The disorder
The third window
Everything about SSCD follows from one idea: a hole that should not be there. Grasp the third-window mechanism and the whole symptom list becomes predictable.
Two windows, then three
The inner ear normally has two flexible windows that let sound waves move through it. In SSCD a third opening appears where bone is missing. Sound and pressure now escape through that opening — stirring the balance organ (dizziness) and stealing energy from hearing.
Normally the stapes drives fluid in at the oval window and the round window relieves it — two windows. A dehiscence adds a third window at the top of the superior canal, a low-impedance path that diverts acoustic and pressure energy.1 Toggle the model:
The third window splits the consequences neatly. Mechanically, sound and pressure deflect the cupula of the superior canal — sound/pressure-induced vertigo with eye movements in that canal’s plane. Acoustically, energy shunted from the cochlea lowers air-conduction thresholds (and the third window enhances bone conduction) — producing the low-frequency air–bone gap and bone-conduction hyperacusis.1
One mechanism, two symptom clusters
The same leak produces a vestibular cluster (sound/pressure vertigo, oscillopsia, chronic imbalance) and an auditory cluster (autophony, bone-conduction hyperacusis, pulsatile tinnitus, the pseudo-conductive gap). Holding the third window in mind lets you predict which tests will be abnormal — explored in Diagnosis & tests. For the normal canal and otolith physiology this builds on, see Physiology and Anatomy.
Why the bone is missing
A temporal-bone survey found frank dehiscence in ~0.5% and critically thin bone in ~1.4%, suggesting a developmental failure of the bone to thicken after birth.2 Most authorities favour a two-hit model: a congenitally thin roof that becomes symptomatic only after a second event — head trauma, a rise in intracranial pressure, or erosion against the middle-fossa dura or superior petrosal sinus.3 The same mechanism over other sites gives the related third-window lesions.
Key points
- SSCD adds a third window to the normal oval and round windows.
- Shunted energy stimulates the superior canal (vertigo) and diverts sound from the cochlea (air–bone gap).
- Enhanced bone conduction explains autophony and bone-conduction hyperacusis.
- A congenitally thin roof plus a second hit (trauma, raised ICP) is the favoured aetiology.