Hydrocephalus, or "water on the brain," is the buildup of excess cerebrospinal fluid (CSF) within the brain. The pressure from the fluid buildup may lead to brain damage if the condition is not treated.

Normally, CSF flows through and out of chambers in the brain called ventricles, and then around the brain and spinal cord, providing nutrition and a protective cushion. Hydrocephalus is caused by an imbalance between the brain's production of CSF and the body's ability to distribute or absorb it properly.

Hydrocephalus is most often present at birth (congenital) and is usually noticeable within the first 9 months of life. Less often, hydrocephalus develops after a serious illness (such as meningitis) or a head injury.

Treatment usually includes draining the fluid spaces (ventricles) of the brain with a tube, called a shunt.

With treatment, babies with hydrocephalus may not have any long-term problems. Some may have only mild problems, such as learning problems. Hydrocephalus can be life-threatening or cause severe intellectual disability if it is not treated.