The causes of hydrocephalus (excess fluid in the brain) are poorly understood.

It's thought hydrocephalus present at birth (congenital hydrocephalus) may be the result of a brain defect restricting the flow of cerebrospinal fluid (CSF).

Hydrocephalus that develops in adults and children (acquired hydrocephalus) is often caused by an illness or injury that affects the brain.

Hydrocephalus that develops in older people (normal pressure hydrocephalus) may also be the result of an infection, illness or injury, but in many cases it's not clear what causes the condition.

Hydrocephalus present at birth (congenital hydrocephalus) can be caused by certain health conditions, such as spina bifida.

It can also develop 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:

• a mutation of the X chromosome – this is known as X-linked hydrocephalus
• rare genetic disorders – such as Dandy Walker malformation
• arachnoid cysts – fluid-filled sacs located between the brain or spinal cord and the arachnoid membrane

In many cases of congenital hydrocephalus the cause is unknown.

Hydrocephalus that develops in children and adults (acquired hydrocephalus) is usually the result of an injury or illness.

Possible causes of acquired hydrocephalus include:

• bleeding inside the brain – for example, if blood leaks over the surface of the brain (subarachnoid haemorrhage)
• blood clots in the brain (venous thrombosis)
• meningitis – an infection of the membranes surrounding the brain and spinal cord
• brain tumours
• head injury
• stroke

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.

Older people can sometimes develop normal pressure hydrocephalus (NPH) after a brain injury, bleeding in the brain or an infection. But it's often not clear why NPH happens.

It may be that NPH is linked to other underlying health conditions that affect the normal flow of blood – for example, diabetes, heart disease, or having a high level of cholesterol in the blood.