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myasthenia gravisMyasthenia gravis (literally "serious muscle-weakness"; from Greek μύς "muscle", σθένεια "weakness", and Latin gravis "serious") is an autoimmune disorder, in which weakness is caused by circulating antibodies that block acetylcholine receptors at the post-synaptic neuromuscular junction,[1] inhibiting the stimulative effect of the neurotransmitter acetylcholine. Myasthenia gravis can lead to fluctuating muscle weakness and fatiguability which can lead to a variety of symptoms, some of which can be fatal.

In neonatal myasthenia, the fetus may acquire immune proteins (antibodies) from a mother affected with myasthenia gravis. Generally, cases of neonatal myasthenia gravis are transient (temporary) and the child's symptoms usually disappear within 2-3 months after birth. Other children develop myasthenia gravis indistinguishable from adults. Myasthenia gravis in juveniles is common. Rarely, children may show signs of congenital myasthenia or congenital myasthenic syndrome. These are not autoimmune disorders, but are caused by defective genes that produce proteins in the acetylcholine receptor or in acetylcholinesterase.

Myasthenia Gravis is associated with various autoimmune diseases, including:

Seropositive and "double-seronegative" patients often have thymoma or thymic hyperplasia. However, anti-MuSK positive patients do not have evidence of thymus pathology.

Risk and Probability of Myasthenia Gravis

  • Myasthenia gravis is a relatively rare disease affected just 200-400 per million people. [1]
  • Myasthenia gravis occurs in all ethnic groups and both genders.
  • Myasthenia gravis is not directly inherited nor is it contagious. Occasionally, the disease may occur in more than one member of the same family. Myasthenia gravis most commonly affects young adult women (under 40) and older men (over 60), but myasthenia gravis can occur at any age.

Causes of Myasthenia Gravis

Myasthenia gravis is caused by a defect in the transmission of nerve impulses to muscles. It occurs when normal communication between the nerve and muscle is interrupted at the neuromuscular junction - the place where nerve cells connect with the muscles they control. Normally when impulses travel down the nerve, the nerve endings release a neurotransmitter substance called acetylcholine. Acetylcholine travels through the neuromuscular junction and binds to acetylcholine receptors which are activated and generate a muscle contraction.

In myasthenia gravis, antibodies block, alter, or destroy the receptors for acetylcholine at the neuromuscular junction which prevents the muscle contraction from occurring. Individuals with seronegative myasthenia gravis have no antibodies at all to receptors for acetylcholine and muscle-specific kinase, which is involved in cell signaling and the formation of the neuromuscular junction.  These antibodies are produced by the body's own immune system. Thus, myasthenia gravis is an autoimmune disease because the immune system - which normally protects the body from foreign organisms - mistakenly attacks itself.

Myasthenia Gravis Signs and Symptoms

The hallmark of myasthenia gravis is fatiguability. Muscles become progressively weaker during periods of activity and improve after periods of rest. Muscles that control eye movement, eyelid movement, facial expression, talking, chewing and swallowing are especially susceptible. The muscles that control breathing and neck and limb movements can also be affected. Often the physical examination is within normal limits.[2]

The onset of myasthenia gravis can be sudden. Often symptoms of myasthenia gravis are intermittent, subtle and/or variable and therefore the diagnosis of myasthenia gravis may be delayed.

In most cases of myasthenia gravis the first noticeable symptom is weakness of the eye muscles. For many others with of myasthenia gravis, slurred speech and/or difficulty in swallowing and chewing may be the first signs of myasthenia gravis.

The commonality of those afflicted with myasthenia gravis is muscle weakness that can improve with rest. The degree of muscle weakness involved in of myasthenia gravis can vary among patients afflicted with myasthenia gravis. Muscle weakness may range from a localized form, limited to eye muscles (ocular myasthenia), to a severe or generalized form in which many muscles are affected.  In severe cases of  myasthenia gravis the muscles that control breathing may be affected. In myasthenic crisis a paralysis of the respiratory muscles occurs, necessitating assisted ventilation to sustain life. In patients whose respiratory muscles are already weak, crises may be triggered by infection, fever, an adverse reaction to medication, or emotional stress. Since the heart muscle is stimulated differently, it is never affected by myasthenia gravis.

