Superior canal dehiscence (SCD)

A third mobile window in the labyrinth. The horizontal canal is intact — and so is the rotational chair test. SCD is the most important example of an RCT-normal vestibular disease.

Clinical picture

Sound- and/or pressure-induced vertigo (the Tullio and Hennebert phenomena), autophony, hearing one's own footsteps and pulse, and chronic disequilibrium are the cardinal symptoms Minor LB 1998. Many patients also report a low-frequency air-bone gap on audiometry with intact stapedial reflexes — a useful clue away from otosclerosis.

Pathophysiology

A bony defect over the superior semicircular canal lets sound and pressure dissipate energy through the labyrinth as a third mobile window. Vibration of the dehiscent canal — not the horizontal canal — drives the vertigo and eye movements Sun DQ 2022.

RCT pattern

Because the horizontal canal is mechanically intact, SHA gain, phase and symmetry are within normal limits in isolated SCD. The step-test Tc is typically normal. This is the most common clinically obvious vestibular disease that produces an entirely unremarkable RCT — which is itself diagnostically useful, because it excludes a co-existing horizontal canal lesion.

Diagnosis & differential

• High-resolution CT of the temporal bone — the anatomical gold standard.
• Reduced cVEMP thresholds (often < 70 dB nHL) and increased oVEMP amplitudes.
• Audiometry showing low-frequency conductive loss with normal stapedial reflex.
• RCT to exclude an additional horizontal canal lesion.