Periodic alternating nystagmus in progressive cerebellar ataxia

Periodic alternating nystagmus (PAN) is a distinctive disorder where horizontal nystagmus beats in one direction for 90-120 seconds, slows and enters a (quiet) null period, reverses and beats in the opposite direction, and reverses again after another 90-120 second cycle. These patients tend to predominantly experience oscillopsia without significant dizziness/vertigo, so the examiner must have a suspicion for PAN in patients with spontaneous horizontal nystagmus (who are not in the midst of a vertigo attack) and know to watch it for 90-120 seconds. PAN localizes well to a lesion (e.g., stroke, mass lesion) involving the cerebellar nodulus/ventral uvula, although it may also occur in patients with spinocerebellar ataxia, Creutzfeldt-Jakob disease, or other causes of more generalized cerebellar disease. Baclofen tends to be a highly effective medication for oscillopsia in many patients, making the recognition of PAN consequential even if the underlying etiology is not amenable to treatment.