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 transient and is frequently associated with symptoms ct dizziness, dysequilibrium, or vertigo. The phenomenon presumably reflects headshake-induced asymmetry in vestibule-ocular reflex pathways, which persists after headshaking stops, We postulated that the same vestibular imbalance that underlies post-headshake nystagmus might produce an equivalent in postural instability, To test this hypothesis, we investigated the effect of headshake on postural control and eye movements in patients who exhibited post-headshake nystagmus, vestibulopathy, or both, Postural instability was quantified with a dynamic platform device, whereas eye movements were recorded with electrooculography. Ten normal controls and 21 patients with a history of post-headshake nystagmus or unilateral vestibulopathy were evaluated, Subjects were tested for 20 seconds before and immediately after passive horizontal headshake (+/-30-degree amplitude) at 2 Hz for 20 seconds, Postural stability was assessed while subjects stood with eyes closed, and the floor was modulated proportionally with sway, The difference in normalized peak-to-peak sway (equilibrium score) before and after headshake was assessed in all subjects and compared between groups, Post-headshake nystagmus was documented by electro-oculography recorded during posturography. Results for 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 caloric asymmetry) showed a robust decline in postural stability. The most dramatic decline, together with an increased number of ''falls'' (step-out responses), occurred in those patients with both a vestibulopathy (caloric response asymmetry) and post-headshake nystagmus, These findings illustrate that the vestibular imbalance underlying post-headshake nystagmus shares a counterpart in the form of headshake-induced postural instability.