Visual pathology and human circadian rhythms

The extent to which visual pathology affects hormonal and behavioral circadian rhythms has been investigated in a blind population. Visually impaired subjects (n = 388) with varying degrees of light perception (LP) or no light perception (NPL) were assessed for sleep disturbances by interview using the Pittsburgh Sleep Quality Index (PSQI). The duration and rapidity of visual loss and visual field defect were also recorded. Overall a high incidence of sleep disorders was noted (58% subjects with PSQI greater than or equal to 5), those with NPL having a higher occurrence and more severe sleep disturbances than subjects with LP. Over a 4 week period two subgroups of blind subjects (n = 15 with LP, n = 15 with NPL) were studied in detail. For 48 h each week urine was collected every 4 h (8 h during sleep) and the melatonin metabolite, 6-sulphatoxymelatonin (aMT6s) measured by direct radioimmunoassay. This is considered a reliable marker of the circadian system. Actigraphs were worn continuously to record activity. The majority of LP subjects (14/15) had entrained aMT6s acrophase times, mostly (12/14) occurring within the range of sighted individuals. Conversely most subjects with NPL had aMT6s acrophase times outside the normal range (12/15), 9 having free running aMT6s rhythms (tau range 24.34 - 24.79 h) and 3 having entrained aMT6s acrophases but abnormally timed. With reduced light perception the incidence of free running aMT6s rhythms increased and the percentage of entrained, normally timed aMT6s rhythms declined. If freerunning activity rhythms were observed (n = 5), these occurred in subjects with free running aMT6s rhythms but with a shorter tau. Our results demonstrate that a lack of light perception is associated with a higher incidence of desynchronisation from 24 h.