Superior semicircular canal dehiscence : auditory mechanisms

Abstract

Superior semicircular canal dehiscence (SCD) syndrome is a recently defined clinical disorder in which patients present to the clinic with vestibular symptoms, auditory symptoms, or both. Understanding the effect of SCD, a hole in the bony superior canal, on hearing will broaden our understanding of the mechanics of the inner ear and lead to better diagnosis and treatment of SCD syndrome. We evaluate the effect of SCD on cochlear responses to both air- and bone- conducted sound. In chinchilla SCD produces reversible changes in cochlear potential: an increased sensitivity to bone-conducted sound and a decreased sensitivity to air-conducted sound. Such differences in air- and bone-conducted sound (air-bone gaps) are typically associated with a conductive hearing loss due to middle-ear pathology; however, a SCD is an inner-ear pathology. We hypothesize that the SCD acts as a 'third window' into the inner ear, shunting volume velocity away from the cochlea and through the dehiscent canal, altering cochlear responses to sound. To qualitatively evaluate this hypothesis we measured sound-induced fluid motion within the SCD as well as the effect of SCD on sound-induced stapes velocity and middle-ear input admittance.

(cont.) Our results are consistent with the hypothesis that the SCD introduces a low-impedance shunt pathway. To quantitatively evaluate the third-window hypothesis we developed an anatomically and physiologically constrained lumped-element mechano-acoustic model that predicts the effect of SCD on cochlear responses. Our model also predicts the effect of anatomical variations, such as dehiscence size and location, on auditory sensitivity. This work demonstrates that an air-bone gap can be caused by pathological changes in inner-ear mechanics. Additionally, our model provides a framework that will be of direct clinical benefit in understanding the variable effects of SCD among patients.