Superior semicircular canal dehiscence syndrome

Key points • The Tullio phenomenon is sound-induced vertigo, nystagmus, or both. • Hennebert sign is pressure-induced vertigo, nystagmus, or both, elicited by insufflation of the external auditory canal. • Typically, in affected patients with superior semicircular canal dehiscence syndrome, there is a several-year history of symptoms that may include sound-induced vertigo (Tullio phenomenon), pressure-induced vertigo (Hennebert sign), or oscillopsia as well as chronic disequilibrium. • Characteristic signs include sound-induced eye movement and head tilt, Valsalva-induced eye movement, Hennebert sign, and postural sway induced by external auditory canal pressure. Other signs can include pulse-synchronous rotatory nystagmus, hyperacusis to bone-conducted sounds, and conductive hearing loss. • Patients with bilateral superior semicircular canal dehiscence may have vertical oscillopsia and impaired vision during locomotion, disequilibrium, spontaneous pulse-synchronous vertical pendular nystagmus, and Valsalva-induced upbeat jerk nystagmus. • Superior semicircular canal dehiscence syndrome is caused by missing bone over the superior aspect of a semicircular canal, creating a third "mobile window" in the bony labyrinth. This may be a developmental abnormality. • High-resolution computed tomography of the temporal bones is usually considered the definitive test for superior semicircular canal dehiscence. • Patients also have lowered vestibular-evoked myogenic potential thresholds and larger vestibular-evoked myogenic potential amplitudes; vestibular-evoked myogenic potential studies are highly sensitive and specific for superior canal dehiscence (although rare patients with posterior canal dehiscence will also have abnormal vestibular-evoked myogenic potentials). • Patients with superior semicircular canal dehiscence and disabling disequilibrium have benefited from surgically plugging or patching (ie, resurfacing or “reroofing”) the dehiscent superior semicircular canal through a middle cranial fossa approach or, more recently, with a less complicated and potentially safer transmastoid approach.