Purpose: To study time of day and seasonal variation of onset of visual loss in nonarteritic anterior ischemic optic neuropathy (AION).
Methods: From 1975 to 1995, we prospectively investigated the time of discovery of visual loss in 635 patients (871 eyes) with AION--a total of 925 episodes. Data were analyzed for two variables: time of day of discovery of visual loss for 544 episodes and seasonal variation of the onset of AION for 839 episodes.
Results: Of 544 episodes, time of day for discovery of visual loss was upon awakening from sleep in the morning or a nap in 282 (51.8%), during the first opportunity to use vision critically early in the morning in 117 (21.5%), and later in the day in 145 (26.7%). AION was significantly (P = .0030) more frequent in summer than winter. Estimated monthly onset rates were 82.7 episodes (95% confidence interval [CI], 71.3 to 95.8) in summer, 58.3 (95% CI, 48.9 to 69.6) in winter, 66.0 (95% CI, 55.9 to 77.9) in spring, and 72.7 (95% CI, 62.1 to 85.1) in fall. Onset significantly (P < .0001) occurred more during hot than cold months, with the estimated monthly rate of onset in hot months of 82.2 episodes (95% CI, 73.7 to 91.8) compared with 59.8 (95% CI, 53.3 to 67.1) in cold months.
Conclusions: In at least 399 (73.3%) of 544 episodes of AION, patients discovered visual loss upon first awakening or at first opportunity to use vision critically after sleeping, suggesting that nocturnal arterial hypotension may play an important role. Also, AION developed more often in summer than in winter.