Firstly, a blood test is taken and the level of a muscle protein measured – the creatine kinase (CK) level. In approximately 40 percent of cases of congenital muscular dystrophy, this level is five to 20 times higher than normal.
An electromyography (EMG) test may also be performed. A small needle is inserted into the muscle and the electric activity recorded. This test, which may provide evidence of an abnormal pattern of electric activity in the muscle, is usually not necessary in children who have markedly elevated serum CK.
At this stage, however, even in the cases with high CK levels, abnormal muscle ultrasound and EMG, an additional test that is required in almost every case is a muscle biopsy.
A muscle biopsy can help to identify the subtype of congenital muscular dystrophy 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 - muscle proteins can be studied in detail with specialised tests. This greatly helps to narrow down the diagnosis.
In the forms of congenital muscular dystrophy in which the gene defect has been identified, genetic tests can provide a definitive diagnosis.