accumulation of cerebrospinal fluid in the ventricles, or cavities, of the brain, causing progressive enlargement of the head. Normally, cerebrospinal fluid continuously circulates through the brain and the spinal cord and is continuously drained into the circulatory system. In hydrocephalus the fluid accumulates in the two large lateral ventricles, and the brain and skull become enlarged because of the accumulation of fluid. The condition may result from an overproduction of the fluid, from a congenital malformation blocking normal drainage of the fluid, or from complications of head injuries, strokes (stroke), or infections.

Hydrocephalus occurs in one or two out of every 1,000 live births. Infants and young children with hydrocephalus have abnormally large heads because the pressure of the cerebrospinal fluid has caused the individual skull bones—which have not fused with each other yet—to bulge outward at their juncture points. Compression of the brain by the accumulating fluid eventually causes convulsions (convulsion) and mental retardation (intellectual disability). Pressure within the brain can be reduced by surgical insertion of a permanent tube called a shunt that drains accumulated cerebrospinal fluid into another area of the body where it can be absorbed.