Superior canal dehiscence (SCD), one form of perilymphatic fistula, is a clinical condition that results in a constellation of auditory and vestibular symptoms, such as sound- and/or pressure-induced vertigo and nystagmus, hearing loss, and/or autophony (hearing one’s own body sounds at an abnormally high sound level). First described by Minor, Solomon, Zinreich, & Zee (1998), SCD occurs when the temporal bone overlying the superior semicircular canal is unusually thin or absent.
Under normal conditions, sound pressure by air conduction enters the fluid-filled inner ear via the oval window and exits via the round window. In superior canal dehiscence, the deficit of bone overlying the semicircular canal results in a third mobile window effect, leading to areas of increased compliance and allowing for the movement of endolymph within the canal in the presence of sound and/or pressure stimuli. These stimuli increase the pressure gradient within the membranous labyrinth resulting from the change in compliance at the dehiscence site, with resulting deflection of the superior canal cupula and a subsequent inhibitory or excitatory response from the vestibular nervous system.