Spinal Muscular Atrophy
The first steps in diagnosis of a neuromuscular disease are usually an in-office physical examination and family history, with some simple tests to distinguish spinal muscular atrophy (SMA) from similar conditions (such as muscular dystrophy).
The doctor may order a blood test for an enzyme called creatine kinase (CK), an enzyme that leaks out of muscles that are deteriorating. This is a nonspecific test because CK levels are elevated in many neuromuscular diseases, but it’s often useful anyway. High blood CK levels aren’t harmful in and of themselves, but they do indicate that muscle damage has occurred.
The doctor probably will recommend genetic testing if SMA is suspected, because this is the least invasive and most accurate way to diagnose chromosome 5-related SMA (types 1-4). Genetic testing requires only a blood sample. However, it has implications for the whole family that must be considered (see Causes/Inheritance).
Genetic tests are available for chromosome 5-related SMA and for some of the other forms of SMA. See Athena Diagnostics, a Massachusetts company that offers genetic testing for many neuromuscular diseases, including SMA; and Gene Tests, a website supported by the National Center for Biotechnology Information and sponsored by the University of Washington-Seattle, that lists available genetic tests.
Reliability and specificity of genetic tests are improving, and the number of tests available is expanding rapidly as knowledge and technology improve. For more on getting a definitive genetic diagnosis, see The Genie's Out of the Bottle: Genetic testing in the 21st century. Your MDA clinic team can guide you toward the right type of genetic testing for your situation.
In rare cases, doctors may order a muscle biopsy, which involves taking a small sample of muscle tissue, usually from the thigh, and looking at it under a microscope.
Other tests sometimes used to diagnose SMA include one that measures nerve conduction velocity — the speed with which signals travel along nerves — and one that measures the electrical activity in muscle, called anelectromyogram, or EMG. Nerve conduction velocity tests involve sensations that feel like mild electric shocks, and EMGs require that short needles be inserted in the muscles.