Hydrocephalus describes an excessive accumulation of cerebrospinal fluid (CSF) within the ventricular system of the brain. This is usually the result of impaired CSF flow or absorption.
Normal CSF Physiology
There are four ventricles in the brain:
- Two lateral ventricles
- A third ventricle
- A fourth ventricle
Each ventricle has a choroid plexus that produces cerebrospinal fluid, although the lateral ventricles produce the majority of CSF. It flows from the lateral ventricles to the third ventricle, then through the cerebral aqueduct to the fourth ventricle. It exits into the subarachnoid space around the brain and spinal cord and is absorbed into the venous system by the arachnoid granulations.
Congenital Causes
Aqueductal stenosis is the most common cause of hydrocephalus. The cerebral aqueduct connects the third and fourth ventricles. Stenosis (narrowing) blocks the normal flow of CSF out of the third ventricle, causing CSF to build up in the lateral and third ventricles.
Other congenital causes of hydrocephalus include:
- Arachnoid cysts blocking the outflow of CSF
- Chiari II malformations (where the cerebellum herniates downwards through the foramen magnum)
- Chromosomal abnormalities and congenital malformations obstructing CSF drainage
Presentation
The cranial bones are not fused at the sutures until around 2 years of age, allowing the skull to expand to fit the cranial contents. Hydrocephalus causes outward pressure on the cranial bones. Therefore, infants with hydrocephalus have an enlarged and rapidly increasing head circumference (occipitofrontal circumference).
Other signs of hydrocephalus include:
- Bulging anterior fontanelle
- Poor feeding and vomiting
- Poor tone
- Sleepiness
Ventriculoperitoneal (VP) Shunt
A ventriculoperitoneal shunt allows CSF to drain from the ventricles into the peritoneal cavity. The surgeon places a small tube (catheter) through a small hole in the skull into one of the ventricles. The catheter runs under the skin to the peritoneal cavity. A valve on the catheter regulates the amount of CSF that drains from the ventricles.
VP shunt complications include:
- Infection
- Blockage
- Malfunction or breakage
- Excessive drainage
Last updated January 2026
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