Fourth Ventricle

The fourth ventricle is a diamond-shaped cavity in the hindbrain that forms part of the ventricular system of the brain. It is filled with cerebrospinal fluid (CSF) and lies between the brainstem anteriorly (pons and medulla) and the cerebellum posteriorly. The fourth ventricle connects the cerebral aqueduct superiorly to the central canal of the spinal cord inferiorly and also communicates with the subarachnoid space through three apertures, allowing CSF to flow around the brain and spinal cord.

Location

The fourth ventricle lies in the posterior part of the brainstem, in the floor of the rhomboid fossa, between the pons and medulla (ventrally) and the cerebellum (dorsally). It spans the entire dorsal surface of the pons and upper medulla and is enclosed by the cerebellar peduncles laterally.

Boundaries

• Floor (rhomboid fossa): Formed by the posterior surfaces of the pons and medulla; contains cranial nerve nuclei and several surface landmarks.
• Roof: Formed by the superior and inferior medullary vela and the cerebellum.
• Superior boundary: Continuous with the cerebral aqueduct (of Sylvius) from the midbrain.
• Inferior boundary: Narrows to form the central canal of the spinal cord at the level of the obex.

Recesses and Apertures

• Lateral recesses: Extend laterally toward the cerebellopontine angle; each terminates at the lateral aperture (foramen of Luschka).
• Median aperture (foramen of Magendie): Located in the inferior part of the roof; allows CSF to exit into the cisterna magna of the subarachnoid space.
• Obex: The caudal tip of the fourth ventricle where it becomes the central canal; an important landmark on the dorsal medulla.

Communication with Other Structures

• Superiorly: Communicates with the third ventricle via the cerebral aqueduct
• Inferiorly: Continues into the central canal of the spinal cord
• Laterally and medially: Opens into the subarachnoid space through the foramina of Luschka (lateral) and Magendie (median)

Choroid Plexus

The fourth ventricle contains a rich choroid plexus, primarily located in its inferior portion. The choroid plexus lies along the roof and secretes CSF into the ventricular cavity. This CSF passes through the foramina to reach the subarachnoid space and bathe the brain and spinal cord.

Floor (Rhomboid Fossa) Features

The floor of the fourth ventricle contains several surface landmarks and cranial nerve nuclei:

• Median sulcus: Divides the floor into symmetrical halves.
• Facial colliculus: Formed by the facial nerve looping over the abducens nucleus.
• Hypoglossal and vagal trigones: Elevations overlying their respective cranial nerve nuclei.
• Locus coeruleus: A bluish area associated with norepinephrine production; involved in arousal and stress response.
• Vestibular area: Lateral region associated with vestibular nuclei.

Relations to Cerebellum

• Superior cerebellar peduncles: Form the lateral boundaries of the superior roof.
• Inferior medullary velum: A thin layer forming the lower part of the roof; helps cover the ventricle posteriorly.
• Flocculonodular lobe: Lies near the lateral recesses and is involved in balance and vestibular function.

Blood Supply

• Posterior inferior cerebellar artery (PICA): Supplies the inferior parts of the fourth ventricle and choroid plexus.
• Anterior inferior cerebellar artery (AICA): Contributes to the lateral recess region.
• Superior cerebellar artery: Supplies the roof and surrounding cerebellar structures.

Venous drainage occurs via the superior and inferior cerebellar veins into the transverse, straight, and sigmoid sinuses.

Development

The fourth ventricle develops from the cavity of the rhombencephalon (hindbrain), which gives rise to the metencephalon (pons and cerebellum) and myelencephalon (medulla). The roof of the neural tube thins to form the membranous roof of the ventricle, while the alar and basal plates contribute to the formation of its floor and walls.

Clinical Significance

• Hydrocephalus: Obstruction at the level of the fourth ventricle or its apertures (Luschka or Magendie) can cause dilation of the entire ventricular system above, leading to non-communicating hydrocephalus.
• Dandy–Walker malformation: A congenital condition involving cystic dilation of the fourth ventricle, hypoplasia of the cerebellar vermis, and enlargement of the posterior fossa.
• Tumors: Ependymomas, medulloblastomas, or astrocytomas may arise in or compress the fourth ventricle, causing obstructive hydrocephalus and brainstem compression.
• Cerebellopontine angle syndrome: Tumors like vestibular schwannomas may compress the lateral recess and adjacent cranial nerves.
• Fourth ventriculostomy: A surgical approach used to access lesions or relieve pressure, though less common than third ventriculostomy.

Imaging

MRI is the modality of choice to visualize the fourth ventricle, its shape, and its communications. Obstruction, compression, or expansion of the ventricle can be evaluated in axial and sagittal views. CT scans may show dilation or mass effect in acute settings. Ventriculography and CSF flow studies may be used for surgical planning or hydrocephalus evaluation.