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.

Hydrocephalus (fluid on the brain) can cause complications, or complications may develop as a result of surgery used to treat it.

Shunt malfunction

A shunt is a delicate piece of equipment prone to malfunction, usually through blockage or infection. It’s estimated that up to four out of 10 shunts will malfunction in the first year after surgery. Sometimes a scan carried out after the operation shows that the shunt isn’t in the best position, and that further surgery may be needed to reposition it.

If a baby or child has a shunt fitted, the shunt may become too small as your child grows, and it will need to be replaced. As most people need to have a shunt for the rest of their life, more than one replacement may be needed.

It’s estimated that most children with hydrocephalus may have an average of two procedures for shunt problems before they’re 10 years old.

Occasionally, when shunt tubes are positioned, bleeding can occur. This can result in nerve problems, such as weakness down one side. There’s also a small risk of seizures (fits) following any surgery on the brain.

In younger children, particularly babies, cerebrospinal fluid (CSF) can run alongside the shunt rather than down it, and it can leak through the skin wound. If this occurs, further stitches will be needed to stop the leak.

Shunt blockage

A shunt blockage can be very serious because it can lead to an excess build-up of fluid on the brain, which can cause brain damage. This will cause the symptoms of hydrocephalus, such as:

• headaches
• feeling sick
• being sick
• confusion
• drowsiness or coma

In babies, you may notice their head growing larger or they may have a bulging or tense fontanelle (the soft spot on the top of their head).

Contact your care team immediately if you or your child has these symptoms. Emergency surgery will be required to replace the malfunctioning shunt.

Shunt infection

Shunt infection is also a relatively common complication. The risk of infection can be around 3-15% and is more likely to occur during the first few months after surgery.

The symptoms of a shunt infection may include:

• redness and tenderness along the line of the shunt
• a high temperature (fever) of 38ºC (100.4ºF) or above
• headache
• being sick
• neck stiffness
• tummy pain (if the shunt drains into your tummy)
• irritability or drowsiness in babies

Contact your care team immediately if you or your child has these symptoms. You may need a course of antibiotics to treat the infection and, in some cases, surgery may be required to replace the shunt.

Complications of endoscopic third ventriculostomy (ETV)

An endoscopic third ventriculostomy (ETV) is a surgical procedure where a small hole is made in the floor of your brain. Complications can occur after this type of surgery, such as:

• the hole can close
• your brain may not be able to absorb the cerebrospinal fluid that’s now draining through it
• you may develop an infection, although this is less likely than after shunt surgery
• you may have bleeding inside your brain (this is usually minor)

If there’s a problem with the hole, it may be possible to repeat the procedure, or you may need to have a shunt fitted (see treating hydrocephalus).

Other risks of ETV include nerve problems, such as weakness down one side of the body, double vision or hormone imbalances. Most nerve problems will get better, but there’s a small risk of permanent problems. There’s also a small risk of epilepsy, and a very small risk of an injury to one of the blood vessels in the brain, which may be fatal.

Long-term complications of congenital hydrocephalus

Many babies born with hydrocephalus (congenital hydrocephalus) have permanent brain damage. This can cause a number of long-term complications, such as:

• learning disabilities 
• impaired speech
• memory problems
• short attention span
• problems with organisational skills
• vision problems, such as a squint and visual impairment
• problems with physical co-ordination
• epilepsy

The different types of hydrocephalus (fluid on the brain) can be diagnosed with brain scans.

Congenital hydrocephalus

In some cases, an ultrasound scan can detect congenital hydrocephalus before your baby is born. An ultrasound scan uses high-frequency sound waves to create an image of your womb and the baby inside.

If your baby has some of the physical characteristics associated with congenital hydrocephalus after they’re born, such as an enlarged head, they may be referred for an ultrasound scan. If the results of the ultrasound are inconclusive, further testing can be carried out using:

• a computerised tomography (CT) scan – this takes a series of X-rays at slightly different angles and uses a computer to put the images together
• a magnetic resonance imaging (MRI) scan – this uses a strong magnetic field and radio waves to produce detailed images of the brain

These scans can examine the brain in greater detail. As well as showing the build-up of fluid on the brain and the increased pressure, the scans can also highlight any defects in the structure of the brain that may be causing the hydrocephalus. 

Acquired hydrocephalus

Hydrocephalus that develops in adults or children (acquired hydrocephalus) can be diagnosed using a combination of CT and MRI scans. The scans can also reveal possible causes of your symptoms, such as a brain tumour.

Normal pressure hydrocephalus

Hydrocephalus that usually develops in older people (normal pressure hydrocephalus or NPH) can be difficult to diagnose for the following reasons:

• the symptoms come on very gradually
• the symptoms are more common to conditions, such as Alzheimer’s disease, which can frequently occur with NPH

It’s important to make a correct diagnosis because, unlike Alzheimer’s disease, it’s possible to relieve the symptoms of NPH with treatment.

