= focal reduction in size of aqueduct at level of superior colliculi / intercollicular sulcus (normal range, 0.2–1.8 mm²)

Embryology:

• aqueduct develops at 6^th week GA + decreases in size until birth ← growth pressure from adjacent mesencephalic structures

Incidence: 0.5–1÷1,000 births; most frequent cause of congenital hydrocephalus (20–43%); 1–4.5% recurrence rate in siblings; M÷F = 2÷1

Manifestation: any time from fetal age to adulthood; age at presentation depends on severity of stenosis and hydrocephalus

Etiology:

1. postinflammatory (50%): perinatal infection (toxoplasmosis, CMV, syphilis, mumps, influenza virus) OR intracranial hemorrhage → destruction of ependymal lining of aqueduct → marked adjacent fibrillary gliosis
2. developmental: aqueductal forking (= marked branching of aqueduct into channels) / narrowing / transverse septum (X-linked recessive inheritance in 25% of males)
3. neoplastic (extremely rare): pinealoma, meningioma, tectal astrocytoma (may be missed on routine CT scans, easily visualized by MR)

May be associated with: other congenital anomalies (16%): thumb deformities

Location: most often proximal aqueduct as congenital / acquired (= postinflammatory aqueductal gliosis) stenosis

• enlargement of lateral + 3^rd ventricles + normal-sized sulci and 4^th ventricle (4^th ventricle may be normal with communicating hydrocephalus)
  • Adult hydrocephalus is in 10% due to aqueductal stenosis!
• rounded anterior recesses extending into suprasellar cistern
• 3^rd ventricular floor displaced inferiorly into prepontine cistern

Prognosis: 11–30% mortality

Rx: 3^rd ventriculostomy = creating a perforation in tuber cinereum → communication with prepontine cistern

N.B.: Alert surgeon prior to ventriculostomy of an anomalous floor contour of 3_rd ventricle (long-standing hydrocephalus displaces floor downward)