Drug facilitates muscle stimulation to block fatigue from myasthenia gravis

ON CALL

October 25, 1994|By Dr. Simeon Margolis | Dr. Simeon Margolis,Special to The Sun

Q: My 27-year-old daughter has been started on medication to treat myasthenia gravis. Although her doctor tried to explain it to us, we are still not sure how the medicine works.

A: Myasthenia gravis is a disorder caused by a defect in the communication between muscles and the nerves that stimulate them to contract. Signals from the brain travel along nerves as electrical impulses. When they reach the muscle, these impulses trigger the release of the chemical substance acetylcholine from the nerve.

The binding of acetylcholine to specific acetylcholine receptors on the muscle cells leads to muscle contraction. The acetylcholine acts for only a short time before it is taken up again by the nerve or broken down by the enzyme acetylcholinesterase.

People with myasthenia gravis produce antibodies that interact with and inactivate the acetylcholine receptors. As a result, the muscles are weak and become fatigued rapidly.

Most often the first muscles affected in myasthenia gravis are those around the eyes and face. Frequent early symptoms are drooping eyelids, double vision and weakness of the facial muscles.

As the disease progresses, patients tend to note difficulty in chewing foods and slurred, nasal-sounding speech due to weakness of the tongue and muscles of the palate and pharynx. The final problem in most patients is weakness of the arms and legs.

The course of the disease is punctuated by cycles of marked improvement alternating with relapses. Most feared during relapses is a myasthenic crisis, a medical emergency caused by great difficulty in breathing or in swallowing due to severe worsening of muscle weakness.

Two general types of medications are used to treat myasthenia gravis. By blocking the acetylcholinesterase enzyme, the group of drugs (anticholinesterases) prevent the breakdown of acetylcholine and allow enough of it to accumulate so that the muscles can be stimulated despite the presence of antibodies against the acetylcholine receptor.

Another common treatment for myasthenia is the use of high doses of corticosteroids, or other immunosuppressive drugs, which work by reducing the formation of antibodies against the acetylcholine receptor.

The majority of individuals with myasthenia gravis have a tumor or overgrowth of the thymus, a gland in the upper part of the chest that normally reaches its largest size just before puberty, and then shrinks progressively with age. Surgical removal of an enlarged thymus improves the symptoms of myasthenia gravis in most patients.

Dr. Margolis is professor of medicine and biological chemistry at the Johns Hopkins School of Medicine.

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