making muscle wasting history

Once a child with DMD has been born in a family, it is often possible to offer prenatal diagnosis in future pregnancies, either for the mother or for other women who are found to be at risk of being carriers. This is normally possible when DNA studies give precise information that allows the status of the foetus to be identified. This is achieved by studying the foetus’s own DNA in a chorion villus biopsy. This test is performed on a tiny piece of the developing placenta usually at about the 11th-12th week of pregnancy.

All affected boys have very abnormally high levels of an enzyme called creatine kinase in their blood. Most hospital laboratories can do this test. But there are other even rarer causes of a positive creatine test, so for a specific diagnosis in families with no previous affected member other tests are needed. These may involve genetic testing (looking for the actual fault in the dystrophin gene) and a muscle biopsy which can be studied to look at the dystrophin protein. Duchenne muscular dystrophy is caused by an X-linked gene (that is, the gene is on the X chromosome; girls have two of these and boys only one). This means that only boys are affected but that their mothers may be carriers. Actually in almost half of all affected boys nowadays it turns out that the faulty gene has arisen by a change in the gene or ‘mutation’ in the boy himself and no other member of the family carries it. But this may be difficult to prove and can be decided only after careful and expert assessment of the family.

In just over half of all cases the mother carries the gene but is usually not herself affected by it. Such women are known as ‘carriers’. Each subsequent son of a carrier has a 50:50 chance of being affected and each daughter has a 50:50 chance of being a carrier herself. A small number of female carriers of the gene have a mild degree of muscle weakness themselves and are then known as ‘manifesting carriers’. One of the most important things that needs to be done soon after the diagnosis of a boy with Duchenne muscular dystrophy is to seek genetic advice and appropriate tests for those members of the family who are at risk of being carriers.