Vertical eye-movement oscillation with a frequency double that of lateral linear acceleration in patients with long-standing unilateral vestibular loss

Objective: To examine our hypothesis that an unusual floating or tilting sensation claimed by patients with long-standing unilateral vestibular loss might be attributed to incomplete central compensation in the otolith system. Methods: Seven patients who were with or without symptoms for 6–101 months after intratympanic gentamicin therapy for unilateral endolymphatic hydrops were sinusoidally exposed to lateral linear acceleration, and their compensatory eye movements were compared with those of 18 normal controls, using electro-oculography (EOG). The subjects, secured firmly in the chair of a linear accelerator (sled), were oscillated at three different G-loadings of 0.1 (0.11 Hz), 0.2 (0.16 Hz) and 0.3 G (0.19 Hz), respectively. During displacement, they gazed at a real (visible) or imaginary target which was located on the wall (earth-fixed), or moved on the wall in sync with the sled (body-fixed). Results: Vertical EOG (V-EOG) with a frequency two-fold that of the stimulus frequency was characteristically evoked in all patients, but in none of the normal controls. One of the doubled V-EOG was larger in amplitude than the other, when the stimulus acceleration was directed from the affected ear to the intact ear in those patients whose symptoms still remained. Such a directional difference tended to be greater with the imaginary target than with the visible target. In the horizontal EOG, there was no marked difference between the patients and normal controls. Nobody reported tilt perception but only horizontal translation during the sled displacement, except one patient who was examined twice; a roll tilt was perceived in the first examination when she still had symptoms, but no tilt sensation in the second one when she no longer had symptoms. Conclusion: It was suggested that incomplete compensation in the otolith system could cause unusual sensations even after long-standing unilateral vestibular loss, and that V-EOG induced by a sled might be helpful in evaluating functional recovery.