Muscle Diseases - Myasthenia gravis
Myasthenia gravis is characterized by muscle weakness and an abnormal muscle fatigability (pathologic fatigue); patients are abnormally weak after exercise or at the end of the day. The disease affects males and females at any period, from infancy to old age, but it is most common during the second to the fourth decades. There is no complete explanation for myasthenia gravis, but it is believed that there is some defect in the transmission of a nerve impulse to the muscle ( myoneural junction defect ). The disease may occur spontaneously, during pregnancy, or following an acute infection, and there appears to be a curious association with diseases in which there is an immunologic abnormality, such as tumors of the thymus gland, increased or decreased activity of the thyroid gland, and rheumatoid arthritis.
Usually there is an insidious onset of generalized weakness or weakness confined to small groups of muscles. Normal muscle power may be present early in the day, but as the hours pass the patient notices that one or both eyelids droop ( ptosis ) or he may see double images ( diplopia ). If he rests and closes his eyes for a short time, the ptosis and the diplopia clear up, only to return after further muscle activity. The weakness can also be seen in the trunk or the limbs, and some patients have involvement of the muscles used in speaking, chewing, or swallowing ( bulbar muscles). Some are weak all the time and have an increase in that weakness the longer they use their muscles. The muscles used in breathing may be affected in patients with severe myasthenia, and these patients must be maintained on a respirator for varying periods of time.
Myasthenia gravis is treated with drugs that assist in the transmission of the nerve impulse to the muscle. Medication is very effective, but the patient should be carefully observed by the physician to determine that the drug dose and the time of administration are adjusted so that the patient may have the benefit of maximal muscle power. Surgical removal of the thymus gland, thymectomy , may prove of benefit to some patients in lessening the symptoms of muscle weakness; however, not all patients have clinical improvement of the disease after thymectomy, and patients must be selected very carefully by the physician. Myasthenia gravis is another chronic disease in which long-term careful observation by the physician is most important in obtaining an optimal medical and psychological outcome.