Myasthenia gravis
What is myasthenia gravis?
Myasthenia gravis (MG) is a chronic muscle disease that produces weakness and abnormally rapid fatigue of the voluntary muscles. The weakness is caused by a defect in the transmission of nerve impulses to muscles. The disease is seldom fatal, though it can be life-threatening when muscle weakness interferes with respiration. Modern methods of treatment give many patients marked relief of symptoms and often allow them to lead full, productive lives.
What are the symptoms of myasthenia gravis?
The onset of MG may be sudden, with severe and generalised muscle weakness, but more often the symptoms in the early stages are subtle and variable, making it difficult to diagnose correctly.
The first noticeable symptom frequently is weakness of the eye muscles. The disease may remain localised there, or progress to muscles involved in swallowing, chewing, talking, or in moving the limbs. Symptoms vary from patient to patient, but can include a drooping of one or both the eyelids (ptosis), blurred or double vision, weakness of the muscles that move the eyeballs, and unstable or waddling gait, weakness in arms, hands, and fingers, difficulty in swallowing, and difficulty in breathing. These last 2 represent a significant danger to the patient.
Muscle weakness may develop over a few days or weeks, or remain at the same level for long periods of time. The severity of weakness varies from patient to patient, and even in the same patient at different times of the day. Weakness tends to worsen with exercise and at the end of the day, and is usually particularly alleviated by rest.
Who may get myasthenia gravis?
The disease can affect people of all ages and either sex, but it is almost twice as common in women. It is not contagious and not thought to be inherited, though recent studies indicate that it occurs within certain families more frequently than can be explained by chance alone.
What causes myasthenia gravis?
The symptoms of myasthenia gravis are caused by a defect in the transmission of nerve impulses to muscles, even though the nerves and muscles themselves may remain normal. The chemical acetylcholine normally transmits nerve impulses to muscle fibres at the neuromuscular junction, causing the muscles to contract. In myasthenia gravis, the number of acetylcholine receptors, or sites at which the chemical can be received, appears to be reduced.
There is evidence to indicate that the receptor deficit is due to an attack on the acetylcholine receptors by the body's immune system, the same system normally involved in fighting infections. Myasthenia gravis is therefore termed an autoimmune disease because the body's immune system appears to be attacking its own tissues. Research has shown that most patients with MG form abnormal immune blood proteins (antibodies) against the acetylcholine receptor. Scientists are investigating what triggers the body to develop an autoimmune response. In many patients, the thymus gland appears to be involved.
What is the role of the thymus gland in myasthenia gravis?
The cells of the thymus gland form a part of the body's normal immune system. Located in the chest, the thymus is normally large in infancy, and shrinks in size so that in the normal adult it is hardly functional. In the myasthenic adult, however, the patients have thymic abnormalities. 10 to 15 per cent have tumours, called thymomas, which are usually relatively benign, but may become malignant. Although the relation of the thymus gland to myasthenia gravis is not totally understood, it appears that the thymus gland is linked to the production of acetylcholine receptor antibodies or other substances that interfere with neuromuscular transmission.
How can myasthenia gravis be treated?
The use of drugs and surgery, alone or in combination, has been quite successful in the treatment of myasthenia gravis, allowing many patients to lead normal lives. The first line of treatment is usually with drugs — called anticholinesterase agents — that strengthen neuromuscular transmission. While these drugs don't repair the basic deficiency in the acetylcholine receptors, they do prolong the effect of the acetylcholine that signals the muscles to act. Treatment with anticholinesterase agents is beneficial, but in most patients not sufficient to allow the patient to return to full activity.
A thymectomy, the removal of the thymus gland, is often the next step. After a thymectomy, remission or marked improvement occurs in more than half the cases.
Steroid treatment may be recommended for more severely ill patients. Steroids and other immuno-suppressive drugs are often very effective and produce remission of symptoms by suppressing the body's immune system. Unfortunately these drugs can also have serious effects, especially after prolonged use.
A method of treatment called plasmapheresis has been put to use in helping people with particularly severe cases of MG.
What is plasmapheresis?
Some patients are not helped by any of the traditional treatments. Their conditions may continue to deteriorate to almost complete paralysis and life-threatening respiratory problems. Some of these patients have returned to normal functioning after plasmapheresis treatments. Plasmapheresis is a blood plasma exchange process largely developed as a treatment for MG by Muscular Dystrophy Association-supported scientists. It works to remove from the blood antibodies and other substances that interfere with the transmission of nerve impulses. While plasmapheresis has a valuable and sometimes life-saving role to play in the treatment of myasthenia gravis, it is expensive, time-consuming and not totally risk-free.
What is a crisis in myasthenia gravis?
A crisis occurs when a person with MG develops difficulty in breathing, requiring hospitalisation and usually a mechanical respirator. The crisis may be brought on by any stress on the body, such as infection, emotional upset, physical activity, menstruation, pregnancy, or even adverse reaction to medication. The patient in crisis requires swift medical attention to treat life-threatening respiratory problems.
Last Reviewed: 13 June 2007
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