Psychophysical evidence for rod vulnerability in age-related macular degeneration

PURPOSE. TO determine whether there is rod system dysfunction in the centra l retina of patients with age-related macular degeneration (AMD). METHODS. Dark-adapted sensitivity (500-nm stimulus) and light-adapted sensi tivity (600 nm) were measured psychophysically at 52 loci in the central 38 degrees (diameter) of retina in 80 patients with AMD, and results were com pared with those from older adult normal controls. All dark-adapted data we re corrected for preretinal absorption. RESULTS. Mean field dark-adapted sensitivity was significantly lower in AMD patients as a group than in normal subjects. Within the AMD group were sub sets of patients with normal mean dark- and Light-adapted sensitivities; re duced dark-adapted sensitivities without detectable light-adapted losses; b oth types of losses; and, least commonly, only light-adapted losses. Region al retinal analyses of the dark-adapted deficit indicated the greatest seve rity was 2 degrees to 4 degrees or approximately 1 mm from the fovea, and t he deficit decreased with increasing eccentricity. CONCLUSIONS, These psychophysical results are consistent with histopatholog ic findings of a selective vulnerability for parafoveal rod photoreceptors in AMD. The different patterns of rod and cone system losses among patients at similar clinical stages reinforces the notion chat AMD is a group of di sorders with underlying heterogeneity of mechanism of visual loss. Dark-ada pted macula-wide testing may be a useful complement to the more traditional outcome measures of fundus pathology and foveal cone-based psychophysics i n future AMD trials.