Downbeat nystagmus with a pseudocycloid waveform: improvement with base-out prisms.

Downbeat nystagmus in primary position and oscillopsia resulted from nutritional deficiency during prolonged intravenous therapy of a patient with hyperemesis gravidarum. Wide bandwidth infrared oculography demonstrated a pseudocycloid nystagmus waveform with an increasing-velocity exponential slow phase. Because the oscillopsia decreased and the nystagmus was damped with convergence, visual acuity improved with the addition of base-out prisms to each spectacle lens.