Purpose: Because simple anisocoria is believed to decrease in bright light, the authors determined the prevalence of simple anisocoria under different lighting conditions. Methods: The authors measured the pupil size of 104 healthy subjects with infrared videography at four clinically accessible light levels: darkness; darkness with a hand-held light shining from below; room light; and room light with the hand light shining from below. Results: Of the 104 subjects, 40 (38%) were men and 64 (62%) were women. The ages ranged from 12 to 71 years (mean, 36.3 ± 12.5 years). The mean decrease in pupillary diameter from darkness to the brightest condition was 1.89 mm. Based on the traditional definition of a pupillary diameter difference of 0.4 mm or greater, the prevalence of simple anisocoria decreased from 18% in darkness to 8% in room light with the hand-held light shining from below. The prevalence of anisocoria varied considerably when other definitions were used. Repeated measures analysis of variance showed that pupillary area difference decreased with brighter conditions (P = 0.026). However, the ratio of the pupillary areas did not change with brighter conditions (P = 0.666). Conclusions: The prevalence of simple anisocoria decreases with brighter conditions based on pupillary diameter difference. However, this decrease is not apparent when anisocoria is expressed as pupillary area ratio. Those clinicians who measure pupils will find that simple anisocoria decreases in bright light. However, with gross observation where perception of an anisocoria may be related more to the ratio of the pupillary areas, simple anisocoria may not seem to change much with brighter conditions.
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