This study describes patients with horizontal aperiodic alternating nystagmus (AAN)  in peripheral vestibulopathy and brainstem lesion and elucidates the possible mechanism of that.

We defined horizontal AAN that it is repeat reversal of spontaneous horizontal nystagmus but markedly asymmetric and irregular unlike PAN(periodic alternating nystagmus). We consecutively recruited patients with horizontal AAN from 5 different dizziness clinics of tertiary hospital from January 2018 to Jury 2019. Eye movements of all patients were recorded using video-oculography for a minimum 2 cycles. All patients had vestibular function test(caloric test, bedside or video head impulse test) and magnetic resonance imaging(MRIs). To determine the most relevant mechanism of horizontal AAN, we analyzed the pattern of nystagmus and classified with vestibular dysfunction and reasonable causes.

In rare cases, horizontal AAN can occur due to various peripheral vestibulopathy from acute to chronic phase. the shortened time constant(impaired velocity storage mechanism) due to peripheral vestibulopathy can destabilize the adaptive process and cause a horizontal AAN.