10Superior Semicircular Canal Dehiscence Repair

FThe third-window concept

The labyrinth normally has two mobile windows: the oval window, where the stapes pushes sound energy in, and the round window, where it escapes. Superior semicircular canal dehiscence (SCD) creates a pathological third window — a gap in the bone of the arcuate eminence that exposes the membranous canal to the floor of the middle cranial fossa. Through this opening, acoustic and pressure energy that should drive the cochlea instead dissipates into the dehiscent canal.

This single anatomical defect explains a remarkably broad symptom cluster. Sound and pressure now move the cupula of the superior canal, generating vertigo and nystagmus; bone-conducted sound is heard abnormally loudly, producing autophony and even the perception of one’s own pulse, eye movements or footsteps. The condition was first defined by Minor and colleagues in 1998 [1998], and two decades of experience have refined both its recognition and its repair [2017].

FWhy SCD causes tinnitus

Pulse-synchronous tinnitus in SCD is a mechanical phenomenon. With the bony roof of the canal absent, the membranous labyrinth lies directly against the pulsating dura and the nearby superior petrosal sinus. Each arterial pulse transmits a small pressure wave into the perilymph, and because the third window makes the inner ear abnormally sensitive to bone-conducted and pressure stimuli, that pulsation is perceived as a rhythmic, heartbeat-locked sound.

The same low-impedance pathway amplifies internal body sounds — hence autophony, hearing the eyes move, and a roaring quality to chewing. Many patients describe the tinnitus as the least of their problems until it is explained that all the symptoms share one cause. Distinguishing this from [2013] ordinary subjective tinnitus matters because SCD tinnitus is potentially curable by closing the window.

TDiagnosis: CT, VEMP and the clinical triad

Diagnosis rests on the convergence of symptoms, physiology and imaging. High-resolution temporal-bone CT reformatted in the planes of the superior canal (Poschl and Stenver) is essential, but thin dehiscences can be overcalled on standard slices — volume averaging makes intact thin bone look absent. Imaging must therefore be correlated with function.

The most useful physiological test is the cervical and ocular vestibular-evoked myogenic potential (VEMP). The third window lowers the threshold and raises the amplitude of the response: cVEMP thresholds fall (often <70 dB) and ocular VEMP amplitudes rise. Audiometry classically shows a low-frequency air–bone gap with negative bone-conduction thresholds and preserved acoustic reflexes — a pattern that, unlike otosclerosis, points away from the middle ear [2017]. Only when imaging, VEMP and symptoms align should repair be considered.

CSurgical repair: plug or resurface

Repair closes the third window. Two corridors are used. The classic middle cranial fossa approach lifts the temporal lobe to expose the arcuate eminence from above, allowing the canal to be plugged (occluded with bone wax or fascia and bone) or resurfaced (covered without obliterating the lumen). Plugging gives the most durable symptom control but slightly raises the risk to canal function; resurfacing is more conservative but can fail if the cover shifts [2024].

The transmastoid approach reaches the canal from below without a craniotomy, plugging it on either side of the dehiscence, and avoids temporal-lobe retraction — an attractive option in older patients or when a low-lying tegmen complicates the middle-fossa route. Round-window reinforcement is a lesser alternative that dampens, rather than closes, the window. Across approaches, autophony and pulsatile tinnitus resolve in the large majority, with sound- and pressure-induced vertigo the most reliably abolished symptom.

COutcomes, risks and counselling

Symptom-specific outcomes differ. Vertigo and pressure sensitivity resolve in roughly 90–95% of operated patients; autophony and pulsatile tinnitus improve in around 80–90%; the low-frequency air–bone gap closes in most. Patients should be counselled that a transient post-operative imbalance is expected as the brain recalibrates to a now-occluded superior canal, usually settling over weeks.

Risks scale with the approach: the middle-fossa route carries small risks of CSF leak, temporary or rarely permanent sensorineural hearing loss, and temporal-lobe retraction effects; the transmastoid route reduces intracranial risk but still threatens hearing if the membranous labyrinth is breached. Because symptoms can be tolerable in many people, surgery is reserved for genuinely disabling disease with concordant imaging and VEMP — treating a CT finding alone is a recognised pitfall.