Disease 05.1

Superior Canal Dehiscence

The textbook indication for VEMP testing — a third-window pathology whose imaging finding is confirmed functionally by lowered thresholds and augmented amplitudes.

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Foundation

What is SCD?

Superior semicircular canal dehiscence is the absence of bone over the top of the superior semicircular canal — usually a paper-thin region of the petrous temporal bone that has eroded or failed to form. The defect creates a "third window" in the inner ear, in addition to the oval and round windows, and changes how sound and pressure travel through it.

Patients can experience vertigo triggered by loud sound (the Tullio phenomenon), hear their own internal sounds unnaturally loudly (autophony), or notice that their eyes move when they hum.

AudiogramThe pure-tone signature

A low-frequency air-bone gap is classic — but the tympanogram is normal because the 'gap' reflects the third window, not middle-ear disease. Bone-conduction thresholds may sit at or below 0 dB HL (a 'supra-normal' bone line). Pair this with an augmented oVEMP to clinch the diagnosis.

Affected ear shows markedly augmented cVEMP amplitude with normal latencies. Thresholds are also lowered (not visible on amplitude alone).

Trainee

VEMP signature of SCD

Lowered cVEMP threshold on the affected side — often below 70 dBnHL, against a normal threshold of 80–90 dBnHL.
Increased cVEMP amplitude on the affected side.
Increased oVEMP amplitude, often more conspicuous than cVEMP.
Preserved high-frequency oVEMP (2–4 kHz) — perhaps the most discriminating single finding, because normal ears no longer respond at those frequencies.^[4]

Clinician

Mechanism — the third window

Normally the inner ear has two windows — the oval window (where the stapes articulates) and the round window. A third window short-circuits acoustic energy: more of it now travels through the dehiscence and stimulates the otolith maculae, while bone-conducted vibration is similarly amplified to the vestibular receptors. The result is that subthreshold stimuli now evoke responses, and suprathreshold stimuli evoke larger ones.

The threshold finding, step by step

The defining VEMP feature of SCD is not the augmented amplitude alone but the lowered threshold. Step the stimulus intensity down in 5 dB increments and watch when each ear stops responding.

95 dBnHL

Step 1 of 7

Suprathreshold — both ears respond robustly.

Welgampola and colleagues showed that VEMP thresholds normalise after surgical plugging of the dehiscence.^[9] That observation effectively closed the circle on causality — the abnormal VEMPs are a downstream consequence of the bone defect, not a coincidental finding.

Reading the report

When the clinical picture suggests SCD (autophony, sound- or pressure-induced vertigo, conductive hyperacusis), a high-resolution CT shows the bone defect and the VEMP confirms it functionally. A patient with imaging "near-dehiscence" but normal VEMPs has not been demonstrated to have functional third-window physiology — useful when counselling about surgery.^[4,9]