What is spina bifida?
This is the most common of a group of abnormalities collectively known as "neural tube defects" (NTDs). The spine is made up of several small bones called vertebrae, which are stuck neatly together. Each vertebra is closed, leaving a hole in the centre. The holes of the spinal vertebrae form a hollow "tube", into which the spinal cord runs. The spinal cord is therefore completely covered all round.
When there is a defect on one or more of the vertebrae, part of the spinal cord will not be covered. This defect is what is known as spina bifida. It is usually on the lower part of the spine.
Are there different types of spina bifida?
There are two major groups. The more serious type - which is immediately obvious - is where only a thin membrane covers the spinal cord from the elements. The less common and less serious "occult" spina bifida is where, even though there is a defect on the bony spine, the skin over the defect is intact. This may be missed antenatally and could conceivably go unrecognized for years.
Only one in ten cases (10%) of spine bifida are of this hidden type.
What are the related abnormalities?
These are known as "cephaloceles". Cephaloceles generally mean bone defects on the skull.
Because the skull closure is supposed to take place quite early in pregnancy (before six weeks of gestation), when there is failure of this occurrence, the growing brain may protrude through the defect. In about 10 per cent of spine bifida cases, there is also a cephalocele.
How is spina bifida or related defects detected?
The screening tests performed early in the second trimester are crucial in this. One of the chemicals looked for in the blood test variably known as the "triple test" or the "double test" is called alpha-fetoprotein or AFP (αFP).
AFP is a chemical produced by the fetal liver. It eventually reaches the maternal circulation by diffusion across the placenta.
When there is an open neural tube defect, the amount of AFP that reaches the maternal bloodstream is quite markedly increased. By checking levels of AFP in maternal blood, one can therefore suspect whether there is a significant possibility of a neural tube defect such as spina bifida.
What if the neural tube defect is of the closed type?
In such a case, the AFP (αFP) levels will be normal. This test is therefore not useful in detecting this kind of defect.
Is AFP raised only in cases of neural tube defects?
No. This is why high levels alone are not diagnostic, only suggestive. Such a result only raises the possibility of the presence of such a defect. Other causes of high levels of AFP (αFP) include:
abdominal wall defects
fetal bowel obstruction
fetal skin disorders
fetal growth restriction.