The recent discovery of superior semicircular canal (SC) dehiscence syndrome as a clinical entity affecting both the auditory and vestibular systems has led to the investigation of the impact of a SC opening on the mechanics of hearing. It is hypothesized that the hole in the SC acts as a “third window” in the inner ear which shunts sound-induced stapes volume velocity away from the cochlea through the opening in the SC. To test the hypothesis and to understand the third window mechanisms the middle-ear input admittance and sound-induced stapes velocity were measured in chinchilla before and after surgically introducing a SC opening and after patching the opening. The extent to which patching returned the system to the presurgical state is used as a control criterion. In eight chinchilla ears a statistically significant, reversible increase in low-frequency middle-ear input admittance magnitude occurred as a result of opening the SC. In six ears a statistically significant reversible increase in stapes velocity was observed. Both of these changes are consistent with the hole creating a shunt pathway that increases the cochlear input admittance.
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