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Myasthenia Gravis

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Myasthenia gravis is an autoimmune disease that affects neuromuscular transmission. Antibodies attack acetylcholine receptors in the muscles resulting in weakness and fatigue. Symptoms typically first present as ocular myasthenia, intermittent ocular muscle fatigue causes diplopia and ptosis of the upper eyelid. Other symptoms may include: difficulty breathing due to fatigue of the chest wall muscles, trouble with chewing and swallowing, difficulty talking, drooping of the head and difficulty performing daily activities (A.D.A.M. Medical Encyclopedia, 2011). While it is the most common neuromuscular transmission disorder, myasthenia gravis only affects around one out of every ten-thousand persons with a higher incidence in young adult females (Cohen, Camilo, Lawrence, & Thomas, 2008).
Normal Neuromuscular Transmission Muscles are controlled by nerve impulses originating in the brain. These impulses are sent via nerve fibers to the muscles being controlled. As the impulse reaches the end of the nerve fiber, vesicles release the neurotransmitter acetylcholine by exocytosis. Acetylcholine crosses the neuromuscular junction to receptor sites on the end plate membrane of the muscle. Muscles contract as a result of the acetylcholine binding to these receptors (Myasthenia Gravis Foundation of America, Inc.). To terminate the neuromuscular transmission, acetycholinesterase and diffusion deactivates the acetylcholine.
Pathophysiology and Treatment of Myasthenia Gravis
In myasthenia gravis, an autoimmune response produces antibodies to acetylcholine receptor sites in muscles resulting in decreased efficiency of neurotransmission (Lindstrom, 2000). Clinical features include a decreased number of acetylcholine receptors and a wider synaptic cleft result in fewer interactions between acetylcholine and its receptor. Combined, these changes to normal neuromuscular transmission lead to the muscular weakness and fatigability associated with myasthenia gravis. As with most autoimmune diseases, symptoms come and go in varying degrees of severity. For example, ptosis of the eyelid, which is a common side effect, is not necessarily a permanent condition. Ptosis may present itself then spontaneously get better. However, due to the variable nature of myasthenia gravis, the ptosis may last days, months, or even years. Symptoms can last indefinitely in severe cases if left untreated.
While there is no cure for myasthenia gravis, the goal is remission. Common treatments include medications, thymectomy, and plasmapheresis. Anticholinesterase inhibitors such as Mestinon are used as a symptomatic treatment option as it only treats the symptoms and does not have an effect on the underlying cause of the disease. Anticholinesterase inhibitors allow acetylcholine to remain at the neuromuscular junction longer so that more receptor sites may be activated. Although it is not clear what role the thymus plays in myasthenia gravis, there are typically abnormalities seen in patients. Removing the thymus frequently reduces and occasionally eliminates symptoms. Plasmapheresis is another available option for treatment. Used for acute therapy, plasmapheresis filters antibody proteins out of the blood. This option shows drastic improvements in patients; however ongoing treatment is required due to the reproduction of antibodies and subsequent recurrence of symptoms (Myasthenia Gravis Foundation of America, Inc.).
Conclusion
Neuromuscular transmission is a relatively simple, straightforward design. In myasthenia gravis, the normal transmission of acetylcholine from the nerve fibers across the neuromuscular junction to the receptor sites of muscles is disrupted causing muscle weakness and fatigue. There is no known cure, however with three separate treatment options available, individualized care aimed at reducing symptoms, controlling the autoimmune response, and removing specific antibodies, can allow people to lead full lives.

References
A.D.A.M. Medical Encyclopedia. (2011, June 18). Mayathenia Gravis. Retrieved from U.S. National Library of Medince: http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001731/
Cohen, J. M., Camilo, F. M., Lawrence, J. M., & Thomas, W. M. (2008). Dartmouth Medical School. Retrieved from Disorders of the Nervous System: http://www.dartmouth.edu/~dons/part_3/chapter_21.html
Lindstrom, J. M. (2000, March 14). Acetylcholine Receptors and Myasthenia. Muscle & Nerve Volume 23 Issue 4 , 453-477.
Myasthenia Gravis Foundation of America, Inc. (n.d.). Treatment for MG. Retrieved March 21, 2013, from http://www.myasthenia.org/WhatisMG/TreatmentforMG.aspx
Myasthenia Gravis Foundation of America, Inc. (n.d.). What is Myathenia Gravis (MG)? Retrieved March 21, 2013, from http://www.myasthenia.org/WhatisMG.aspx

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