Myasthenia gravis
Myasthenia gravis (MG) is an autoimmune disease that causes muscle weakness.MG affects the neuromuscular junction, interrupting the communication betweennerve and muscle, and thereby causing weakness. A person with MG may have difficulty moving their eyes, walking, speaking clearly, swallowing, and even breathing, depending on the severity and distribution of weakness. Increased weakness with exertion, and improvement with rest, is a characteristic featureof MG. MG affects "voluntary" muscles, which are those muscles under conscious control responsible for movement. It does not affect heart muscle or the "smooth" muscle found in the digestive system and other internal organs.
About 30,000 people in the United States are affected by MG. It can occur atany age, but is most common in women who are in their late teens and early twenties, and in men in their sixties and seventies.
While the trigger for the autoimmune attack is unknown, about 10% of those with MG also have thymomas, or benign tumors of the thymus gland. The thymus isa principal organ of the immune system, and researchers speculate that thymic irregularities are involved in the progression of MG, even in many people without overt thymomas. Other possible factors that may contribute to MG are infections, genes, and abnormal immune system development.
The earliest symptoms of MG often result from weakness of the extraocular muscles, which control eye movements. Symptoms involving the eye (ocular symptoms) include double vision (diplopia), especially when not gazing straight ahead, and difficulty raising the eyelids (ptosis). A person with ptosis may needto tilt their head back to see. Eye-related symptoms remain the only symptoms for about 15% of MG patients. Another common early symptom is difficulty chewing and swallowing, due to weakness in the bulbar muscles, which are in themouth and throat. Choking becomes more likely, especially with food that requires extensive chewing.
Weakness usually becomes more widespread within several months of the first symptoms, reaching their maximum within a year in two-thirds of patients. Weakness may involve muscles of the arms, legs, neck, trunk, and face, and affectthe ability to lift objects, walk, hold the head up, and speak.
Symptoms of MG become worse upon exertion, and better with rest. Heat, including heat from the sun, hot showers, and hot drinks, may increase weakness. Infection and stress may worsen symptoms. Symptoms may vary from day to day andmonth to month, with intervals of no weakness interspersed with a progressive decline in strength.
While there is no cure for myasthenia gravis, there are a number of treatments that effectively control symptoms in most people.
Pyridostigmine (Mestinon) is usually the first drug tried. Like edrophonium,pyridostigmine blocks acetylcholinesterase. It is longer-acting, taken by mouth, and well-tolerated. Loss of responsiveness and disease progression combine to eventually make pyridostigmine ineffective in tolerable doses in many patients.
Thymectomy, or removal of the thymus gland, has increasingly become standardtreatment for MG. Up to 85% of people with MG improve after thymectomy, withcomplete remission eventually seen in about 30%. The improvement may take months or even several years to fully develop. Thymectomy is not usually recommended for children with MG, since the thymus continues to play an important immune role throughout childhood.
Immune-suppressing drugs are used to treat MG if response to pyridostigmine and thymectomy are not adequate. Drugs include corticosteroids such as prednisone, and the non-steroids azathioprine (Imuran) and cyclosporine (Sandimmune).
Plasma exchange may be performed to treat myasthenic crisis or to improve very weak patients before thymectomy. In this procedure, blood plasma is removedand replaced with purified plasma free of autoantibodies. It can produce a temporary improvement in symptoms, but is too expensive for long-term treatment. Another blood treatment, intravenous immunoglobulin therapy, is also usedfor myasthenic crisis. In this procedure, large quantities of purified immuneproteins (immunoglobulins) are injected. For unknown reasons, this leads tosymptomatic improvement in up to 85% of patients. It is also too expensive for long-term treatment.
People with weakness of the bulbar muscles may need to eat softer foods thatare easier to chew and swallow. In more severe cases, it may be necessary toobtain nutrition through a feeding tube placed into the stomach (gastrostomytube).
Most people with MG can be treated successfully enough to prevent their condition from becoming debilitating. In some cases, however, symptoms may worseneven with vigorous treatment, leading to generalized weakness and disability.MG rarely causes early death except when untreated.
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