The causes of hydrocephalus are poorly understood.
It's thought hydrocephalus present at birth may be the result of a brain defect restricting the flow of cerebrospinal fluid (CSF).
Hydrocephalus that develops in adults and children is often caused by an illness or injury that affects the brain.
Normal pressure hydrocephalus (NPH) that develops in older people may also be the result of an infection, illness or injury, but in many cases it's not clear what causes the condition.
Congenital hydrocephalus can also occur in babies born prematurely, before week 37 of the pregnancy.
Some premature babies have bleeding in the brain, which can block the flow of CSF and cause hydrocephalus.
Other possible causes of congenital hydrocephalus include:
In many cases of congenital hydrocephalus, the cause is unknown.
Hydrocephalus that develops in adults or children (acquired hydrocephalus) is usually the result of an injury or illness.
Possible causes of acquired hydrocephalus include:
Some people are born with narrowed passageways in their brain that restrict the flow of cerebrospinal fluid, but do not cause any symptoms until years later.
Hydrocephalus that develops in older people (normal pressure hydrocephalus, or NPH) can occur after a brain injury, bleeding in the brain or an infection. However, in most cases, there's no clear reason.