Classifications and Causes

Hydrocephalus is a condition, not a disease. It can develop for a variety of reasons, sometimes as part of another condition.

Congenital hydrocephalus means the condition is present at birth, caused by a complex interaction of genetic and environmental factors during fetal development. Congenital hydrocephalus is now often diagnosed before birth through routine ultrasound.

Compensated hydrocephalus is hydrocephalus that is diagnosed in adulthood, but may have existed since birth. It can still be considered congenital.

Acquired hydrocephalus develops after birth as a result of neurological conditions such as head trauma, brain tumor, cyst, intraventricular hemorrhage or infection of the central nervous system.

Normal pressure hydrocephalus occurs in older adults when the ventricles of the brain are enlarged, but there is little or no increase in the pressure within the ventricles. Sometimes the cause of NPH is known – but most often it is idiopathic, which means the cause is not known.

Another set of terms you may hear from a doctor to describe hydrocephalus are “communicating” and “non-communicating.” If the cerebrospinal fluid (CSF) flows freely throughout the brain’s ventricular system and the subarachnoid space, it is referred to as communicating hydrocephalus. If there is a physical blockage such as a tumor somewhere in that system, it is called non-communicating hydrocephalus.

Common Causes of Congenital Hydrocephalus

Aqueductal stenosis – The most common cause of congenital hydrocephalus is an obstruction called aqueductal stenosis. This occurs when the long, narrow passageway between the third and fourth ventricles (the aqueduct of Sylvius) is narrowed or blocked, perhaps because of infection, hemorrhage, or a tumor. Fluid accumulates “upstream” from the obstruction, producing hydrocephalus.

Neural tube defect – Another common cause of hydrocephalus is a neural tube defect (NTD). An open NTD, where the spinal cord is exposed at birth and is often leaking CSF, is called a myelomeningocele, and is often referred to as spina bifida. This kind of NTD usually leads to the Chiari II malformation, which causes part of the cerebellum and the fourth ventricle to push downward through the opening at the base of the skull into the spinal cord area, blocking CSF flow out of the fourth ventricle and causing hydrocephalus. Please see the National Institute of Neurological Disorders and Stroke (NINDS)  for more information on neural tube defects.

Arachnoid cysts – Congenital hydrocephalus can also be caused by arachnoid cysts, which may occur anywhere in the brain. In children, they’re often located at the back of the brain (posterior fossa) and in the area of the third ventricle. These cysts are filled with CSF and lined with the arachnoid membrane, one of the three meningeal coverings. Some arachnoid cysts are self-contained, while others are connected with the ventricles or the subarachnoid space. The fluid trapped by the cysts may block the CSF pathways, causing hydrocephalus.

Dandy-Walker syndrome – In Dandy-Walker syndrome, another cause of congenital hydrocephalus, the fourth ventricle becomes enlarged because its outlets are partly or completely closed and part of the cerebellum fails to develop. Dandy-Walker syndrome may also be associated with abnormal development in other parts of the brain and sometimes leads to aqueductal stenosis. In some instances, two shunts are placed in the child’s ventricles — one in the lateral ventricle and another in the fourth ventricle to manage the hydrocephalus. Please see the National Institute of Neurological Disorders and Stroke (NINDS) for more information on Dandy-Walker syndrome.

Chiari malformation – There are two types of Chiari malformation. Both types occur in the bottom of the brain stem where the brain and spinal cord join. The lowest portion of the brain is displaced and is lower than normal pushing down into the spinal column. Please see the National Institute of Neurological Disorders and Stroke (NINDS) for more information on Chiari malformation.

Common Causes of Acquired Hydrocephalus

Intraventricular hemorrhage – An intraventricular hemorrhage, which most frequently affects premature newborns, may cause an acquired form of hydrocephalus. When small blood vessels alongside the ventricular lining rupture, blood may block or scar the ventricles or plug the arachnoid villi, which allow CSF to be absorbed. When the CSF can’t be absorbed, hydrocephalus results.

Meningitis – Meningitis is an inflammation of the membranes of the brain and spinal cord. Caused by a bacterial or (less frequently) viral infection, meningitis can scar the delicate membranes called meninges that line the CSF pathway. An acquired form of hydrocephalus may develop if this scarring obstructs the flow of CSF as it passes through the narrow ventricles or over the surfaces of the brain in the subarachnoid space.

Head injury – A head injury can damage the brain’s tissues, nerves, or blood vessels. Blood from ruptured vessels may enter the CSF pathway, causing inflammation. Sites of CSF absorption might then be blocked by scarred membranes – meninges – or by blood cells. The CSF flow is restricted, and hydrocephalus develops.

Brain tumors – In children, brain tumors most commonly occur in the back of the brain which is referred to as the posterior fossa. As a tumor grows, it may fill or compress the fourth ventricle, blocking the flow of CSF and causing hydrocephalus. A tumor somewhere else in the brain might also block or compress the ventricular system.  Please see the National Institute of Neurological Disorders and Stroke (NINDS) for more information on brain tumors.

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