What tests are available?

The gold standard is to identify the underlying genetic change in an affected child and look for this in female relatives. Unfortunately, it sometimes takes a considerable time to identify this change in a new case - it is like looking for a needle in a haystack. Until it is found, another method has to be employed.

When we know that the condition has been passed down through a family, we can use DNA markers that lie close to the gene to track the damaged copy (see figure below). In smaller families and especially if there is just one affected boy in a family, this technique is less helpful because it does not take into account the possibility that the genetic change has arisen anew in some generation. It is important to discuss with a clinical geneticist the specific circumstances in a family and the most appropriate form of testing that is available.

fig1

For couples wanting prenatal diagnosis this may be feasible and again advice should be sought from a clinical geneticist. Such testing can be performed from about 11 wks of pregnancy. A sample of the placenta that is made by the developing foetus is obtained under local anaesthetic by passing a needle, under ultrasound guidance, through the mother's abdominal wall and into the womb. The laboratory analysis then takes 1-2 weeks. Initially the sex of the baby is determined: if female, no further testing is performed; if male, the sample is checked to determine whether or not the baby has inherited the damaged ATRX gene.