Estimating loss of canal function in the video head impulse test (vHIT)

BACKGROUND: The vestibulo-ocular reflex (VOR) gain is the primary parameter for quantifying and interpreting the video head impulse test (vHIT). Yet, the relationship between the VOR gain and the extent of canal function is not clear. OBJECTIVE: The goal of this paper was to determine if the loss of canal function in vHIT can be estimated from the VOR gain. METHODS: A model of the VOR was developed that included linear components for the cupula and the velocity storage mechanism as well as nonlinear components for the vestibular nerve and the vestibular nuclei. Multiple simulations were carried out as the level of function for the right and left VOR pathways was varied systematically over their entire range. RESULTS: Simulation results were similar to the typical findings in normal individuals as well as in patients with unilateral and bilateral loss of canal function. Based on these simulations, a relationship between the canal function and the VOR gains was established. This relationship was surprisingly independent of most model parameters. CONCLUSIONS: The sum of right and left VOR gains (or 2 times the mean of VOR gains) at a given head velocity is an estimate of the total function of the involved canals. This simple formula can estimate the loss of canal function in purely unilateral lesions. For bilateral lesions, the same formula can estimate the total loss of bilateral function but contributions from individual canals cannot be determined without additional information.