Healthcare professionals have devised a diagnostic checklist which examines:

• how you walk (your gait)
• your mental ability
• symptoms that affect your bladder control such as urinary incontinence
• the appearance of your brain during CT, MRI and ultrasound scans

You may be diagnosed with NPH if you have the combination of an impaired gait, slowing of the normal mental processes and urinary incontinence, and scans have shown that your cerebrospinal fluid (CSF) is at a higher level than usual.

However, you may not have all of the symptoms on the checklist.

Further tests may also be carried out to decide whether you would benefit from having surgery, such as:

• a lumbar puncture
• a lumbar drainage test
• a lumbar infusion test

These procedures are briefly described below.

Lumbar puncture

A lumbar puncture, also known as a spinal tap, is a procedure used to take a sample of CSF from your lower back.

A hollow needle is inserted between your back bones (vertebrae), and a small amount of the fluid is removed. The pressure of the CSF sample can then be checked.

Removing some CSF during a lumbar puncture may help to improve your symptoms. If this is the case, it’s a good indication that you may benefit from treatment with surgery (see treating hydrocephalus).

Lumbar drain

If having a lumbar puncture doesn’t improve your symptoms, this doesn’t mean that you do not have NPH. If the lumbar puncture test is negative, you may have a lumbar drain.

A lumbar drain involves inserting a tube between your back bones to drain a large amount of CSF. This is done over a few days to see if this improves your symptoms, such as your ability to walk around. This is usually done under local anaesthetic to numb the area, or sedation to relax you.

Lumbar infusion test

A lumbar infusion test can also be used to help diagnose NPH and decide whether you need surgery. The procedure should be carried out under local anaesthetic so it shouldn’t be painful.

The test involves slowly injecting fluid into your lower back while measuring the pressure. The additional fluid should be absorbed by your body so the pressure stays low. However, if your body can’t absorb the extra fluid, the pressure will rise. This could indicate that you have NPH and that surgery will be beneficial. 

Hydrocephalus is a build-up of fluid on the brain. The excess fluid puts pressure on the brain, which can damage it.

The damage to the brain can result in a wide range of symptoms, including:

• headache
• being sick
• blurred vision
• difficulty walking

Read more about the symptoms of hydrocephalus.

Hydrocephalus can usually be treated using a piece of equipment known as a shunt. A shunt is a thin tube that’s surgically implanted in the brain and drains away the excess fluid.

Read more about how hydrocephalus is treated.

Causes

In the past, hydrocephalus was often referred to as ‘water on the brain’. However, the brain is not surrounded by water but by a special fluid called cerebrospinal fluid (CSF).

CSF has three important functions, it:

• protects the brain from damage
• removes waste products from the brain
• provides the brain with the nutrients it needs to function properly

The brain constantly produces new CSF (about a pint a day), while old fluid is released from the brain and absorbed into the blood vessels. However, if this process is interrupted, the level of CSF can quickly build-up, placing pressure on the brain.

Read more about the causes of hydrocephalus.

Types of hydrocephalus

There are three main types of hydrocephalus:

• hydrocephalus that’s present at birth (congenital hydrocephalus)
• hydrocephalus that develops after birth (acquired hydrocephalus)
• hydrocephalus that usually only develops in older people (normal pressure hydrocephalus or NPH)

These are briefly described below.

Congenital hydrocephalus

Congenital hydrocephalus is present in babies when they’re born and can be caused by birth defects, such as spina bifida, or as a result of an infection the mother develops during pregnancy, such as mumps or rubella (German measles).

It’s estimated that spina bifida affects one baby in every 1,000 born in Britain. Most of them will have hydrocephalus.

Congenital hydrocephalus carries the risk of long-term mental and physical disabilities as a result of permanent brain damage.

Read more information about complications of hydrocephalus.

Acquired hydrocephalus

Acquired hydrocephalus can affect children or adults. It usually develops after an injury or illness. For example, it may occur after a serious head injury or as a complication of a medical condition, such as a brain tumour.

Read more about the causes of hydrocephalus.

Normal pressure hydrocephalus

Normal pressure hydrocephalus (NPH) is a poorly understood condition that usually only affects people over 50 years old.

It can sometimes develop after an injury or a stroke, but in most cases the cause is unknown.

The average age of people with NPH is 75, although it’s a rare condition. In 2009-2010, there were just under 1,000 recorded cases in England. However, the true number may be higher.

Hydrocephalus (fluid on the brain) causes slightly different symptoms depending on the type of hydrocephalus and the age of the person affected.

Congenital hydrocephalus

Babies born with hydrocephalus (congenital hydrocephalus) often have distinctive physical characteristics. Physical signs include:

• your baby’s head may appear unusually large
• your baby’s scalp may be thin and shiny with easily visible veins
• your baby may have a bulging or tense fontanelle (the soft spot on the top of their head) 
• the baby’s eyes appear to be looking down; this is known as the ‘setting-sun sign’ because the eyes resemble the sun setting below the horizon
• the muscles in your baby’s lower limbs may appear stiff and be prone to muscle spasms (contractions)

As well as these physical signs, congenital hydrocephalus can also cause symptoms such as:

• poor feeding
• irritability
• being sick
• drowsiness

Acquired hydrocephalus

Hydrocephalus that develops in adults or children (acquired hydrocephalus) can cause headaches. The headache may be worse in the morning after waking up, as the fluid in your brain doesn’t drain so well while you’re lying down and may have built up overnight. Sitting up for a while may improve your headache. However, as the condition progresses, the headaches may become continuous. If hydrocephalus is not treated, it can be life-threatening. 

