Rod plateaux during dark adaptation in Sorsby's fundus dystrophy and vitamin A deficiency

To investigate the transitory plateaux observed during dark adaptation after partial bleaches in Sorsby's fundus dystrophy (SFD) and in systemic vitamin A deficiency (VAD).

Psychophysical dark adaptation functions were measured after bleaching exposures isomerizing from 2% to 99% of the rhodopsin. Narrow-band stimuli of 1.7 degrees diameter and 200 msec duration were presented at an eccentricity of 30 degrees.

After a full bleach, the patients showed typical dark adaptation abnormalities reported for these diseases. The cone recovery 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 final rod threshold. After partial bleaches, short wavelength stimuli produced a biphasic recovery function, with an initial rapid component and plateau, followed by a subsequent break-off and eventual return to prebleach thresholds. Action spectra obtained during the plateaux were consistent with thresholds for shorter wavelength stimuli mediated by rods and thresholds for longer wavelength stimuli mediated by cones. In the patient with VAD, vitamin A supplementation led to accelerated recovery and disappearance of the transitory rod plateaux.

The biphasic dark adaptation functions resulting from fractional bleaches in SFD and VAD appear superficially similar to the classic biphasic adaptation functions obtained with full bleaches. However, thresholds during the plateaux are lower than the cone threshold, and action spectra indicate rod mediation. These transitory rod plateaux may increase our understanding of the normal visual cycle and its perturbation in retinal disease.