Spinal-Bulbar Muscular Atrophy (SBMA)

Diagnosis

The age distribution and symptoms of spinal-bulbar muscular atrophy (SBMA) overlap with those of another motor neuron disease, amyotrophic lateral sclerosis (ALS), so the two are sometimes confused early in the diagnostic work-up. It’s worth knowing which disorder affects you or your family member, since ALS is a much more profound and rapidly progressing condition than SBMA.

The first steps in diagnosis of a neuromuscular disease are usually an in-office physical examination and family history, with some nonpainful tests to distinguish SBMA from similar conditions.

The doctor may order a simple blood test for an enzyme called creatine kinase (CK). This enzyme leaks out of muscles that are deteriorating. Although it’s a nonspecific test (since CK levels are elevated in many neuromuscular diseases), it’s often useful anyway. High blood CK levels aren’t harmful; they’re just an indicator of muscle damage.

The doctor probably will recommend genetic testing if SBMA is suspected. Requiring only a blood sample, genetic testing is the most accurate way to diagnose the condition. For more on getting a definitive genetic diagnosis, see The Genie's Out of the Bottle: Genetic testing in the 21st century.

Other tests that may be requested are those that measure nerve conduction velocity — the speed with which signals travel along nerves — and the electrical activity in muscle. The latter is called an electromyogram, 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.

An SBMA diagnosis has implications for the whole family that must be considered. It’s a good idea to talk with a genetic counselor when someone in the family is found to have a genetic disease or when testing is undertaken. Your MDA clinic can help you find one.