Disorders · Introduction
Superior semicircular canal dehiscence
A pinhole of missing bone over the superior canal turns the inner ear into a leaky instrument — the patient grows dizzy at loud sounds and hears their own eyes move. A third window, hiding in plain sight.
What SSCD is
In SSCD a small piece of bone covering one of the balance canals is missing. That opening lets sound and pressure reach the balance organ in a way they normally cannot — so loud noises or straining cause dizziness, and the patient may hear their own voice, footsteps or pulse unusually loudly.
SSCD is the prototypical third-window syndrome: a dehiscence of the bone over the superior semicircular canal lets acoustic and pressure energy be shunted abnormally, producing sound- and pressure-induced vertigo (Tullio and Hennebert), autophony and bone-conduction hyperacusis, with a low-frequency air–bone gap.1
Two ideas anchor the chapter. First, the third window explains every symptom and every test in one mechanism. Second, the diagnosis is one of concordance: radiographic dehiscence is common and over-called, so symptoms, physiology and imaging must agree before the label — and certainly before surgery.3
By the numbers
A dehiscence is found in roughly 0.5% of temporal bones, with critically thin bone in more — a developmental substrate that usually needs a second hit (trauma or raised pressure) to become symptomatic.2
Temporal-bone survey
A dehiscence is found in ~1 in 200 temporal bones; the symptomatic syndrome is rarer still — which is why a scan alone cannot make the diagnosis.
How this chapter is organised
- The third window — one mechanism that explains all the symptoms and signs.
- Clinical features — the auditory and vestibular clusters and the bedside provocations.
- Diagnosis & tests — the air–bone gap with reflexes, VEMP, CT, and an interactive confidence checker.
- Differential diagnosis — above all, telling it from otosclerosis.
- Treatment & surgery — conservative care, and plugging/resurfacing for disabling disease.
Key points
- A bony defect over the superior canal creates a third window into the inner ear.
- Sound/pressure-induced vertigo (Tullio, Hennebert), autophony and bone-conduction hyperacusis are the hallmarks.
- It causes a low-frequency air–bone gap with PRESERVED reflexes and supranormal bone conduction — not otosclerosis.
- VEMP shows low cervical thresholds / high ocular amplitudes; CT (Pöschl/Stenvers) confirms the defect.
- Diagnosis needs concordant symptoms, physiology and imaging — never a scan alone.