Hydrocephalus - Causes of hydrocephalus

To understand the causes of hydrocephalus (fluid on the brain), it is first useful to understand how cerebrospinal fluid (CSF) circulates through the brain.

Cerebrospinal fluid and the brain

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

Excess cerebrospinal fluid 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 cerebrospinal fluid 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 cerebrospinal fluid (this is very rare)

Congenital hydrocephalus

Congenital hydrocephalus, when a baby is born with the condition, may be the result of a brain defect restricting the flow of cerebrospinal fluid.

These defects in the development of the brain can be caused by certain health conditions. For example, most children born with the most serious type of spina bifida will develop hydrocephalus.

Congenital hydrocephalus can also occur in babies born prematurely (before week 37 of the pregnancy). Some premature babies experience bleeding in their brain, which can block the flow of cerebrospinal fluid and cause hydrocephalus.

Other possible causes of congenital hydrocephalus include:

X-linked hydrocephalus – where the condition occurs as a result of 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, which is one of the three membranes surrounding the brain and spinal cord

In many cases of congenital hydrocephalus, the cause is unknown. This is medically referred to as idiopathic.

Acquired hydrocephalus

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

Possible causes of acquired hydrocephalus include:

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

It's also possible for someone to be 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 infection. However, in most cases, there's no clear reason why the condition occurs.

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

Problems with the arachnoid villi

One idea is that NPH occurs when something goes wrong with the arachnoid villi, which is the layer of tissue that allows cerebrospinal fluid to filter into the blood vessels. This means that the blood vessels don't reabsorb the fluid. This creates a gradual increase in pressure, which can cause progressive brain damage.

Underlying health conditions

NPH may be caused by underlying health conditions affecting 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 blood vessels within the brain or that supply blood to the brain (cerebrovascular disease) may be linked to NPH.