Moving Ears and Eyes: Quantifying the Head-Impulse Test in the Clinic

Abstract

Visual fixation is maintained during a rapid passive ‘head-impulse’ stimulus by the vestibulo-ocular reflex (VOR) while saccades change fixation. This thesis examines these eye movement characteristics with the monocular video head impulse test (vHIT) of the right eye. In eighty healthy controls, the VOR gain was found to depend on analysis method and the stimulus peak velocity, with age affecting only the left posterior and anterior canals. Minuscule compensatory saccades were common towards lateral and posterior canals, but rare towards anterior canals, and became larger and more common in older subjects. In forty subjects before complete unilateral vestibular loss (UVL) for schwannoma, the VOR gains of the lateral and posterior canals of the ipsilesional ear reduced. After surgery, the OFF direction of the remaining ear was reduced for all canals. The dynamic range (ON minus OFF) of the posterior canal was only 39% of the lateral and anterior canals. After surgery, from one week to one year the first compensatory saccade frequency was permanently saturated at unchanged amplitudes, while the second saccade decreased in both measures. Saccade latency was earlier with lower gains. In twenty-two UVL subjects, early saccades occurred at similar frequency, magnitude, and latency with and without visual fixation. However, in eighteen healthy and eight bilateral vestibular loss (BVL) subjects, visual fixation evoked more late compensatory saccades than without fixation. Late saccades in UVL were smaller in magnitude and increased in latency. BVL subjects showed more frequent and larger early and late saccades with fixation. A case-series of twenty-seven subjects after sudden hearing loss with vertigo commonly showed loss of the posterior canal VOR and cochlear function. Audio-vestibular tests cannot distinguish between ischaemic and non-ischaemic causes. In summary, quantification of both the VOR and compensatory saccades are useful for identifying lesions involving each semicircular canal.