This condition, also known as "hydrocephaly", is an abnormal enlargement of the head that is caused by abnormal increase in the amount of cerebro-spinal fluid. Hydrocephalus is not a neural tube defect. However, it is frequently associated with these.
What is "cerebro-spinal fluid" (CSF)?
There are channels within the brain which contain a circulating fluid. This is called cerebro-spinal fluid, by virtue of its location. The short form is CSF.
This fluid is continually produced by some parts of the brain and it circulates continuously. If there is an obstruction to the flow of this fluid during fetal life or in early infancy, the fluid builds up. This will squeeze the brain, effectively impeding its growth and causing enlargement of the head, hence hydrocephalus.
Is this occurrence always associated with a neural tube defect such as spine bifida?
Not at all. In the UK, about three per 10,000 babies are born with hydrocephalus not associated with any neural tube defect.
Are there any known causes of congenital hydrocephalus?
There are several known causes. Hydrocephalus may result from a structural .abnormality during brain formation, where the fluid channels do not have a free passage.
Another known cause is infection affecting the fetal brain. This may lead to scarring and blockage of the fluid channels.
Obstruction of the fluid channels may also result from bleeding inside the brain or from growth of a tumour.
The list above gives causes of obstruction of the fluid channels. Is this the only mechanism through which hydrocephalus develops?
Channel obstruction is the most common but not the only cause of hydrocephalus. Occasionally, the cause is a defect in the membrane that is responsible for absorption of the fluid. In this case, the fluid continues to be produced but is not removed. The inevitable consequence is fluid build-up and hence hydrocephalus.
There are many cases of hydrocephalus that remain unexplained.
How is hydrocephalus diagnosed?
By using ultrasound. Mild forms of hydrocephalus may be difficult to ascertain and may require several scans over a number of weeks to confirm the diagnosis. When hydrocephalus is advanced, the diagnosis is relatively easier and may not need a repeat scan to confirm.
What happens after the diagnosis of hydrocephalus is confirmed?
A detailed ultrasound examination of the fetus will be made to check whether there are any other associated or concurrent abnormalities. These could be in other organs such as the heart and lungs. A sample of amniotic fluid from around the baby will be taken for analysis. This is to check whether there is any underlying chromosomal disorder.
Once the status has been established, the relevant experts will carefully evaluate the situation to try and determine the possible prognosis. This will then be discussed with the prospective parents, who will be helped to decide on how best to proceed.
What is the likelihood that a hydrocephalic baby has an underlying chromosomal disorder?
About one in ten 10%). That is why this is an important test as it will influence advice given regarding future pregnancies as well.