Abnormal accumulation of cerebrospinal fluid within the ventricles of the brain. It is sometimes known as "water on the brain." Within the center of our brains each of us has two fluid-filled areas called cerebral ventricles. Cerebrospinal fluid is made within these ventricles and distributed over the surface of the brain and spinal cord. When the normal circulation of cerebrospinal fluid is interrupted, fluid can accumulate within the ventricles, which leads to increased pressure and enlargement of the ventricles, a condition called obstructive hydrocephalus. Abnormal re-absorption of the cerebrospinal fluid can also lead to fluid accumulation, a condition known as communicating hydrocephalus.
The accumulation of fluid puts pressure on the brain, forcing it against the skull and enlarging the ventricles. In infants, this fluid accumulation often results in bulging of the fontanelle (soft spot) and abnormally rapid head growth. The head enlarges because the bony plates that make up the skull have not yet fused together. In premature infants the ventricles can enlarge without the head getting bigger.
Hydrocephalus can be caused by birth defects in the brain development and may be associated with other congenital anomalies, such as spina bifida. In prematurely born infants the most common cause of the hydrocephalus is intraventricular hemorrhage.