Delayed manifestation of infantile nystagmus following a vertigo attack

Infantile nystagmus syndrome (INS), formerly referred to as congenital nystagmus, typically presents in early infancy. We report a rare case in which nystagmus consistent with INS became clinically apparent after a vertigo attack during adolescence. A 17-year-old female developed acute rotational vertigo. Although the vertigo gradually subsided, she continued to perceive rightward visual motion. Five months later, she was referred to our institution because of persistent spontaneous and gaze-evoked nystagmus. There was no prior history or documentation of nystagmus. Examination revealed jerk-type horizontal nystagmus with a predominantly right-beating fast phase (slow-phase velocity [SPV] ~5°/s). Gaze testing showed direction-changing nystagmus, predominantly left-beating (SPV 10-30°/s), with a rightward null position. Horizontal smooth pursuit was saccadic, whereas vertical pursuit was preserved. Optokinetic testing demonstrated poor development and inversion of horizontal responses. Vestibular testing showed mild right-sided hypofunction. Over a 4-year follow-up, mild oscillopsia persisted, and ocular motor findings remained unchanged. This case suggests that a vertigo attack may have contributed to the clinical manifestation of latent INS in adolescence. Persistent characteristic ocular motor findings support the possibility that pre-existing but clinically silent INS may have become manifest following vestibular dysfunction.