Head injury sometimes has the same effect. 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.
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.