Vestibular perception' in subjects with congenital nystagmus

Abstract

Subjects with congenital nystagmus (CN), 18 otherwise healthy and 3 with cochleo-vestibular disease, were assessed on a test of ‘vestibular perception’. Subjects were seated on a motorised chair and tasked with returning themselves to a starting position, using a joystick control, in response to passive rightwards or leftwards displacements which had raised cosine velocity profiles of 3o7s or 6o7s and amplitudes varying from 30o to 180o. On average, normal subjects return to the starting position for displacements up to circa 150o and are slightly hypometric for larger displacements, undershooting the start because of the low frequency characteristic of vestibular transduction. Eight of the healthy CN subjects were normal. Six had responses which were mildly hypometric in one direction. Three subjects were hypometric in both directions. One subject had mildly hypermetric responses for one direction overshooting the start. The three CN subjects with otological disease had marked hypometricity in one direction, presumed to be towards the affected side, comparable with patients with unilateral vestibular neurinectomies. The abnormal findings in CN subjects are explained in terms of a reduction of ‘velocity storage’ in vestibular processing and mild asymmetries of vestibular tone. The results on the test clearly differentiate CN subjects with otological disease and help substantiate their diagnoses. A model based on the processing of vestibular ocular reflex eye movements with an additional integrator for ‘perception’ of position is utilised to explain the findings.