Superior canal dehiscence - Mechanisms of pressure sensitivity in a chinchilla model

Background: Patients with superior canal dehiscence syndrome may experience vertigo and nystagmus when pressure changes occur in the external auditory canal, the middle ear, or the intracranial space. The cause is a defect in the bone of the superior canal. Objective: To study the mechanisms of pressure sensitivity of the labyrinth in superior canal dehiscence syndrome and its surgical repair in a chinchi lla model. Methods: We investigated the changes in firing rates of vestibular nerve af ferents in the chinchilla in response to changes in external auditory canal pressure before and after fenestration of the superior canal, and after re pair of the fenestra. Results: Before superior canal fenestration, external auditory canal pressu re changes caused no responses in horizontal canal or otolith afferents, an d only 1 of 9 superior canal afferents responded to pressure. After fenestr ation, all superior canal afferents were excited by positive pressure and i nhibited by negative pressure. Half of 18 otolith and most (21 of 33) horiz ontal canal afferents were unaffected by pressure. The superior canal affer ents had higher pressure gain than the horizontal canal afferents (P = .03) . Pressure responses could be abolished only by applying a rigid seal to th e fenestra. Conclusions: Fenestration of the superior canal rendered all superior canal afferents sensitive to pressure, whereas less than half of the other affer ents became pressure sensitive. The direction of the superior canal afferen t responses agreed with the predictions of our model of endolymph flow with in the superior canal. A rigid seal applied to the fenestra abolished press ure sensitivity while maintaining physiologic rotational sensitivity.