Hydrocephalus is an increase in volume of cerebrospinal fluid (CSF) in the ventricles of the brain. It is a condition that exerts dangerous pressure on brain tissue. It can be due either to obstruction, preventing the CSF from draining away, or to a failure of natural reabsorption. In babies it is congenital, often associated with spina bifida, and causes a characteristic enlargement of the head; in adults, it may arise from brain injury or disease. Hydrocephalus is treated by insertion of a shunting system to drain the CSF harmlessly into the abdominal cavity.