Cerebral Aqueduct

The cerebral aqueduct, also known as the aqueduct of Sylvius, is a narrow, canal-like structure in the midbrain that connects the third ventricle superiorly with the fourth ventricle inferiorly. It serves as a critical conduit for cerebrospinal fluid (CSF) flow through the ventricular system. Due to its narrow diameter, it is a common site of obstruction, making it clinically significant in the development of non-communicating hydrocephalus.

Location

The cerebral aqueduct is located in the midbrain, running longitudinally through the tegmentum beneath the tectum (which contains the superior and inferior colliculi). It lies dorsal to the substantia nigra and red nucleus, and ventral to the periaqueductal gray matter that surrounds it.

Structure and Features

• Length: Approximately 15–17 mm
• Diameter: Very narrow, typically less than 1 mm
• Shape: Can vary between round, slit-like, or irregular; cross-sectional shape may change with CSF pulsations and pathology
• Lined by: Ependymal cells, similar to other ventricular surfaces

Function

The primary function of the cerebral aqueduct is to facilitate the flow of CSF from the third ventricle into the fourth ventricle. From there, CSF enters the subarachnoid space via the lateral and median apertures of the fourth ventricle. The aqueduct plays a key role in maintaining normal CSF circulation and intracranial pressure balance.

Relations

• Dorsally: Tectum (superior and inferior colliculi)
• Ventrally: Tegmentum of the midbrain (red nucleus, substantia nigra)
• Laterally: Surrounded by periaqueductal gray matter, which is involved in pain modulation and autonomic functions

Periaqueductal Gray

The gray matter surrounding the cerebral aqueduct is called the periaqueductal gray (PAG). It plays a crucial role in pain inhibition through descending pathways, autonomic regulation, and behavioral responses to stress. The PAG is also involved in vocalization, micturition control, and modulation of cardiovascular and respiratory functions.

Blood Supply

• Posterior cerebral artery (PCA): Provides branches that supply the midbrain, including the area surrounding the aqueduct
• Superior cerebellar artery: Also contributes to perfusion of the dorsal midbrain

Venous drainage occurs via small veins of the midbrain, which drain into the basal vein of Rosenthal and eventually into the deep venous system (internal cerebral veins → vein of Galen → straight sinus).

Development

The cerebral aqueduct develops from the cavity of the mesencephalon (midbrain) during embryogenesis. As the neural tube forms, the lumen narrows in the midbrain region, resulting in the aqueduct. Congenital narrowing or failure of canalization can lead to aqueductal stenosis and hydrocephalus.

Clinical Significance

• Aqueductal stenosis: A common cause of non-communicating (obstructive) hydrocephalus. May be congenital, due to gliosis, infection, hemorrhage, or tumors compressing the aqueduct. Symptoms include headache, vomiting, and signs of increased intracranial pressure.
• Hydrocephalus: Obstruction at the aqueduct prevents CSF flow from the third to fourth ventricle, causing ventricular dilation above the blockage. Managed by endoscopic third ventriculostomy or shunt placement.
• Aqueductal web or forking: Thin membranes or duplications within the aqueduct can obstruct CSF flow, often detected via advanced MRI sequences.
• Tumors: Pineal region tumors or midbrain gliomas may compress the aqueduct externally, leading to hydrocephalus.
• Infections or inflammation: Conditions like ependymitis or post-hemorrhagic gliosis can lead to aqueductal narrowing and scarring.

Imaging

MRI is the preferred imaging modality for visualizing the cerebral aqueduct. High-resolution T2-weighted sequences show its patency and surrounding structures. Cine phase-contrast MRI can assess CSF flow dynamics. Obstruction may be inferred from dilation of the third and lateral ventricles with a normal or collapsed fourth ventricle. CT scans may show hydrocephalus but are less specific for aqueductal detail.