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Brain Education: Hydrocephalus

Cerebrospinal Fluid and the Ventricles

The brain and spinal cord are surrounded by a clear fluid called cerebrospinal fluid (CSF). This fluid is produced and stored in 4 cavities in the brain called ventricles, which are connected by narrow pathways. CSF is in constant production and absorption; it has a defined pathway from the lateral ventricles to the third ventricle and then into the 4th ventricle. Once the fluid is in the 4th ventricle, the CSF passes to the outside of the brain through three small holes near the base of the brain. It then circulates down and up the spinal column and over the outside of the brain. The purposes of the fluid are to cushion and protect the brain and spinal cord, to supply them with nutrients, and to remove some of their waste products. The CSF is then absorbed into the blood system where it is filtered and discarded by the body.

Hydrocephalus

Hydrocephalus is a condition in which there is too much CSF in the ventricles. Hydrocephalus occurs when there is an imbalance of production and/or absorption. The ventricles enlarge to accommodate the extra fluid and then press on different parts of the brain, causing a number of different symptoms.

Frequency

The overall incidence of hydrocephalus is unknown. When cases of spina bifida are included, congenital hydrocephalus occurs in 2–5 births per 1000 births. Incidence of acquired types of hydrocephalus is unknown.

Etiology

The etiology of hydrocephalus in congenital cases is unknown. Very few cases (<2%) are inherited (X-linked hydrocephalus). The most common causes of hydrocephalus in acquired cases are tumor obstruction, trauma, intracranial hemorrhage, and infection.

Types of Hydrocephalus

Hydrocephalus can be subdivided into the following 3 forms:

• Disorders of CSF production: This is the rarest form of hydrocephalus. Choroid plexus papillomas and choroid plexus carcinomas can secrete CSF in excess of its absorption.
• Disorders of CSF circulation: This form of hydrocephalus results from obstruction of the pathways of CSF circulation. This can occur at the ventricles or arachnoid villi. Tumors, hemorrhages, congenital malformations (such as aqueductal stenosis), and infections can cause obstruction at either point in the pathways.
• Disorders of CSF absorption: Conditions, such as the superior vena cava syndrome and sinus thrombosis, can interfere with CSF absorption.
• Some forms of hydrocephalus cannot be classified clearly. This group includes normal pressure hydrocephalus and pseudotumor cerebri.