The human brain is surrounded by cerebro-spinal fluid contained in a covering sac, and designed to protect the brain from injury. Hydrocephaly describes a condition where an abnormal accumulation of cerebro-spinal fluid occurs inside or around the brain.

There is an imbalance of fluid formation inside the brain and absorption of the fluid into the blood sinuses inside the skull. As a result, the fluid is under increased pressure, causing it to push against the brain. Hydrocephalic may result from pre-natal malformations of the brain; it may be caused by trauma during or after birth or by infections. Advanced forms of hydrocephaly are immediately recognisable.