Genetic testing is always necessary even if DMD is first confirmed by muscle biopsy. Different types of genetic tests are able to provide specific and more detailed information about the change in the DNA or mutation. Having genetic confirmation of the diagnosis is important for several reasons. It will help to determine if the boy may be eligible for a number of mutation-specific clinical trials and will help the family with decisions related to prenatal diagnosis and future pregnancies.
Once the exact mutation or change in the DNA in the dystrophin gene is known, mothers should be offered the opportunity for genetic testing to check whether they are carriers or not. This information will be important for other female family members on the mother’s side (sisters, daughters, aunts, cousins) to understand if they may be carriers as well.
Genetic testing and a referral to a genetic counsellor will help the family understand the results of the testing and the potential impact on other family members (see here).
Your doctor may recommend a muscle biopsy (taking a small sample of muscle for analysis). The genetic mutation in DMD means the body cannot produce the protein dystrophin, or doesn’t produce enough of it. Tests on the muscle biopsy can provide information on the amount of dystrophin present in the muscle cells.
Muscle biopsy; Left: normal muscle showing dystrophin round the fibres, Right: dystrophic muscle with absent dystrophin.
If confirmation of the diagnosis has already been achieved by genetic testing, a muscle biopsy may not be required. However, at some centres, the diagnosis of DMD may be made by muscle biopsy analysis. Genetic testing after a positive biopsy diagnosis of DMD is still essential to determine the specific change in the DNA or genetic mutation causing DMD.
There are two types of tests normally performed on a muscle biopsy. They are immunocytochemistry and immunoblotting for dystrophin. These tests are done to determine the presence or absence of dystrophin and can help to distinguish DMD from a milder form of the condition.
In the past, the tests known as electromyography (EMG) and nerve conduction studies (needle tests) have been a traditional part of the assessment of a child with a suspected neuromuscular disorder. The experts agree that these tests are NOT appropriate or necessary for the evaluation of DMD.
Information based on consensus statement (published in January 2010)