Periodic alternating nystagmus.

Three patients with periodic alternating nystagmus (PAN) are described in detail. Digital computer methods were used to quantify their disordered eye movement in an attempt to understand the pathophysiology. One of the patients was unusual in showing rebound nystagmus with fixation and PAN without fixation. Each patient had hyperactive vestibular responses and the phase and gain of the PAN cycles were altered in a predictable fashion by vestibular stimuli. It is postulated that PAN represents cyclical firing between reciprocally connected groups of inhibitory neurons within the vestibular and oculomotor nuclei. The cyclical firing is initiated by a critical imbalance of tonic input to either group of normally functioning neurons.