Cerebrospinal fluid and the brain
In order to understand the causes of hydrocephalus, it is first useful to understand how cerebrospinal fluid (CSF) circulates through the brain.
Most CSF is created in a part of the brain called the choroid plexus. CSF flows around the brain through a series of chambers, known as ventricles.
Excess CSF is moved out of the brain through a series of channels known as the dural venous sinuses. These channels run down to a layer of tissue called the arachnoid villi.
The arachnoid villi act like a one-way valve. They allow the 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:
- the choroid plexus starts to produce too much CSF,
- there is a blockage in one of the ventricles of the brain, so that excess CSF cannot move out of the brain, or
- there is a problem with the arachnoid villi, so that the CSF is unable to filter into the blood vessels.
Congenital hydrocephalus
The most common cause of congenital hydrocephalus is that a baby is born with a blockage in a long passage known as the cerebral aqueduct. The cerebral aqueduct is a connection between two of the larger ventricles of the brain. Less commonly, a baby may be born with a choroid plexus that produces too much CSF.
These defects in the development of the brain can be caused by health conditions that are known to cause birth defects. For example, 70% of children born with the most serious type of spina bifida will develop hydrocephalus.
Infections that a mother develops during pregnancy can also interfere with the normal development of a baby’s brain. Infections that can cause congenital hydrocephalus include:
- rubella (German measles)
- mumps,
- syphilis,
- cytomegalovirus (CMV), the virus that is responsible for glandular fever, and
- toxoplasmosis, an infection that is caused by parasites.
Acquired hydrocephalus
Acquired hydrocephalus is usually caused when injury or illness results in a blockage between the ventricles of the brain.
Conditions that can cause acquired hydrocephalus include:
- stroke,
- bleeding inside the brain (haemorrhage),
- meningitis,
- brain lesions, and
- brain tumours.
Normal pressure hydrocephalus
In some cases, normal pressure hydrocephalus (NPH) develops after a brain injury, stroke or infection. However, in most cases of NPH, there is no clear reason why the condition occurs (this is known as idiopathic normal pressure hydrocephalus).
There are two theories to explain what happens to the brain in cases of NPH.
The first theory is that NPH occurs when something goes wrong with the arachnoid villi, so that the CSF is not reabsorbed into the veins. The brain compensates for this by scaling back the production of new CSF.
As a result of this, there is not the sharp rise in pressure that is associated with other types of hydrocephalus, but a gradual increase in pressure that can cause progressive brain damage.
The second theory is that NPH is caused by underlying health conditions that affect the normal flow of blood, such as diabetes, heart disease or having a high level of cholesterol in the blood.
The alteration in blood flow that is caused by these conditions may lead to a softening of the brain tissue. The softened brain may be placed under increasing pressure, leading to the symptoms of NPH.
