Teaching Video NeuroImages: periodic alternating nystagmus evident only in darkness.

An 81-year-old man with retinitis pigmentosa had severe, slowly progressive visual loss that began at age 20. Intermittent nystagmus was observed for 1 decade. Examination revealed severe retinal atrophy and periodic alternating nystagmus (PAN) that occurred only in darkness (video on the Neurology® Web site at www.neurology.org). PAN refers to horizontal nystagmus that periodically reverses direction. It is caused by instability of brainstem velocity storage mechanisms that regulate rotationally induced nystagmus, with intact vestibular “repair mechanisms” that are calibrated by visual input. PAN may emerge with impairment of central visual stabilization mechanisms or deficient visual input. This case demonstrates that the presence of PAN may correlate with ambient lighting and maintenance of visual fixation.