Other symptoms of acquired hydrocephalus include:

• neck pain
• feeling sick
• being sick – which may be worse in the morning
• drowsiness, which can progress to a coma
• changes in your mental state, such as confusion
• blurred vision or double vision 
• difficulty walking
• not being able to control your bladder (urinary incontinence) and, in some cases, your bowel (bowel incontinence)

Normal pressure hydrocephalus

Unlike the other two types of hydrocephalus, the symptoms of hydrocephalus that develop in older people (normal pressure hydrocephalus or NPH) usually develop slowly, over the course of many months or years.

There are three sets of distinctive symptoms. They affect:

• how you walk
• your urinary system
• your mental abilities

These are discussed below.

How you walk 

Usually, the first noticeable symptom of normal pressure hydrocephalus (NPH) is a change in how you walk (your gait). You may find it increasingly difficult to take the first step when you want to start walking. Some people have described it as feeling as though they’re frozen to the spot. You may also shuffle rather than take proper steps.

As the condition progresses, you may become increasingly unsteady on your feet. You may be more likely to fall, particularly when turning.

Urinary symptoms

The change in the way that you walk is often followed by bouts of urinary incontinence, which may include symptoms such as:

• a frequent need to urinate
• an urgent need to urinate
• loss of bladder control

Mental abilities

The normal thinking process also starts to slow down. This can take the form of:

• being slow to respond to questions
• reacting slowly to situations
• being slow to process information

These may be confused with the symptoms of dementia. You may actually have mild dementia, which will improve when the normal pressure hydrocephalus (NPH) is treated. 

Hydrocephalus (fluid on the brain) is treated with surgery.

Congenital and acquired hydrocephalus

Babies who are born with hydrocephalus (congenital hydrocephalus) and adults or children who develop hydrocephalus (acquired hydrocephalus) usually require prompt treatment to reduce the pressure on their brain. If the hydrocephalus is not treated, the rise in pressure will damage the brain.

Both congenital and acquired hydrocephalus will be treated with either shunt surgery or neuroendoscopy (see below).

Normal pressure hydrocephalus

Hydrocephalus that usually develops in older people (normal pressure hydrocephalus or NPH) can also be treated with a shunt. However, experience has shown that not everyone with NPH will benefit from shunt surgery.

Due to the risks of complications occurring as a result of surgery, you will need tests to assess whether the potential benefits of surgery outweigh the risks. A lumbar drainage test or lumbar infusion test, or both, can be used to find out whether shunt surgery will benefit you. See diagnosing hydrocephalus for more information about these tests.

Shunt surgery will be recommended if testing reveals that it would be beneficial.

Shunt surgery

Shunt surgery involves implanting a thin tube, called a shunt, in the brain. The excess cerebrospinal fluid (CSF) in the brain runs through the shunt to another part of the body, usually the abdomen. From here the fluid is absorbed into your blood stream. The shunt has a valve inside to control the flow of CSF and to ensure it does not drain too quickly. You can feel the valve as a lump under the skin of your scalp.

The operation

Shunt surgery is carried out by a neurosurgeon (a specialist in surgery of the brain and nervous system). You will be given a general anaesthetic before the operation so that you will be asleep throughout the procedure, which usually takes one to two hours. 

After the operation, you may need to spend a few days in hospital to recover. If you have stitches in the wound in your head, they may dissolve on their own, or you may be advised about when these will be removed. Some surgeons use skin staples to close the wound. Like stitches, these will need to be removed after a few days.

Once the shunt has been installed, further treatment for hydrocephalus may be required if the shunt becomes blocked or infected. Shunt repair surgery will then be necessary. See complications of hydrocephalus for more information.

Endoscopic third ventriculostomy (ETV)

An alternative procedure to shunt surgery is an endoscopic third ventriculostomy (ETV).

ETV involves making a hole in the floor of the brain, allowing the trapped CSF to escape to the surface of the brain where it can be absorbed, instead of inserting a shunt.

An ETV is not suitable for everyone. However, it could be a possible treatment option if the build-up of CSF in your brain is the result of a blockage (obstructive hydrocephalus). The CSF will be able to drain through the hole, avoiding the blockage.

The operation

You will be given general anaesthetic before the operation. The neurosurgeon will then make a small hole in your skull and use an endoscope to look inside the chambers of your brain. An endoscope is a thin, long tube that has a light and a video camera at one end. A small hole will be made inside your brain with the help of the endoscope. After the endoscope has been removed, the wound will be closed using stitches. The procedure takes around one hour.

There is less risk of an infection developing after an ETV than with shunt surgery. However, as with all surgical procedures, there are some risks associated with ventriculostomy (see complications of hydrocephalus). 

The long-term results for treatment with ETV are very similar to those for a shunt operation. As with shunts, ETVs may block months or years after surgery, resulting in your symptoms reoccurring.