cerebral aqueduct

The cerebral aqueduct runs through the midbrain (mesencephalon), situated between the tectum (superior and inferior colliculi) dorsally and the tegmentum ventrally. It carries CSF produced primarily by the choroid plexus in the lateral and third ventricles onward to the fourth ventricle, from which CSF exits via the foramina of Luschka and Magendie into the subarachnoid space for reabsorption by arachnoid granulations. Because the aqueduct is the narrowest part of the ventricular system, it is highly susceptible to obstruction. Congenital aqueductal stenosis is the most common cause of congenital hydrocephalus, presenting with macrocephaly, bulging fontanelle, and split sutures in infants. Acquired obstruction can result from tumors (e.g., pinealoma, tectal glioma), hemorrhage, or inflammation (e.g., post-meningitis scarring), leading to noncommunicating hydrocephalus with dilation of the lateral and third ventricles but a normal-sized fourth ventricle. Clinically, obstruction presents with signs of increased intracranial pressure: headache, vomiting, papilledema, and in children, an enlarging head circumference. Parinaud syndrome (upward gaze palsy, light-near dissociation) can occur if a pineal region mass compresses the tectum and aqueduct. MRI is the imaging modality of choice to visualize the aqueduct and identify obstruction. Treatment often involves CSF diversion via ventriculoperitoneal shunt or endoscopic third ventriculostomy (ETV), which creates an alternate pathway bypassing the obstructed aqueduct.