Unexpected non-linearities in the VOR responses of patients with bilateral loss of peripheral vestibular function

Twelve patients with bilateral loss of peripheral vestibular function exhibited less than 50/s peak slow-phase eye velocity for each of 30 degrees, 44 degrees and iced water irrigations. They were exposed to harmonic accelerations of 90, 150, 180, and 2100/s peak head velocities with 1/6Hz passive whole-body rotations. X-Y plots of hc;id vs nystagmus slow-phase eye velocity showed low or no eye responses at low head velocities (<similar to 500/s). At higher velocities, however, the slow-phase eye velocity increased bilaterally exponentially with increasing head velocity, In sonic cases, the VOR gain at head velocities above 1500/s could even reach the range observed in normals, Our bilateral mathematical model explains these peculiar responses to harmonic accelerations in these patients, by an increase in the dead-zone of the peripheral vestibular response (high threshold), and an overall decrease in afferent sensitivity From the clinical standpoint, there are two major implications. First, an absence of caloric responses cannot be used as an indicator of VOR deficits in all behavioural protocols. Second, such patients may even have near-normal VOR gains, when head speeds are sufficiently high. The form of X-Y plots relating eye and head velocity in the VOR of patients can be useful in confirming the presence of bilateral peripheral low vestibular function and in assisting the design of more meaningful rehabilitation exercises.