Overview

Length≈1.5–2 cm

Diameter≈1–2 mm

The aqueduct is ≈1.5–2 cm long and runs through the midbrain between the tectum behind and the tegmentum in front. It is surrounded by the periaqueductal grey, and the oculomotor and trochlear nuclei lie just in front of it. It contains no choroid plexus.

Because the aqueduct is so narrow, small lesions can obstruct it:
• congenital stenosis or forking;
• gliosis after intraventricular haemorrhage or infection;
• compression by tumours of the tectal plate or pineal region.
When it is blocked, CSF made by the lateral and third ventricles cannot escape. The result is triventricular hydrocephalus: dilated lateral and third ventricles with a normal-sized fourth ventricle.

Location: Midbrain, between the tectum (dorsally) and tegmentum (ventrally); connects the posterior third ventricle (below the posterior commissure) with the upper fourth ventricle.

Anatomy

Relations: Surrounded by the periaqueductal grey; superior and inferior colliculi dorsally; CN III and IV nuclei and the MLF ventrally.

Function

Conducts CSF from the third to the fourth ventricle

Clinical significance

Aqueductal stenosis

The commonest cause of congenital hydrocephalus. One form is X-linked (L1CAM mutation), with adducted thumbs and intellectual disability. Acquired causes include post-haemorrhagic or post-infective gliosis, tectal plate gliomas and pineal-region tumours. Treatment is endoscopic third ventriculostomy or shunting.

Parinaud syndrome with hydrocephalus

Pineal-region masses compress both the dorsal midbrain (upgaze palsy, light–near dissociation) and the aqueduct (hydrocephalus).

Test yourself

CSF passes from the third ventricle to the fourth ventricle through the:

Interventricular foramen (of Monro)
Central canal
Median aperture (of Magendie)
Cerebral aqueduct (of Sylvius)

Answer: Cerebral aqueduct (of Sylvius)

The pathway is lateral ventricles → interventricular foramina → third ventricle → cerebral aqueduct (in the midbrain) → fourth ventricle → median and lateral apertures → subarachnoid space. The aqueduct is the narrowest point and the commonest site of congenital obstruction.