Causes of hydrocephalus 

The causes of hydrocephalus are poorly understood.

It's thought that congenital hydrocephalus may be the result of a brain defect restricting the flow of cerebrospinal fluid.

Acquired hydrocephalus is often caused by an illness or injury that affects the brain.

Normal pressure hydrocephalus (NPH) may also be the result of an infection, illness or injury, but in many cases it's not known why the condition occurs.

Cerebrospinal fluid

Cerebrospinal fluid (CSF) is created in the brain. It flows around the brain through a series of passageways called ventricles.

Excess CSF moves out of the brain, where it's absorbed back into the bloodstream by a specialised tissue called the arachnoid villi. The arachnoid villi act like a one-way valve, allowing excess CSF to leave the brain and filter into blood vessels while preventing the blood from leaking into the brain and damaging it.

Hydrocephalus can develop if:

  • there's a blockage in one of the ventricles so that excess fluid can't move out of the brain
  • there's a problem with the arachnoid villi so that fluid is unable to filter into the blood vessels
  • the brain starts to produce too much CSF (this is very rare)

Congenital hydrocephalus

Congenital hydrocephalus (when a baby is born with the condition) may be the result of a defect in the development of the brain that restricts the flow of CSF. This can be caused by certain health conditions, such as spina bifida.

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:

  • X-linked hydrocephalus – caused by a mutation (change in the genetic material) of the X chromosome
  • rare genetic disorders – such as Dandy Walker malformation
  • arachnoid cysts – fluid filled sacs located between the brain or spinal cord and the arachnoid membrane (one of the three membranes surrounding the brain and spinal cord)

In many cases of congenital hydrocephalus, the cause is unknown. This is known as idiopathic.

Acquired hydrocephalus

Hydrocephalus that develops in adults or children (acquired hydrocephalus) is usually the result of an injury or illness that causes a blockage in the ventricles of the brain.

Possible causes of acquired hydrocephalus include:

Some people are born with narrowed passageways in their brain that restrict the flow of cerebrospinal fluid, but don't cause any symptoms until years later. 

Normal pressure hydrocephalus

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.

There are several theories to explain what happens to the brain in cases of NPH. Some are outlined below.

Problems with the arachnoid villi

It's thought that NPH occurs when something goes wrong with the arachnoid villi (the tissue that allows CSF to filter into the blood vessels). If the blood vessels don't reabsorb the fluid it will gradually increase pressure, leading to brain damage.

Underlying health conditions

NPH may be caused by conditions that affect the normal flow of blood. For example, diabetes, heart disease, or having a high level of cholesterol in the blood.

The exact cause is unknown, but conditions that affect the blood vessels within the brain, or those that supply blood to the brain (cerebrovascular disease), may be linked to NPH.


Page last reviewed: 19/01/2015

Next review due: 19/01/2017