Aging Begins at 30

Weak Muscles Without Paralysis: Myasthenia Gravis

Ian Maclean Smith, M.D.
Emeritus Professor
Department of Internal Medicine
University of Iowa Hospitals and Clinics
Creation Date: June 2001
Last Revision Date: June 2001
Peer Review Status: Internally Peer Reviewed

In 1672 Thomas Willis described a woman with difficulty speaking due to lazy or fatiguing voice muscles. Her voice was partly nasal due to palatal weakness and partly mushy due to tongue weakness. It is now recognized as an outstanding example of a disease of the nerve cell at the junction of two nerve cells, (the synapse), and as an autoimmune disease, where antibodies are produced against normal body substances as if they were foreign invaders.

The disease presents as weakness and fatigability of skeletal muscle becoming more evident with exertion. It often presents as lazy drooping eyelids (called ptosis) or by seeing double. The normal smile may be replaced with a snarl like grimace. In addition there may be trouble speaking, chewing or swallowing, rarely neck weakness leads to a drooping head. Sometimes breathing difficulties can be life threatening. The muscles are weak not wasted. The onset can be sudden, sub acute or may come and go. In mild cases it may be necessary to fatigue the symptomatic muscle such as up gazing for a minute to precipitate the eyelid droop.

My classmate, who by the way, when he retired gave up neurology entirely in favor of a new career in music, suggested that antibodies might be interfering with the firing of involved muscles. And so it turned out to be. The antibodies are formed against the acetylcholine receptor (the "docker" protein of the chemical neuromessenger that fires a muscle action). The numbers of these receptors is reduced and so is the firing of the muscle.

About one person per 100,000 is affected each year and that has been constant in those patients, especially women, under 40. There has been a substantial increase in patients over 40, especially men. There is some evidence of particular tissue types (transplant compatibility markers) amongst those affected. The disease is somewhat different in occidental and oriental patients. Many of the latter have the problem confined to the eye muscles only. About 10% of patients have a benign tumor of the thymus gland, a lymph gland that usually disappears at puberty. Another 65% have other abnormalities of the thymus gland. There are muscle-like cells there which may be the origin of the muscle antibodies. About 45% of thymomas are associated with myasthenia gravis. Some people on certain drugs, for example penacillimine, antimalarials, beta-blockers, verapamil, some general anesthetics, or certain antibiotics called aminoglycosides develop a disease indistinguishable from myasthenia, which usually goes away when the treatment is stopped.

As this intriguing mechanism has unfolded, certain similar but rare muscle diseases have been shown to be due to antibodies to other essential neuromessenger intermediaries. Sometimes they enter the differential diagnoses of myasthenia gravis, that is they have to be ruled out. Diagnosis can be with edrophonium that has a rapid onset of about 30 seconds in normalizing muscle action and rapid span of 5 minutes before its effect is over.

The treatment that is now available is highly effective though the specific cure still eludes us.

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