Superior Canal Dehiscence

Thin or missing bone over the top semicircular canal

Overview

Superior canal dehiscence is a thinning or absence of the bone overlying the superior semicircular canal, creating a third mobile window into the inner ear. As a result, sound and pressure changes can stimulate the balance system and bone conducted sounds are transmitted abnormally well. People may experience dizziness or eye movements triggered by loud sounds, known as the Tullio phenomenon, or by straining, coughing, or pressure in the ear, along with autophony, hearing their own voice, breathing, footsteps, or even eye movements loudly. The audiogram may show an apparent conductive loss with unusually good bone conduction. It was first described by Lloyd Minor and colleagues in 1998. High resolution CT shows the defect, and VEMP testing typically shows enlarged responses. Mild cases are often managed by avoiding triggers, while severe cases may be treated by surgical plugging or resurfacing of the canal. It can be mistaken for otosclerosis, Meniere disease, or patulous Eustachian tube.