Intermittent oscillopsia in a case of congenital nystagmus. Dependence upon waveform.

Spontaneous reports of oscillopsia are rare in cases of congenital nystagmus (CN). We examined the relationship between nystagmus waveform characteristics and oscillopsia in one such case. To reduce the patient's nystagmus, she was fitted with contact lenses. We examined the effects of tactile feedback by applying local anesthetic while she wore the lenses. When she was without lenses, we provided tactile feedback by applying gentle finger pressure to one eyelid. She was also asked to look at a peripheral afterimage. Nystagmus was analyzed for frequency, amplitude, foveation duration, and drift velocity, if foveation was not perfectly stable. Perceived target stability was recorded. The patient noted oscillopsia during the initial baseline recording and with lid pressure. The image was stable with contact lenses with and without anesthesia and during the second session baseline; at these times, drift velocity was less than 4 degrees/sec and foveation duration was greater than 100 msec. No oscillopsia of the afterimage in dark was noted; she perceived it moving with her gaze as she attempted to look at it. It appears that in some CN patients, the suppression of oscillopsia operates only within fixed limits of foveation stability and duration. When, because of internal or external factors, their nystagmus exceeds these, oscillopsia results.