THE SUBJECTIVE VISUAL HORIZONTAL AFTER STAPEDOTOMY - EVIDENCE FOR AN INCREASED RESTING ACTIVITY IN OTOLITHIC AFFERENTS

The subjective visual horizontal (SVH) was measured by means of a smal l rotatable luminous line in darkness in the upright body position and at 10, 20 and 30 degrees of body tilt to the right and left prior to, and during a follow-up period after, stapedotomy in 12 patients with otosclerosis. In the acute stage after surgery, SVH in the upright bod y position was significantly tilted away from the operated side. In ad dition, the perception of roll till towards the operated side (K-op) w as significantly increased after stapedotomy, while the perception of roll lilt towards the healthy side (K-he) showed a slight but not sign ificant reduction. After exclusion of two outliers, a statistically si gnificant correlation was found between changes in K-op and in K-he. T he slope of the regression line was 1.8:1, probably corresponding to a preference of the utricle for ipsilateral as opposed to contralateral head tilt. In four patients there was a weak (< 1 degrees/s) spontane ous nystagmus, not systematically related to the side of surgery, whil e in most cases there were no nystagmus or subjective vertigo symptoms . These specific changes in the subjective horizontal show that the ot olithic effects on perception can be dissociated from canal effects. F urther, the results are opposite to those for patients with unilateral loss of vestibular function. The till of SVH after stapedotomy indica tes an increase in resting activity of utricular afferents. In additio n, based on recent theories on otolith function, we suggest that an in creased activity in saccular afferents is of major importance for the changes in roll-tilt perception because of its interaction with the ut ricle on the central nervous level.