NYSTAGMUS AND POSTURAL INSTABILITY AFTER HEADSHAKE IN PATIENTS WITH VESTIBULAR DYSFUNCTION

Nystagmus after rapid head-shaking (post-headshake nystagmus) is often seen in patients with vestibulopathy. Post-headshake nystagmus is tra nsient and is frequently associated with symptoms ct dizziness, dysequ ilibrium, or vertigo. The phenomenon presumably reflects headshake-ind uced asymmetry in vestibule-ocular reflex pathways, which persists aft er headshaking stops, We postulated that the same vestibular imbalance that underlies post-headshake nystagmus might produce an equivalent i n postural instability, To test this hypothesis, we investigated the e ffect of headshake on postural control and eye movements in patients w ho exhibited post-headshake nystagmus, vestibulopathy, or both, Postur al instability was quantified with a dynamic platform device, whereas eye movements were recorded with electrooculography. Ten normal contro ls and 21 patients with a history of post-headshake nystagmus or unila teral vestibulopathy were evaluated, Subjects were tested for 20 secon ds before and immediately after passive horizontal headshake (+/-30-de gree amplitude) at 2 Hz for 20 seconds, Postural stability was assesse d while subjects stood with eyes closed, and the floor was modulated p roportionally with sway, The difference in normalized peak-to-peak swa y (equilibrium score) before and after headshake was assessed in all s ubjects and compared between groups, Post-headshake nystagmus was docu mented by electro-oculography recorded during posturography. Results f or normal controls and vestibulopathic subjects without post-headshake nystagmus showed only a small transient decline in postural stability after headshake, Those with post-headshake nystagmus (regardless of c aloric asymmetry) showed a robust decline in postural stability. The m ost dramatic decline, together with an increased number of ''falls'' ( step-out responses), occurred in those patients with both a vestibulop athy (caloric response asymmetry) and post-headshake nystagmus, These findings illustrate that the vestibular imbalance underlying post-head shake nystagmus shares a counterpart in the form of headshake-induced postural instability.