A diagnosis of CMD can be confusing because for many years the term was used as a “catch-all” name to describe conditions that looked like other muscular dystrophies, but started much earlier or followed different patterns of inheritance.
In recent years, doctors have agreed that there are several categories of “true” CMD, caused by specific gene mutations, and they’re distinct from other muscular dystrophies. It’s possible that some people who received diagnoses of CMD many years ago may actually have some other known form of muscular dystrophy with an unusually early onset.
In diagnosing any form of muscular dystrophy, a doctor usually begins by taking a patient and family history and performing a physical examination. Much can be learned from these, including the pattern of weakness. The history and physical go a long way toward making the diagnosis, even before any complicated diagnostic tests are done.
The doctor also wants to determine whether the patient’s weakness results from a problem in the muscles themselves or in the nerves that control them. Problems with muscle-controlling nerves, or motor nerves, originating in the spinal cord and reaching out to all the muscles, can cause weakness that looks like a muscle problem but really isn’t.
Usually, the origin of the weakness can be pinpointed by a physical exam. Occasionally, special tests called nerve conduction studies and electromyography (EMG) are done.
In these tests, electricity and very fine pins are used to stimulate and assess the muscles or nerves individually to see where the problem lies. Electromyography is uncomfortable but not usually very painful.
Early in the diagnostic process doctors often order a special blood test called a CK level. CK stands for creatine kinase, an enzyme that leaks out of damaged muscle. When elevated CK levels are found in a blood sample, it usually means muscle is being destroyed by some abnormal process, such as a muscular dystrophy or inflammation. Therefore, a high CK level often suggests that the muscles themselves are the likely cause of the weakness, but it doesn’t tell exactly what the muscle disorder might be.
To determine which disorder is causing CK elevation, a doctor may order a muscle biopsy, the surgical removal of a small sample of muscle from the patient. By examining this sample, doctors can tell a great deal about what’s actually happening inside the muscles. Modern techniques can use the biopsy to distinguish muscular dystrophies from infections, inflammatory disorders and other problems.
Other tests on the biopsy sample can provide information about which muscle proteins are present in the muscle cells, and whether they’re present in the normal amounts and in the right locations. This can reveal what’s wrong with the cells’ proteins and provide likely candidates as to which genes are responsible for the problem. The correlation between missing proteins on the muscle biopsy and genetic flaws isn’t perfect, however. An MDA clinic physician can help you understand these results.
Genetic (DNA) tests, using a blood sample, can analyze the person’s genes for particular defects that cause CMD, help predict the likely course of a disease and help families assess the risk of passing on the disease to the next generation. For more on genetic patterns in CMD, see Causes/Inheritance. And, for more on getting a definitive genetic diagnosis, see The Genie's Out of the Bottle: Genetic testing in the 21st century.