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John G. Edwards, Gregory R. Jackson; A 5-minute Dark Adaptation Protocol For Screening Of Early AMD. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5173.
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Impairment of dark adaptation is a clinical hallmark of early age-related macular degeneration (AMD). Previous studies have shown that rod-mediated dark adaptation is a sensitive marker for AMD. A drawback to measurement of dark adaptation has been that test durations are often 30 minutes or longer. The purpose of this study was to optimize a rod-mediated dark adaptation protocol that would screen for AMD with a test duration of 5 minutes.
Dark adaptation was measured with the AdaptDx, an automated dark adaptometer designed for detection of AMD. To create a 5-minute protocol sensitive to AMD required optimization of the test location and bleaching intensity, and use of the rod intercept to characterize adaptation speed. To maintain maximum sensitivity, a test location of 5 deg on the inferior visual meridian was selected because, based on prior studies, it corresponds to the retinal area most affected by early AMD. The bleaching intensity was set to a relatively low level of 18,000 scot cd/m2 sec to achieve fast recovery. Use of the rod intercept eliminated the need for generating a full dark adaptation curve and insured that characterization of adaptation speed reflected rod-mediated function. The resulting 5-minute screening protocol was compared with a previously described 20-minute protocol in subjects with normal retinal health as verified by grading of fundus photographs.
Ten normal subjects (mean age 55 yrs; range 34-67 yrs) were tested using both the 5-minute and 20-minute protocols. The mean rod intercept for the 5-minute screening protocol was 4.55 min (SD 0.63; 95% CI 4.1 - 5.0). The mean rod intercept for the 20-minute protocol was 8.28 min (SD 1.1; 95% CI 7.5 - 9.0). The difference was significant (p<0.001). The responses were highly correlated (r =0.70, p <0.0001). The upper limit of the normal reference range (mean + 2SD) was 5.7 minutes and the upper limit of the 95% confidence interval was 5 minutes. A case series of AMD patients was also tested. All of the AMD patients had rod intercepts longer than the upper limit of the normal reference range.
Rod-mediated dark adaptation impairment is a sensitive marker for early AMD. Its clinical utility has been hampered by the time necessary to measure a quality dark adaptation function. Furthermore, use of traditional parameters such as the rod-cone break to characterize adaptation speed is compromised by the fact that dark adaptation curves for patients with different severities of AMD exhibit markedly different shapes at low bleaching intensities. Our approach overcomes these limitations. The 5-minute screening protocol is being validated in an ongoing study of 186 participants to characterize the protocol’s diagnostic sensitivity, specificity and reliability.
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