The Anatomy of the Third Ventricle

The brain consists of elaborate, interconnected fluid-filled cavities called ventricles. The ventricular system is composed of two lateral ventricles, the third ventricle, the cerebral aqueduct, and the fourth ventricle. They are filled with cerebrospinal fluid (CSF) produced by the choroid plexus.

The third ventricle is a narrow funnel-shaped cavity of the brain that is located in the midline between the two hemispheres of the diencephalon of the forebrain. The third ventricle makes up the central part of the brain and facilitates communication between other ventricles.

Similar to the other brain ventricles, the main function of the third ventricle is to produce, secrete, and convey CSF. It also has several very important secondary roles, such as protection of the brain from trauma and injury and transport of nutrients and waste from the body’s central nervous system.

Anatomy

Structure

The third ventricle is a cuboid-shaped structure that has a roof, floor, and four walls—the anterior, posterior, and two lateral walls, respectively.

The roof is made up of the choroid plexus where CSF is produced by ependymal cells. The floor is made up of the hypothalamus, subthalamus, mammillary bodies, the infundibulum, and the tectum of the midbrain. 

Location

The third ventricle is a midline structure. It is found between the cerebral hemispheres.

It communicates directly with each lateral ventricle via the foramen of Monro and with the fourth ventricle via the aqueduct of Sylvius.

The third ventricle is situated between the right and the left thalamus. It has two protrusions on its top surface—the supra-optic recess (located above the optic chiasm) and the infundibular recess (located above the optic stalk).

Anatomical Variations

There are several variations of the third ventricle. The most common variations are:

• Masses: Deformities of the different segments of the floor can be caused by tumors of the posterior fossa and hydrocephalus.
• Longstanding hydrocephalus and increased intracranial pressure: The third ventricle is a common site for anatomical variations in people with congenital hydrocephalus, a hereditary condition that causes excess CSF production. Hydrocephalus enlarges the third ventricle.
• Infections: Some infections, such as neurocysticercosis (an infection of the central nervous system with the larval cysts of the pork tapeworm Taenia solium), preferentially attack the central nervous system, particularly the third ventricle.
• Congenital conditions: Genetic malformations such as congenital aqueductal stenosis can cause enlargement of the third ventricle.
• Chromosomal abnormalities: Infants with Down syndrome (trisomy 21) have been shown to have significant enlargement of the third ventricle, in both width and length, despite the smaller overall head circumference. The impact of the enlarged third ventricle in these infants is unknown.
• Inflammation of the brain: This includes meningitis and ventriculitis. 
• Stroke: The third ventricle can be affected by the bleeding in the brain that occurs when a person has a stroke.

Enlarged Ventricles in Infants

It is also common for infants to have mildly enlarged ventricles. Ventriculomegaly can be a benign finding, particularly in isolated cases. However, it can also be indicative of a more severe condition such as chromosomal abnormalities, congenital infection, cerebral vascular accident, or another fetal abnormality associated with developmental delay.

Colloid Cysts

Colloid cysts are benign, slow-growing lesions of the third ventricle. They can present as incidental findings on imaging or with symptoms of obstructive hydrocephalus.

The causes of colloid cysts are unknown, but they seem to run in families. Researchers have pointed to a potential association between familial colloid cysts and congenital inguinal hernia.

Schizophrenia

One study found that people with schizophrenia and their healthy siblings have larger third ventricles. The findings suggest that there could be related genetic defects that produce a susceptibility to the mental health condition.

Function

The third ventricle is the main site for CSF production. CSF has three main roles in the brain:

• Protection: CSF acts as a cushion for the brain, limiting neural damage in cranial injuries.
• Buoyancy: CSF allows structures to float in the brain. By being immersed in CSF, the net weight of the brain is reduced to approximately 25 grams, preventing excessive pressure on the brain.
• Chemical stability: The chemical environment of the brain and body likes to stay within a very narrow range. Whether pH or hormones, the body functions optimally when it is in homeostasis. CSF creates an environment that allows for the proper functioning of the brain—for example, by maintaining low extracellular potassium levels that help nerve cells optimally communicate with one another.  

Associated Conditions

Abnormalities of the third ventricle are associated with other medical conditions. Some of the most common conditions associated with the third ventricle are:

• Hydrocephalus: Hydrocephalus is a condition that leads to excessive buildup of CSF in and around the brain. In children, it can cause progressive enlargement of the head, potentially causing convulsions, tunnel vision, slowing of mental capacity, cognitive delay, headaches, neck pain suggesting tonsillar herniation, vomiting, blurred vision, double vision, difficulty in walking secondary to spasticity, drowsiness, and other forms of mental disabilities. In adults, hydrocephalus can also lead to an unstable gait, changes in personality, forgetfulness, changes in personality and behavior, and bladder control problems like urinary frequency and urgency.
• Meningitis: This is an inflammation of the membranes that cover the brain and spinal cord.
• Congenital malformations: Hereditary conditions can cause the third ventricles to become misshapen.
• Ventriculitis: This is inflammation of the ependymal lining of the cerebral ventricles, usually as a result of an infection such as meningitis or device-related trauma during brain surgery.

Tests

Ventriculomegaly can be detected through prenatal tests or after the baby is born. Tests include:

• Prenatal ultrasound
• Amniocentesis
• Magnetic resonance imaging (MRI)

In adults, if there is suspicion of a tumor, hydrocephalus, or congenital malformation, a doctor may use the following to help diagnose the condition:

• Physical examination
• Eye examination
• CT scan
• MRI scan
• Transcranial sonography
• Lumbar puncture or spinal tap: A thin needle is placed in the back and 50cc of CSF is removed to see if symptoms improve. CSF pressure is measured and the fluid is analyzed.

Mild hydrocephalus can be treated with a wait-and-see approach, but more severe cases may need treatment with medication or surgery, such as a ventriculoperitoneal shunt or an endoscopic third ventriculostomy.