How are metabolic diseases of muscle diagnosed?
Because these diseases are rare, many people with metabolic disorders of muscle have to spend some time figuring out what is causing their muscle weakness, myoglobinuria or other symptoms. But it's important to get an accurate diagnosis of a specific metabolic myopathy so the affected person can modify diet and exercise, and monitor potentially serious disease effects.
The diagnostic process usually begins with a careful medical history, a physical exam and a neurological exam to test reflexes, strength and the distribution of weakness. Several specialized tests are used to confirm a suspected diagnosis of metabolic disease:
Blood tests can be used to detect the presence of certain chemicals in the blood that may indicate some metabolic diseases.
An exercise test is used to monitor a person’s response to intense or moderate exercise. Blood samples are taken during exercise for testing.
Electromyography (EMG) uses small needle electrodes to measure the electrical currents in a muscle as it contracts. While an EMG can’t definitively diagnose metabolic disease, it can be used to rule out a number of other types of neuromuscular disease that cause similar patterns of weakness.
A muscle biopsy requires the removal of a small piece of muscle tissue for microscopic analysis. The procedure is done either surgically, with an incision to expose the target muscle or with a needle. A skin biopsy also is sometimes performed.
Other tests that may be needed include an electrocardiogram to test heart function, and brain imaging studies such as CT or MRI scans.
Genetic tests, using a blood sample, can analyze the person’s genes for particular defects that cause metabolic disease, but these tests often aren’t necessary for diagnosis or for determining treatment. For more on getting a definitive genetic diagnosis, see The Genie's Out of the Bottle: Genetic testing in the 21st century.
In May 2013, a U.S. federal committee voted to add the metabolic muscle disorder acid maltase deficiency (Pompe disease) to the list of conditions that states screen for in newborn babies. The list is known as the Recommended Uniform Screening Panel (RUSP). To learn more, see Federal Committee Recommends Newborn Screening for Pompe Disease. The recommendation was based on the availability of reliable tests for screening and diagnosis and of an effective treatment (enzyme replacement therapy).