By Dr Roger Bayston MMedSci MRCPath
ASBAH's Honorary Consultant in Hydrocephalus
Part 1 - Hydrocephalus: what it is and what causes it
The term 'hydrocephalus' is from two Greek words meaning 'water in the head'. In fact the 'water' is cerebrospinal fluid (CSF), a clear liquid which is produced all the time in the cavities or ventricles inside the brain. It passes from one ventricle to the next (four in all) through narrow pathways, then circulates around the surface of the brain - a little also goes down the spinal cord - and is absorbed back into the bloodstream. The absorption takes place through specialised veins inside the skull which have a sieve-like surface. Though much slower than the circulation of the blood, the CSF is constantly being produced, circulated and reabsorbed.
Hydrocephalus can result when either too much CSF is produced (very rare), or when it is prevented from circulating or being reabsorbed. As in these circumstances CSF is constantly produced but cannot get out, it accumulates and causes raised pressure inside the brain. The ventricles swell and the brain tissue is stretched and squashed. The skull bones in babies and young children are not fixed together as they are in later life, and the pressure causes the head to increase in size. However it is important to realise that hydrocephalus can also arise in older children and in adults. A later article will deal with the effects of hydrocephalus on those who have it.
A number of conditions can give rise to hydrocephalus. Uncommonly it can be due to a genetic disorder, in which case 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. About 20 years ago the commonest form of hydrocephalus was that associated with spina bifida. At least 80% of people with spina bifida have some degree of hydrocephalus though 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 be made to 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 absorption system leading to post-haemorrhagic hydrocephalus. Brain haemorrhages 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. 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 in which, as the name suggests, the CSF pressure is not raised and which is probably not hydrocephalus at all, are also seen in adults.
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.
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