Walsh & Hoyt: Periodic Alternating Nystagmus

Periodic alternating nystagmus is a spontaneous horizontal nystagmus, present in central gaze, that reverses direction approximately every 90120 seconds. Because the period of oscillation is about 4 minutes, the disorder may be missed unless the examiner observes the nystagmus for several minutes. As the nystagmus finishes one half-cycle (e.g., of right-beating nystagmus), a brief transition period occurs during which there may be upbeating or downbeating nystagmus or saccadic movements before the next half-cycle (e.g., of left-beating nystagmus) starts. A congenital form of PAN also exists (discussed in the section ""Congenital Nystagmus""), but this is usually much less regular in the timing of reversal of direction and shows slow-phase waveforms typical of congenital nystagmus. PAN must also be differentiated from ""ping-pong gaze,"" an ocular deviation that reverses direction not over several minutes but every few seconds and that is encountered in unconscious patients with large bihemispheric lesions.