a central scotoma is present, two alternative strategies may be used. One is to have the patent look at the fixation spot by eccentric viewing. Patients with central scotomas often develop a preferred retinal locus (PRL) that they use for giving direct visual attention to an object. This may be many degrees away from where the anatomical fovea was located. Allowance for this displacement must be made when interpreting the visual field results. The central scotoma region will be located to one side of the fixation point. The second alternative is to use a fixation cross centered on the fixation point. Elastic cord, masking tape, or chalk may be used on the tangent screen to provide a cross through the central point. The patient is instructed to look toward the center of the cross, even though he or she may not see the actual intersection. Flashing the target can make it easier for the patient to maintain central fixation because patients are less tempted to move their eyes to check on the presence of the target when it is flashing. Functional visual field testing is important in low vision patients. Whenever frank scotomas are found on the tangent screen or bowl perimeter, a much coarser test of functional detection ability should be made by using larger and more visible targets. A hand or piece of paper may be used as a target against a black screen to establish whether the scotoma is truly absolute. Coarse screening for peripheral field loss may be performed by using confrontation or similar techniques.10 With confrontation testing, the patient’s visual field is compared to that of the clinician. The clinician closes one eye, the patient closes the opposite eye, and they each look toward the open eye of the other. In a vertical plane midway between their eyes, the clinician introduces the test stimulus, which may be a handheld target or the clinician’s fingers. It is though the clinician and the patient are looking at clear glass screen from opposite sides, and the anatomical limitations of the peripheral fields from the brow, nose, and cheek should project to similar locations for the patient and the examiner. If no substantial peripheral field deficits are present, the point of appearance and disappearance of the test target should be at approximately the same location for both clinician and patient. An alternative technique is to move the target along an arc beginning from behind the patient and simulating a sweep across a bowl perimeter. This pseudoperimeter technique enables better testing of the more peripheral regions of the field, especially on the temporal side. The Amsler grid can be a useful test of central visual function, characterizing disturbances of central vision. 29 Patients may report absences, fading, or distortion in parts of grid pattern while they maintain fixation on its center. When patients report observable changes, the practitioner may gain insights into the nature of the visual disturbance and perhaps may be better able to predict or understand the patients’ functional difficulties. However, a patient often will not recognize scotomas because “filling in” seems to occur. Indeed, the normal physiological blind spot usually cannot be observed on the Amsler grid pattern. Useful information is obtained about the patient’s vision function when visual disturbances are reported on the Amsler grid test. When no visual disturbance is observed by the patient, however, no definite conclusion should be made about the presence or absence of scotomas. COLOR VISION TESTING The purpose of testing color vision is twofold. First, the identification of color vision anomalies can assist in the diagnosis or detection of pathological changes in the visual system. Second, altered color vision can cause some difficulties with color discrimination tasks, and the possibility of such functional difficulties should be discussed with the patient. Color discrimination usually changes slightly as the patient ages because of yellowing of the crystalline lens and physiological and pathological changes in the macular region. Such acquired defects tend to be tritanopic, the most obvious manifestations being a reduction of color discrimination ability for the blue and blue-green regions of the spectrum. The congenital color defects found in approximately 8% of the male population (0.5% in women) are almost always of the deuteranopic or protanopic types, and the main color discrimination difficulties are in the “tomato” region of the spectrum (green, yellow, orange, red). The congenital color vision defects do not cause new functional problems in older patients. The test of choice for the routine assessment of color vision in older patients is the Farnsworth Panel D-15 test, in which 15 colored chips are arranged so that they appear to be in order according their chromatic similarity. Patients with normal aging changes affecting color vision typically make only a few smallmagnitude errors of the tritanopic type. When retinal disease is present, however, the number and magnitude of errors in arranging the D-15 targets are greater. In cases of substantial retinal pathology, the magnitude of errors in arranging the D-15 test targets is large, and the pattern of the errors is more random (see Chapter 2 for further discussion of age-related vision changes). OTHER TESTS OF OCULAR OR VISUAL FUNCTION A variety of clinical tests of visual functions can be useful in identifying the presence of ocular pathological changes, for making diagnostic distinctions, or for explaining functional difficulties resulting from the disease. Contrast sensitivity losses of small magnitude are common in older adults, and more severe losses of contrast sensitivity accompany many of the ocular and visual pathway disorders associated with aging. No close relationship between visual acuity and contrast sensitivity exists; sometimes one function may be significantly reduced while the other may be scarcely affected. Mobility and driving performance and many other tasks of daily living are more affected by impaired contrast sensitivity than by impaired visual acuity. Contrast sensitivity measurements are mainly useful for predicting functional abilities, but they can also have value in making diagnostic decisions and in understanding the nature of a person’s vision loss. Three basic approaches for measuring contrast sensitivity are taken. The traditional method is to present sinusoidal grating targets at selected spatial frequencies. Then contrast is varied to determine the minimal contrast required for detection of the striped grating pattern for each of the selected spatial frequencies. The contrast sensitivity function is a graph showing how contrast sensitivity varies with spatial frequency (see Chapter 2). The grating displays may be presented on oscilloscope or video screens or on printed chart displays.16 The second method is to measure visual acuity with low- contrast letter charts that effectively determine the spatial frequency limit for resolution at selected contrast levels. 27 The third method is to present a sequence of large targets, such as large letters or edge targets, in which a progression of reducing contrast is shown, and the lowest contrast at which the target can be recognized is the measure of contrast sensitivity.4,26,34 The most widely used contrast sensitivity test is the Pelli-Robson chart, which has a series of large letters (49-mm high) with a progressive reduction in contrast in which each successive set of three letters becomes lower in contrast by 0.15 log units (70%). Patients read as far as possible down the chart until reaching their threshold contrast. Disability glare is more of a problem in older patients because all develop increased intraocular light scatter as a result if inevitable aging changes in the lens of the eye. Light scatter becomes more pronounced with the development of cataract or with disorders affecting the cornea or the vitreous. Tests of disability glare can be useful for monitoring the development of cataract or other medical opacities or for the prediction of functional difficulties that may be experienced under glare conditions. The common feature is that all glare tests measure a visual function, visual acuity or contrast sensitivity, tested with and without the presence of a glare source. The Brightness Acuity Tester is a device that has a 6-cm diameter hemispherical bowl of controlled luminance held over the eye.18 The patient observes a visual acuity or contrast sensitivity chart through an aperture in the center of the bowl. A more analytical assessment of light scatter and glare can be made by a method introduced by Van den Berg33 in which scattered light from a flashing bright annulus of light can induce a flickering appearance in a steady, central, spot target. Introducing counterphase flicker of variable intensity into the central spot provides a means of nulling the flickering appearance induced by the light scattered from the annulus. This effectively quantifies the scattered light. Low-contrast letter charts with a surrounding field of glare have been shown to be sensitive measures of disability glare.5,28 Most clinicians do not have glare tests at hand, and a less-controlled assessment of disability glare can be made by shining a penlight at the patient’s eye as a visual acuity chart is being read. Retinal adaptational mechanisms may be impaired by some age-related eye diseases, and dark adaptation and glare recovery tests may identify such losses. Sophisticated instrumentation is available for testing dark adaptation and glare recovery, but some relative or functional assessments may be made by testing the patient’s ability to see objects in very dim light or by measuring the time taken for maximal visual acuity to return after exposure to a strong light such as from a penlight held close to the eye. Differential diagnosis of pathological conditions may be facilitated by the use of electroretinograms, electro-oculograms, fluorescein angiography, measurement of responses to flicker, special tests of color vision function, and visually evoked cortical potentials.