Myasthenia Gravis (MG) is a disorder of the junction between motor nerves and muscles that results in weakness and fatigue of muscles. MG is an "autoimmune" disease, which means that the body’s immune system inappropriately attacks a part of the individual’s own body. In MG, the attack is directed against the part of the muscle that receives the chemical signal from the nerve instructing it to contract, thereby causing the weakness and fatigue that are the hallmarks of this disease. The immune attack in MG is due to antibodies that attack the receptor for the chemical signal (Acetylcholine receptor, or AChR) that are detectable in the blood in nearly 90 percent of patients, or other structures near the AChR. MG can occur at any age -- from infancy to late adulthood, but typically young adult women and older men are most commonly affected.
MG affects the eye muscles early in the course, resulting in double vision and drooping of the lids. However, it can also affect virtually any muscle group in the body, giving rise to weakness of the arms or legs, difficulty speaking, swallowing, and in some severe cases breathing. The weakness is variable, and is usually better after rest or early in the morning and worse after exercise, or later in the day.
MG is now the best understood autoimmune disease. If properly diagnosed and treated, nearly all affected individuals are able to lead normal lives. It is important that they be under the care of a neurologist who is familiar with the management of MG, however. The diagnosis of myasthenia gravis is suspected on the basis of the clinical features of weakness and muscle fatigue (not "tiredness",) and can be confirmed by a number of blood tests or electrical tests. Antibodies to AChR, when detected in the blood, confirm the diagnosis of MG. However, in some patients these antibodies are not detectable; about five percent of patients have antibodies to another structure at the neuromuscular junction called MuSK. Electrical stimulation studies may be helpful in making the diagnosis of MG. In some cases, the injection of a drug called Tensilon7, which produces rapid temporary improvement in the symptoms of MG, can contribute to the diagnosis.
Treatment of MG must be individualized, depending on the special features of the patient. A series of tests are done to evaluate the individual’s related medical conditions, which play an important role in selecting the method of management. In general, there are three types of treatments:
- Drugs that prolong the signal from the nerve to the muscle, and provide short-term benefit, which can last for hours
- Drugs or treatments that alter or suppress the abnormal antibody response
- Surgical removal of the thymus gland.
The most important advance in the treatment of MG has been the development of the immune modulating treatments. Immunomodulatory drugs that are used for MG include cortisone-like agents, and several other immunosuppressive drugs. Other treatments that are often useful for temporary benefit (weeks to months) include intravenous administration of gamma globulin (IVIg), or plasma exchange, which temporarily reduces the level of antibodies in the system. Most often, the physician will use combinations of these treatments to produce improvement over the short term as well as the long term, while minimizing adverse side effects. The thymus gland is a structure in the front of the chest behind the breast bone that is concerned with immune regulation, and is abnormal in about 75 percent of patients with MG. In about 15 percent of these individuals there may be a tumor of the thymus gland, which can be visualized by CT or MRI scanning of the chest. If present, the tumor, which is usually benign, should be removed to prevent local spread. Surgical removal of the thymus gland may provide benefit in many patients without tumors. It should be carried out by a team of surgeons and neurologists with experience in managing MG patients who have this procedure.
At Johns Hopkins, our Muscular Dystrophy team has an extensive record of managing patients with MG. Many of the treatments now in widespread use have been developed by our experts. Our center is generally considered to be the premier center for management of MG.