Under-drainage, in which the fluid is not removed quickly enough and the symptoms of hydrocephalus return, is one of the commonest problems. It is usually due to blockage of the upper or lower tubes of the shunt tissue, though it can be due to the shunt breaking or its parts becoming disconnected from each other. It is rarely due to the valve itself, which usually continues to function in the same way for years. Pressure may sometimes build up rapidly, resulting in loss of consciousness, and treatment is required as an emergency. However, in most cases the onset is more gradual, and can follow a minor illness such as a cold.
Headaches increase in frequency and severity, often worse on waking in the morning.
Vomiting and dizziness also occur, and sometimes other symptoms which vary from patient to patient. In these cases the parents or carers will be able to recognise the symptoms from previous episodes. Specialist hospital staff are now fully aware of the various presentations of 'blocked shunt' but non-specialists and family doctors may not be. Shunt blockage can also have much more subtle consequences and the headaches may be infrequent, the main problem being behavioural deterioration. In older children this might take the form of increased irritability, 'laziness', poor or disruptive school performance or even more antisocial activity.