Mistaken Diagnosis? ALS And SBMA Can Be Confused

by Dr. Kenneth Fischbeck on Thu, 1997-08-07 17:00

Mistakes in diagnosing ALS are rare these days, but they do occur. One disease that's sometimes confused with ALS is SBMA, or spinal-bulbar muscular atrophy, also known as Kennedy's disease.

SBMA is a form of motor neuron disease that can mimic ALS. Many of the symptoms are the same. SBMA causes weakness, particularly in the hip and shoulder muscles, with muscle atrophy and twitching (fasciculations). SBMA patients often have muscle cramps, and they may have difficulty swallowing.

SBMA differs from ALS in that the weakness is usually symmetrical and more slowly progressive, and there is no muscle stiffness (spasticity) or increase in reflexes. Also, SBMA is a hereditary disorder, with sex-linked (X-linked) inheritance. In general, only men are affected, and women in the family (mothers, sisters and daughters of the affected men) may carry the disease gene without showing any symptoms. Because the onset is late, SBMA patients are often unaware of others who are affected in their families.

The cause of SBMA is a mutation in the gene for the androgen receptor (the protein that binds the male sex hormone, testosterone). With this androgen receptor mutation, men with SBMA often have signs of androgen insensitivity — breast enlargement or reduced fertility. Because the mutation is known, SBMA can be diagnosed very easily with a genetic blood test.

SBMA has a better prognosis than ALS, with slow progression over 10 to 20 years. There is some indication that treatment with androgens (male hormones) may be beneficial, but this therapy is considered experimental.

If you're a man with slowly progressive motor neuron disease and the symptoms described above, particularly if other men in your family are affected, it might be worthwhile to be tested for SBMA. Ask your doctor!

Dr. Kenneth Fischbeck is an associate professor of neurology at the University of Pennsylvania School of Medicine in Philadelphia and a long-time MDA research grantee. In 1991, his group identified the gene that, when mutated, causes spinal-bulbar muscular atrophy.

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