Myasthenia gravis is an autoimmune disease in which antibodies attack normal skeletal muscle tissue and render it weak. It occurs in both children and adults and can affect different muscle groups in the body. Symptoms include double vision, droopy eyelids, easy muscular fatigue and breathing/swallowing difficulty. The symptoms tend to worsen with increased activity and improve with rest. The eyelid and extra-ocular muscles are commonly involved in myasthenia, either in isolation or in conjunction with other skeletal muscles [See figure 1].
The condition develops when auto-antibodies attack normal skeletal muscle tissue. Nerve endings that innervate skeletal muscle typically release a chemical, acetylcholine, to induce the muscles to contract. In myasthenia, antibodies attach themselves to the receptors for the acetylcholine on the skeletal muscle. Thus, when acetylcholine is released from a nerve and tries to induce contraction in a muscle, the receptors cannot attach to the acetylcholine so muscle weakness and fatigue results.
Myasthenia is an uncommon disease. The incidence is estimated at 20/100,000. There appears to be no racial or sex predilection in myasthenia. It is not a directly hereditary condition.
Approximately 90% of people with myasthenia gravis have ocular involvement and 50% present with ocular symptoms. Double vision from eye misalignment (strabismus) and drooping of the eyelids (ptosis) are the most common symptoms. These symptoms classically vary and worsen throughout the day. The double vision/ptosis may be present in the evening while not present in the morning. Less common symptoms are facial weakness, chewing and swallowing difficulty, respiratory weakness, and arm and leg muscle fatigue.
The diagnosis of myasthenia can be challenging. No one test is positive 100% of the time.
Examination findings are an important tool to make the diagnosis. Ice testing is another way to diagnose myasthenia and can sometimes be done in your ophthalmologist’s office. Most other testing such as administration of medications like Tensilon or Prostigmin, or electromyography (EMG) are most often performed by a neurologist. Laboratory testing such as acetlylcholine receptor antibody panels can be done but are unfortunately positive in only about 50% of patients with myasthenia. [See figure 2].
Myasthenia can occur in association with other autoimmune diseases most commonly thyroid disease, rheumatoid arthritis, and diabetes. A thymoma in the chest cavity is another association.
Myasthenia is rarely cured, but the symptoms can be improved with a number of different medical or surgical interventions. The main goal of treatment from a biochemical perspective is to increase the amount of acetylcholine available to the receptors on the nerve endings in the muscle. This can be done either by increasing the amount of the acetylcholine itself by minimizing its breakdown, or by destroying the antibodies that are attacking the acetylcholine receptors. Mestinon, or pyridostigmine, is a cholinesterase inhibitor which blocks the breakdown of acetylcholine. This is a commonly used medication to treat the symptoms of myasthenia, but it has been demonstrated that this medication is more effective at alleviating the non-ocular symptoms than the ocular ones. Immunosuppressive agents such as prednisone can be effective at blocking the immune response involved in myasthenia. Plasmapheresis is a procedure whereby the offending antibodies are removed directly from the bloodstream. This treatment frequently needs to be repeated as symptoms can return after time. IVIg is an intravenous treatment that affects the production and/or function of the abnormal antibodies. In some cases surgical removal of a thymoma can improve symptoms.
The ptosis and strabismus related to myasthenia can at times be helped with surgery; Surgical intervention is sometimes advised if the ptosis and/or strabismus deviation has been stable for at least one year.
Contact the Myasthenia Gravis Foundation of America, Inc.
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