The semicircular ducts are three endolymph-filled membranous tubes lying within the bony semicircular canals: anterior (superior), posterior and lateral (horizontal). They sit at roughly right angles to each other. Each has a swelling at one end, the ampulla, that contains its sensory receptor.
| Location | Within the bony semicircular canals, posterior and superior to the vestibule |
|---|---|
| Parts |
|
| Nerve Supply | Superior vestibular nerve (anterior and lateral ducts); Inferior vestibular nerve via the posterior ampullary nerve (posterior duct) |
| Blood Supply | Anterior vestibular artery; Posterior vestibular artery |
Detect rotational (angular) movements of the head in three planes.
The ducts on the two sides work in pairs in the same planes, so each turn of the head excites one side and inhibits the other. This drives the vestibulo-ocular reflex that keeps vision steady. The head impulse test checks the lateral duct, and loose otoconia in any duct produce positional vertigo with a nystagmus pattern that reveals which duct is involved.
Otoconia in the lateral semicircular duct cause vertigo when turning in bed, with horizontal nystagmus. It is treated with roll manoeuvres.
Loss of semicircular duct function on one side causes vertigo and a positive head impulse test until the brain compensates.
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