PLATEAUS DURING DARK-ADAPTATION IN SORSBYS FUNDUS DYSTROPHY AND VITAMIN-A-DEFICIENCY

Purpose. To investigate the transitory plateaux observed during dark a daptation after partial bleaches in Sorsby's fundus dystrophy (SFD) an d in systemic vitamin A deficiency (VAD). Methods, Psychophysical dark adaptation functions were measured after bleaching exposures isomeriz ing from 2% to 99% of the rhodopsin. Narrow-band stimuli of 1.7 degree s diameter and 200 msec duration were presented at an eccentricity of 30 degrees. Results, After a full bleach, the patients showed typical dark adaptation abnormalities reported for these diseases. The cone re covery was slowed, and the time to the rod-cone break was delayed; the final phase of rod recovery was also slowed but led to a normal Anal rod threshold. After partial bleaches, short wavelength stimuli produc ed a biphasic recovery function,with an initial rapid component and pl ateau, followed by a subsequent break-off and eventual return to prebl each thresholds. Action spectra obtained during the plateaux were cons istent with thresholds for shorter wavelength stimuli mediated by rods and thresholds for longer wavelength stimuli mediated by cones. In th e patient with VAD, vitamin A supplementation led to accelerated recov ery and disappearance of the transitory rod plateaux. Conclusions, The biphasic dark adaptation functions resulting from fractional bleaches in SFD and VAD appear superficially similar to the classic biphasic a daptation functions obtained with full bleaches. However, thresholds d uring the plateaux are lower than the cone threshold, and action spect ra indicate rod mediation. These transitory rod plateaux may increase our understanding of the normal visual cycle and its perturbation in r etinal disease.