Variation of congenital nystagmus with viewing distance.

Involuntary oscillations of the eyes in persons with congenital nystagmus often dampen in intensity (amplitude x frequency) with near viewing. This study examined how changes in viewing distance affect the duration and position variability of foveation periods (intervals of low eye velocity when the target's image is at or near the fovea), as these parameters generally correlate better with acuity than nystagmus intensity. To do so, the horizontal positions of both eyes were recorded by infrared limbal tracking in 4 subjects with congenital nystagmus and 4 normals during monocular and binocular fixation at 200, 40, and 20 cm. At nearer distances, position variability (the standard deviation of horizontal eye position) increased in normal subjects, as well as during foveation periods in the subjects with nystagmus. Other parameters of the nystagmus varied idiosyncratically with viewing distance. In none of the subjects with nystagmus was visual acuity (measured psychometrically) better at 40 cm than at distance. The results show that characteristics of congenital nystagmus do not necessarily improve and, in some instances worsen, with near viewing.