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Posted Mar 28 2010 12:00am

          Myasthenia gravis (MG) is an autoimmune disease, which means the body’s defense mechanism, the immune system, begins to attack the body’s own tissues instead of foreign invaders, such as viruses.  It is a chronic (long-lasting) and rare disease that affects the way muscles respond to signals from nerves, leading to muscle weakness.  In myasthenia gravis, the immune system attacks the acetylcholine receptors with specific antibodies.  Some of the receptors are destroyed or blocked, which means that the chemical message cannot be received.  Therefore, muscles do not contract properly and become weak.  It has been estimated that up to 80% of the receptors can be damaged in this disease.  The disease can occur at any age, but it mainly affects women between ages 20 and 40.  After age 50, men are more likely to get the disease.

          In early stages, the disease primarily affects muscles that control eye movement (extraocular muscles) and those that control facial expression, chewing and swallowing.  If untreated, the disorder may affect muscles that control breathing (respiration), causing acute respiratory failure.  Symptoms of myasthenia gravis vary in severity, may come and go, and often worsen with activity and improve with rest.  These signs can take many forms, and they may be more or less severe for different people.  Symptoms may include some or all of the following:

  • Muscles that become progressively weaker with prolonged use
  • Muscle weakness that seems better in the morning or after resting and worse after activity or at the end of the day
  • Drooping eyelids
  • Blurred or double vision
  • A change in facial expressions, such as a smile that looks more like a grimace
  • Jaw muscles that tire easily, such as when chewing gum or meat
  • Difficulty swallowing
  • Slurred or impaired speech
  • Weakness in arms, legs, fingers, hands or neck
  • Shortness of breath

          No one knows what causes the body to attack its own cells.  The thymus, a gland located in the chest just above the heart, is involved in immunity early in life and seems to play an important part in myasthenia gravis, although its precise role is not yet understood.  The thymus is abnormal in most people with MG, and about 10% to 15% of people with the disorder have a benign (noncancerous) tumor of the thymus.  Although myasthenia gravis is not believed to be inherited or contagious, babies born to mothers who have myasthenia gravis have an increased risk of having the disease at birth.  This is most likely because the baby acquires the antibodies that attack the body’s cells from the mother during gestation.  Usually the baby’s symptoms go away within a few weeks after birth. 

          Myasthenia gravis is one of the most treatable neuromuscular disorders.  The body attacking its own muscles is hard to grasp and choices of treatment depend on several factors, including age, overall health, severity of disease and rate of disease progression.  Anticholinesterase medications such as neostigmine (Prostigmin®) and pyridostigmine) are usually prescribed.  These drugs prevent ACh destruction and increase the accumulation of ACh at neuromuscular junctions, improving the ability of the muscles to contract.  Corticosteroids (e.g., prednisone) suppress the antibodies that block AChR at the neuromuscular junction and may be used in conjunction with anticholinesterase. Corticosteroids improve symptoms within a few weeks and once improvement stabilizes, the dose is slowly decreased.  Immunosuppressants such as azathioprine (Imuran®) and cyclophosphamide (Neosar®) are used to treat generalized MG when other medications fail to reduce symptoms.  Plasmapheresis, or plasma exchange, is used to modify the immune system malfunction.  It can be used to treat severe worsening of symptoms (exacerbations) or in preparation for surgery (thymectomy) in patients who have myasthenia gravis.

          Because the cause of myasthenia gravis is unknown, there is no way to prevent it. However, once the disease has developed, there may be ways to help reduce episodes of worsening symptoms or flare-ups.  These prevention tips include giving yourself plenty of rest, avoiding strenuous activities, staying away from excessive heat or cold, lessening exposure to emotional stress and infection.  You should also be vaccinated against common infections, such as influenza.  Working with your doctor is extremely important so that your reactions to prescription medications can be monitored.  Some drugs commonly prescribed for other problems, such as infections, heart disease or hypertension, may make myasthenia gravis worse.  You may need to choose alternative therapies or avoid some medications entirely.

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