Myasthenia gravis is a chronic, progressive neuromuscular, autoimmune disease marked by varying degrees of weakness of the skeletal (voluntary) muscles of the body. The body’s immune system attacks and destroys receptors in the muscles. These receptors bind acetylcholine, the neurotransmitter released from motor neurons. (Turkington & Harris, 2009) The main calling card of myasthenia gravis is muscle weakness that increases during periods of activity and improves after periods of rest. Muscles that control eye and eyelid movements, facial expression, chewing, talking, and swallowing are often, but not always, involved.
The muscles that control breathing and neck and limb movements may also be affected. (Beers, 2003) Causes Myasthenia gravis is thought to be caused by the immune system which produces antibodies that attack one type of receptor in the transmission of nerve impulses, especially on the muscle side of the neuromuscular junction. (Beers, 2003) This impairment is what causes the body to attack its own acetylcholine receptors, which in turn causes an autoimmune reaction. (Beers, 2003) The antibodies destroy the receptor sites more rapidly than the body can replace them.
Muscle weakness occurs when acetylcholine cannot activate enough receptor sites at the neuromuscular junction. (Howard Jr. M. D. , 2006) Why the body attacks it own acetylcholine receptors is unknown. One theory is that the thymus gland may be involved. The thymus gland contains muscle cells with acetylcholine receptors. In the thymus gland, certain cells of the immune system learn to distinguish between the body and foreign substances. The theory is that the thymus tells the immune system to attack acetylcholine receptors. (Beers, 2003) Prevalence
Myasthenia gravis is not directly inherited nor is it contagious and it occurs in all races. (Howard Jr. M. D. , 2006) The disorder occurs most frequently in 20- to 30 year old women. After the age of 40 the frequency is the same in men and women. (Reed, 1991) The disease affects 5 to 14 per 100,000 Americans each year. (Turkington & Harris, 2009) However, MG is considered under-diagnosed and the prevalence is thought to be much higher. (Howard Jr. M. D. , 2006) Pregnant women can pass antibodies against acetylcholine receptors, which circulate in the blood, through the placenta to the fetus.
Neonatal myasthenia happens in 12% of babies born to women with this disorder. The baby has muscle weakness which disappears several days to a few weeks after birth. The remaining 88% of babies are not affected. (Beers, 2003) Myasthenia gravis does not affect the normal growth and development of the fetus. (Howard Jr. M. D. , 2006) Signs and Symptoms The first noticeable symptoms of myasthenia gravis are weakness of the eye muscles, ptosis (drooping eyelid), diplopia (double vision), difficulty in swallowing, or slurred speech. Myasthenia gravis patients usually have expressionless faces and nasal vocal tones. Loeb, 1992) Symptoms vary in type and severity. The severity of weakness fluctuates during the day, usually being least severe in the morning and worse as the day progresses (Howard Jr. M. D. , 2006).
If muscles are used repetitively, the muscles become weak. For example, a construction worker that once used a hammer well becomes weak after hammering for several minutes. However, muscle weakness varies in intensity from hour to hour and day to day, and the course of the disease varies widely. Between 15-20% of people with myasthenia gravis complain of arm and hand weakness; leg muscle weakness is less common. Turkington & Harris, 2009) Hand grip may alternate between weak and normal. Sensation is not affected. Neck muscles may become too weak to support the head in a person with myasthenia gravis. (Loeb, 1992) Progression In mild cases, a person with myasthenia gravis can lead a comparatively normal life. In a few patients, the progression of the disease cannot be stopped. Muscle weakness is progressive; more and more muscles become weak and some muscles lose their function altogether. Progressive weakness of the diaphragm and nearby muscles may produce breathing problems or myasthenia crisis, which can be fatal.
Symptoms of such a crisis include sudden breathing problems and an inability to swallow or speak. (Turkington & Harris, 2009) A crisis also may result from progression of the disease, emotional upset, infection, drugs, surgery, or trauma. (Loeb, 1992) Symptoms depend on the muscle group affected; becoming more intense during menses and after prolonged exposure to sunlight or cold. (Loeb, 1992) Symptoms fluctuated over a relatively short period of time and then became progressively severe for several years (active stage). The active stage is followed by an inactive state in which fluctuations in strength still occurred.
