Anatomy of the Ventricular System

The ventricular system is a communicating network of four fluid-filled cavities (ventricles) located within the brain parenchyma. It is lined by simple cuboidal to columnar ciliated epithelial cells called ependyma, filled with cerebrospinal fluid (CSF), and continuous with the central canal of the spinal cord and the subarachnoid space.

Anatomical Components

1. Lateral Ventricles (Left and Right)

• The largest cavities, located within the cerebral hemispheres.
• Each ventricle is C-shaped and consists of five main regions:
  • Anterior (Frontal) Horn: Projects forward into the frontal lobe, bounded medially by the septum pellucidum and laterally by the head of the caudate nucleus.
  • Body: Extends posteriorly through the parietal lobe under the corpus callosum.
  • Posterior (Occipital) Horn: Extends back into the occipital lobe.
  • Inferior (Temporal) Horn: Curves ventrally and anteriorly into the temporal lobe.
  • Atrium (Trigone): The junction connecting the body, posterior horn, and inferior horn; contains the glomus, a prominent tuft of choroid plexus.

2. Third Ventricle

• A narrow, vertical midline slit situated in the diencephalon between the left and right thalami and hypothalami.
• Often traversed by the interthalamic adhesion (massa intermedia), a bridge of grey matter connecting both thalami.
• Contains small recesses: optic, infundibular, suprapineal, and pineal recesses.

3. Fourth Ventricle

• A tent-shaped cavity located in the hindbrain (dorsal to the pons and upper medulla, ventral to the cerebellum).
• The floor is diamond-shaped (rhomboid fossa) and forms important cranial nerve landmarks (facial colliculus, hypoglossal and vagal trigones).
• The apex (fastigium) points dorsally toward the cerebellum.
• It tapers inferiorly into the obex, opening directly into the central canal of the spinal cord.

Connecting Passages and Apertures

• Interventricular Foramina (Foramina of Monro):
• Paired oval apertures connecting each lateral ventricle to the anterodorsal aspect of the third ventricle.

• Bordered by the fornix anteriorly and the thalamus posteriorly.

• Cerebral Aqueduct (Aqueduct of Sylvius):

• A narrow channel (approx. 1.5 mm wide, 15 mm long) passing through the dorsal midbrain (mesencephalon) beneath the tectum (superior and inferior colliculi).

• Connects the third ventricle directly to the fourth ventricle. Lacks choroid plexus. Due to its narrow lumen, it is the most common site of intraventricular obstruction.

• Outflow Apertures of the Fourth Ventricle:

• Median Aperture (Foramen of Magendie): A single midline aperture in the roof of the fourth ventricle emptying into the cisterna magna.
• Lateral Apertures (Foramina of Luschka): Paired lateral apertures at the lateral extremities of the fourth ventricle emptying into the pontocerebellar cisterna.
• These three apertures are the sole exit points through which CSF leaves the internal ventricular system to enter the subarachnoid space.

Choroid Plexus Distribution

The choroid plexus is a vascular fringe of pia mater covered by modified, secretory ependymal cells responsible for the production of CSF:

• Lateral Ventricles: Present along the floor of the body and the roof of the inferior horn, extending to the atrium; absent in the anterior and posterior horns.
• Third Ventricle: Located on the roof, continuous with that of the lateral ventricles through the foramina of Monro.
• Fourth Ventricle: Located on the inferior medullary velum (lower roof), projecting out through the foramina of Luschka (known as Bochdalek's flower basket).
• Cerebral Aqueduct: Contains no choroid plexus.

Clinical Pathology

• Non-Communicating (Obstructive) Hydrocephalus:
• Blockage within the ventricular channels preventing CSF from reaching the subarachnoid space.

• Common sites:

  • Foramen of Monro: Colloid cyst causing unilateral or bilateral lateral ventricle dilation.
  • Cerebral Aqueduct: Aqueductal stenosis (congenital, post-infectious, or tectal glioma) causing dilation of lateral and third ventricles with a normal fourth ventricle.
  • Fourth Ventricle Outlets: Dandy-Walker malformation or Chiari malformation.

• Ventriculomegaly:

• Enlargement of the ventricles, which may be secondary to increased pressure (hydrocephalus) or compensatory to brain volume loss / parenchymal atrophy (hydrocephalus ex vacuo, seen in Alzheimer's disease).

• Intraventricular Hemorrhage (IVH):

• Common in premature infants originating from the germinal matrix; in adults, typically caused by trauma, ruptured vascular malformations, or hypertensive intracerebral hemorrhages bursting into the ventricles.