DIAGNOSTIC CRITERIA

A baby with congenital MD is usually first diagnosed as a 'floppy baby'. Doctors can see the symptoms described above, but as these could be due to a number of different conditions, they have to conduct a series of tests to try to make an accurate diagnosis.

Firstly a blood test is taken and the level of a muscle enzyme assessed (the creatine kinase or CK level). In approximately 40% of cases of congenital MD this level is 5-20 times higher than normal.

Muscle ultrasound may also help to detect abnormalities of the muscle. The technique is very simple, similar to the ultrasound studies carried out in pregnancy and may provide further evidence of the involvement of the muscle.

An electromyography (EMG) test may also be done. A small needle is inserted into muscle and the electric activity recorded. This test may provide evidence of an abnormal pattern of electric activity in the muscle.

At this stage however even in the cases with high CK levels, abnormal muscle ultrasound and EMG, an additional test which is required in almost every case is a muscle biopsy.

Muscle biopsy can help to identify the subtype of CMD to provide a precise diagnosis in several ways:

When the muscle is studied under the microscope, it will show variation in the size of muscle fibres and that some of these fibres are replaced by fat and fibrous tissue.

In addition, the production of individual components of the muscle fibre can be studied in detail with specialised tests. This greatly helps to narrow down the diagnostic possibilities.

In the forms of CMD in which the gene defect has been identified, genetic tests will provide the ultimate diagnosis.

Prenatal diagnosis

Prenatal diagnosis is possible in several types of CMD. It is based on the ability to detect the genetic abnormality in the developing fetus. This however can only be used in the forms of congenital MD associated with a recognised gene defect or a specific protein deficiency.