After 15 to 20 years, weakness often becomes fixed and the most severely involved muscles are frequently atrophic (burnt-out stage). (Howard Jr. M. D. , 2006) Diagnosis The first steps in diagnosing myasthenia gravis include a review of the individual’s medical history and physical and neurological examinations. If the doctor suspects myasthenia gravis, several diagnostic tests are available to confirm the diagnosis, including a special blood test that can detect the presence of immune molecules or acetylcholine receptor antibodies. Turkington ; Harris, 2009)Several other tests can be administered to aid in the diagnosis of myasthenia gravis; Edrophonium test, nerve conduction test/repetitive stimulation, and single fiber electromyography (EMG). (Turkington ; Harris, 2009) Sometimes all of these tests are negative in someone whose history and examination still seem to point to a diagnosis of myasthenia gravis. The positive clinical findings should probably take precedence over negative test results. (Howard Jr. M. D. , 2006) Treatment Myasthenia gravis can be symptomatically controlled.
Treatment goals must be individualized according to the severity of disease, the patient’s age and sex, and the degree of functional impairment. The response to any form of treatment is difficult to assess because the severity of symptoms fluctuates. Spontaneous improvement, even remissions, occurs without specific therapy, especially during the early stages of the disease. (Howard Jr. M. D. , 2006) Some medications, such as antichloinesterase drugs, can improve neuromuscular transmission and increase muscle strength, and some suppress the production of abnormal antibodies.
These medications must be used with careful medical follow up because they may cause major side effects and they seem to become less effective as the disease worsens. Corticosteroids can also be beneficial in relieving symptoms. (Turkington & Harris, 2009) Thymectomy, the surgical removal of the thymus gland, improves symptoms in certain individuals, possibly by re-balancing the immune system. This may cause remission in some cases. (Loeb, 1992) When drugs do not provide relief during a myasthenia crisis, plasmapheresis may be used. Beers, 2003) Plasmapheresis is a procedure in which abnormal antibodies are removed from the blood, which temporarily modifies the immune system and provides the body with normal antibodies from donated blood. (Beers, 2003) Prognosis/Prevention With treatment, the outlook for most patients with myasthenia is bright: they can expect to lead normal or nearly normal lives. Some case of myasthenia gravis may go into remission temporarily, and muscle weakness may disappear so that medications can be discontinued. But in a crisis the patient will require immediate emergency medical care.
There is no known cure for MG, but there are effective treatments that allow many people with MG to lead full lives. (Howard Jr. M. D. , 2006) Careful baseline assessment, early recognition and treatment of potential crises, supportive measures, and thorough patient teaching can minimize complications. Continued care is essential because myasthenia gravis is a lifelong condition. No methods of prevention are known.
Beers, M. H. (2003). Myasathenia Gravis. In M. H. Beers, The Merck Manual of Medical Information (pp. 577-579). Whitehouse Station: Merck & Co. , Inc. Howard Jr. M. D. , J. F. 2006, Sept 30). Myasthenia Gravis. Retrieved Dec 1, 2010, from Myasthenia Gravis Foundation of America, Inc. : http://myasthenia. org/hp_clinicaloverview. cfm Loeb, S. (1992). Myasthenia Gravis. In S. Loeb, Professional Guide to Diseases (pp. 610-612). Springhouse: Springhouse Corporation. Reed, K. L. (1991). Myasthenia Gravis. In K. L. Reed, Quick Reference to Occupational Therapy (pp. 337-341). Gaithersburg: Aspen Publishers. Turkington, C. , & Harris, J. R. (2009). Myasthenia Gravis. In C. Turkington, & J. R. Harris, The Encyclopedia of the Brain and Brain Disorders (pp. 245-246). New York: Facts on File, Inc.