Types and Causes of Hydrocephalus

Types and Causes of Hydrocephalus

What is hydrocephalus?

The term hydrocephalus is derived from the Greek words "hydro" meaning water and "cephalus" meaning head. As its name implies, it is a condition in which the primary characteristic is excessive accumulation of fluid in the brain. Although hydrocephalus was once known as "water on the brain," the "water" is actually cerebrospinal fluid (CSF) - a clear fluid surrounding the brain and spinal cord. The excessive accumulation of CSF results in an abnormal dilation of the spaces in the brain called ventricles. This dilation causes potentially harmful pressure on the tissues of the brain.

The ventricular system is made up of four ventricles connected by narrow pathways. Normally, CSF flows through the ventricles, exits into cisterns (closed spaces that serve as reservoirs) at the base of the brain, bathes the surfaces of the brain and spinal cord, and then is absorbed into the bloodstream.

CSF has three important life-sustaining functions: 1) to keep the brain tissue buoyant, acting as a cushion or "shock absorber"; 2) to act as the vehicle for delivering nutrients to the brain and removing waste; and 3) to flow between the cranium and spine to compensate for changes in intracranial blood volume (the amount of blood within the brain).

The balance between production and absorption of CSF is critically important. Ideally, the fluid is almost completely absorbed into the bloodstream as it circulates; however, there are circumstances which, when present, will prevent or disturb the production or absorption of CSF, or which will inhibit its normal flow. When this balance is disturbed, hydrocephalus is the result.

What are the different types of hydrocephalus?

Hydrocephalus may be congenital or acquired. Congenital hydrocephalus is present at birth, and may be caused by either environmental influences during fetal development or genetic predisposition. Acquired hydrocephalus develops at the time of birth or at some point afterward. This type of hydrocephalus can affect individuals of all ages and may be caused by injury or disease.

Hydrocephalus may also be communicating or non-communicating. Communicating hydrocephalus occurs when the flow of CSF is blocked after it exits from the ventricles. This form is called communicating because the CSF can still flow between the ventricles, which remain open. Non-communicating hydrocephalus - also called "obstructive" hydrocephalus - occurs when the flow of CSF is blocked along one or more of the narrow pathways connecting the ventricles. One of the most common causes of hydrocephalus is "aqueductal stenosis." In this case, hydrocephalus results from a narrowing of the aqueduct of Sylvius, a small passageway between the third and fourth ventricles in the middle of the brain. There are two other forms of hydrocephalus which do not fit distinctly into the categories mentioned above and primarily affect adults: hydrocephalus ex-vacuo and normal pressure hydrocephalus.

Hydrocephalus ex-vacuo occurs when there is damage to the brain caused by stroke or traumatic injury. In these cases, there may be actual shrinkage (atrophy or wasting) of brain tissue. Normal pressure hydrocephalus commonly occurs in the elderly and is characterized by many of the same symptoms associated with other conditions that occur more often in the elderly, such as memory loss, dementia, gait disorder, urinary incontinence, and a general slowing of activity.

Who gets this disorder?

Incidence and prevalence data are difficult to establish as there is no existing national registry or database of people with hydrocephalus and closely associated disorders; however, hydrocephalus is believed to affect approximately 1 in every 500 children. At present, most of these cases are diagnosed prenatally, at the time of delivery, or in early childhood. Advances in diagnostic imaging technology allow more accurate diagnoses in individuals with atypical presentations, including adults with conditions such as normal pressure hydrocephalus.

What causes hydrocephalus?

The causes of hydrocephalus are not all well understood. Hydrocephalus may result from genetic inheritance (aqueductal stenosis) or developmental disorders such as those associated with neural tube defects including spina bifida and encephalocele. Other possible causes include complications of premature birth such as intraventricular hemorrhage, diseases such as meningitis, tumors, traumatic head injury, or subarachnoid hemorrhage blocking the exit from the ventricles to the cisterns and eliminating the cisterns themselves.