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I. L. Bailey, A. Jackson, J. Zwelling, R. B. Greer; Quantifying the Effect of Illumination on Visual Acuity and Contrast Sensitivity in Low Vision Patients. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1510.
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Low vision patients frequently have extra-ordinary responses to changes in task luminance. Clinicians typically rely on history taking and the patients’ reports of their visual experience when changes are made to the quality or quantity of lighting. We have identified an easy-to-administer method for quantifying individual patients’ responses to changes in light levels, which involves the measurement of visual acuity and contrast sensitivity with and without a standard neutral gray filter.
Subjects are recruited from the Low Vision Clinic of the School of Optometry at UC Berkeley. Visual acuity measurements are made with a retro illuminated Bailey-Lovie Chart at a test distance of 3 or 4 meters. Contrast sensitivity is measured using a computer-based test in which the subject is required to locate a large blinking square (about 5 degrees) that appears in one of four marked quadrants. The touch screen provides a measure of response time as a function of target contrast. These measurements are made with and without a NOIR U23 neutral-gray filter. This widely available fit-over sunglass filter has a 1.8 ND. Thus we determine the magnitude of the increase or decrease in VA and CS in response to a standard reduction (to 1.6%) in retinal illuminance.
Normally sighted subjects show, on average, a 0.22 log unit reduction (11 letters) in visual acuity and a 0.13 log unit reduction (25%) in contrast sensitivity. Within low vision patients having the same diagnosis, there can be considerable diversity, but some strong patterns emerge. Compared to normals, glaucoma patients show about the same reduction in VA (0.23 log units) but twice the reduction in CS (0.27), RP patients show more pronounced reductions in both visual functions, particularly for CS. On the other hand, patients with albinism generally do not show much change in acuity but have moderate reductions in CS. AMD patients show more change than normals and this is can be most pronounced for visual acuity. Some patients show a visual improvement with the filter.
Low vision patient responses to reductions in illumination can be characterized quantitatively by measuring VA and CS with and without a dense neutral filter. This information can be obtained quickly, simply and with minimal expense, and it can be useful in guiding the clinician's treatment decisions and prediction of functional abilities.
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