This is also sometimes called ‘water on the brain.’ It occurs when extra fluid builds up in the ventricles – the inner chambers of the brain. It is often caused by a block in one of the chambers of the brain that does not allow the fluid to drain. When fluid builds up, it causes the ventricles and the skull to expand. It can be present at birth or can happen later in life. If it is not treated, it can cause learning disabilities or intellectual disabilities.