The causes of Hydrocephalus

A number of conditions can give rise to hydrocephalus. Uncommonly it can be due to a genetic disorder, in which case some other members of the family are usually affected. Non-genetic causes are more common. In babies who are born with hydrocephalus the condition is said to be congenital, and it must be realised that this means simply that it is present at birth, and not that it is hereditary. In congenital hydrocephalus the actual cause is usually impossible to determine but it is assumed to be due to events during the baby's development before birth, such as damage to local blood supply or infection. A known cause of congenital hydrocephalus, uncommon in this country, is toxoplasmosis, which is caught from animals or from eating undercooked meat. Most of us have had this infection and are now immune but a woman who becomes pregnant and who is not immune might catch the infection. The baby is then at risk of catching the disease in the womb and developing hydrocephalus and eye problems.

About 25 years ago the commonest form of hydrocephalus was that associated with spina bifida. At least 80-90% of people with spina bifida have some degree of hydrocephalus, although it is not always in need of treatment. Here the interference with CSF flow is due to abnormalities of the brain structure at the back of the head which develop at an early stage of the pregnancy. This is often called the Arnold-Chiari malformation.

In recent years far fewer babies have been born with spina bifida, and the largest number of cases of hydrocephalus in infancy occur in babies born prematurely, due sometimes to maternal ill-health or smoking. Even very small babies can now survive, but their vital functions, normally taken care of during pregnancy by the mother, have to be controlled artificially. Unavoidable rapid changes in blood pressure can cause bleeding in the brain, and the blood from this haemorrhage blocks the sieve-like CSF absorption system leading to post-haemorrhagic hydrocephalus. Brain haemorrhage in adults (stroke) is similar and can also lead to hydrocephalus in survivors. Head injury sometimes has the same effect.

Another way in which the CSF pathways can be blocked is by debris and pus in acute infections of the membranes surrounding the brain (meningitis). This can arise in previously healthy babies if they are unlucky enough to develop a serious bloodstream infection soon after birth, but it can happen to anyone in any age group. Fortunately, meningitis is not common and one major cause, haemophilus, has been greatly reduced by the introduction of the HIB vaccine. Vaccine is available for another type of meningitis, meningococcus C (but not yet for type B). College and university students are now offered type C vaccine. Hydrocephalus is not a reason to refuse vaccination, but if in doubt consult your doctor.

Other types of hydrocephalus can be due to obstruction by cysts in the ventricles. Dandy-Walker cysts, which block the fourth ventricle at the base of the brain are an example of these swellings. Brain tumours can obstruct the CSF flow, either by their bulk or by causing brain swelling. In adults, blockage of one of the connecting pathways between the ventricles (aqueduct stenosis) can cause the onset of hydrocephalus.

Two other conditions, benign intracranial hypertension (not very benign for those who have it) in which the CSF pressure is raised, and normal pressure hydrocephalus (NPH) in which, as the name suggests, the CSF pressure is not raised, are also seen in adults. NPH usually affects those in the third part of life, and can give rise to memory loss, walking difficulties and urinary incontinence.

Finally, hydrocephalus is often classified as either communicating or non-communicating. In the first type, the problem is usually failure to absorb the CSF at the end of the system, whereas in the second there is blockage of the CSF pathways within the ventricular system.