A serious disorder in which the chief symptoms are muscular weakness and a special tendency for fatigue to rapidly develop after effort. The prevalence is around 1 in 30,000. Two-thirds of the patients are women, in whom it develops in early adult life. In men it tends to develop later.

It is a classical example of an autoimmune disease (see AUTOIMMUNITY). The body develops ANTIBODIES which interfere with the working of the nerve endings in muscle, acted on by ACETYLCHOLINE. It is acetylcholine that transmits the nerve impulses to muscles: if this transmission cannot be effected, as in myasthenia gravis, then the muscles are unable to contract. Not only the voluntary muscles, but those connected with the acts of swallowing, breathing, and the like, become progressively weaker. Rest and avoidance of undue exertion are necessary, and regular doses of neostigmine bromide, or pyridostigmine, at intervals enable the muscles to be used and in some cases have a curative effect. These drugs act by inhibiting the action of cholinesterase – an ENZYME produced in the body which destroys any excess of acetylcholine. In this way they increase the amount of available acetylcholine and compensate for the deleterious effect of antibodies on the nerve endings.

The THYMUS GLAND plays the major part in the cause of myasthenia gravis, possibly by being the source of the original acetylcholine receptors to which the antibodies are being formed. Thymectomy (removal of the thymus) is often used in the management of patients with myasthenia gravis. The incidence of remission following thymectomy increases with the number of years after the operation. Complete remission or substantial improvement can be expected in 80 per cent of patients.

The other important aspect in the management of patients with myasthenia gravis is IMMUNOSUPPRESSION. Drugs are now available that suppress antibody production and so reduce the concentration of antibodies to the acetylcholine receptor. The problem is that they not only suppress abnormal antibody production, but also act on normal antibody production. The main groups of immunosuppressive drugs used in myasthenia gravis are the CORTICOSTEROIDS and AZATHIOPRINE. Some patients receive immunoglobulin or are treated by plasma exchange. A gram-positive (see GRAM'S STAIN) rod-like genus of aerobic BACTERIA, some species of which are harmful to humans and animals. For example, M. tuberculosis (Koch's bacillus) and M. leprae cause, respectively, TUBERCULOSIS and LEPROSY.