Symptoms may vary in type and severity. Myasthenia gravis symptoms may include asymmetrical ptosis (a drooping of one or both eyelids), diplopia (double vision) due to weakness of the muscles that control eye movements, shortness of breath and dysarthria (impaired speech, often nasal due to weakness of the velar muscles) weakness in arms, hands, fingers, legs, and neck, a change in facial expression, dysphagia (difficulty in swallowing), and unstable gait, waddling gait.

Myasthenia gravis is more common in families with other autoimmune diseases. A familial predisposition is found in 5% of the cases. This is associated with certain genetic variations such as an increased frequency of HLA-B8 and DR3. People with MG suffer from co-existing autoimmune diseases at a higher frequency than members of the general population. Of particular mention is co-existing thyroid disease where episodes of hypothyroidism may precipitate a severe exacerbation.

The acetylcholine receptor is clustered and anchored by the Rapsyn protein, research in which might eventually lead to new treatment options [3].

Diagnosing Myasthenia Gravis 

Myasthenia gravis can be a difficult diagnosis, as the symptoms can be subtle and hard to distinguish from both normal variants and other neurological disorders.[2]  Because weakness is a common symptom of many other disorders, the diagnosis is often missed in people who experience mild weakness or in those individuals whose weakness is restricted to only a few muscles.

Unfortunately, a delay in diagnosis of one or two years is not unusual in cases of myasthenia gravis.

The first steps of diagnosing myasthenia gravis include a review of the individual's medical history, and physical and neurological examinations. The signs a physician must look for are impairment of eye movements or muscle weakness without any changes in the individual's ability to feel things. If the doctor suspects myasthenia gravis, several tests are available to confirm the diagnosis.

Physical Examination for Myasthenia Gravis Diagnosis

Muscle fatigability can be tested for many muscles[4]. A thorough investigation includes:
  • looking at the feet while lying on the back for 60 seconds
  • looking upward and sidewards for 30 seconds: ptosis and diplopia.
  • keeping the arms stretched forward for 60 seconds
  • 5 situps, lying down and sitting up completely
  • 10 deep knee bends
  • walking 30 steps on both the toes and the heels
  • "Peek sign": after complete initial apposition of the lid margins, they quickly (within 30 seconds) start to separate and the sclera starts to show[2]

Blood Test for for Myasthenia Gravis Diagnosis

A special blood test can detect the presence of immune molecules or acetylcholine receptor antibodies [3]. Most patients with myasthenia gravis have abnormally elevated levels of these antibodies. However, antibodies may not be detected in patients with ocular myasthenia ( myasthenia gravis limited to the eye muscles). The test may be negative in up to 50% of the cases.

Another test is called the single fiber electromyography (EMG), in which single muscle fibers are stimulated by electrical impulses, can also detect impaired nerve-to-muscle transmission. Electromyography (EMG) measures the electrical potential of muscle cells.  Muscle fibers in myasthenia gravis, as well as other neuromuscular disorders, do not respond as well to repeated electrical stimulation compared to muscles from normal individuals. By finding two muscle fibers belonging to the same motor unit and measuring the temporal variability in their firing patterns (i.e. their 'jitter'), the diagnosis can be made. The single fiber electromyography test is considered to be the most sensitive (although not the most specific) test for myasthenia gravis[2]

Edrophonium Test for Myasthenia Gravis Diagnosis

The "edrophonium test" is infrequently performed to identify myasthenia gravis; its application is limited to the situation when other investigations do not yield a conclusive myasthenia gravis diagnosis. The edrophonium test approach requires the intravenous administration of edrophonium chloride (Tensilon, Reversol), a drug that blocks the degradation (breakdown) of acetylcholine and temporarily increases the levels of acetylcholine at the neuromuscular junction. In people with myasthenia gravis involving the eye muscles, edrophonium chloride will briefly relieve weakness[5]. Other methods to confirm the diagnosis include a version of nerve conduction study which tests for specific muscle "fatigue" by repetitive nerve stimulation. This test records weakening muscle responses when the nerves are repetitively stimulated. Repetitive stimulation of a nerve during a nerve conduction study may demonstrate decrements of the muscle action potential due to impaired nerve-to-muscle transmission.

Pulmonary Function Testing for Myasthenia Gravis

A special examination called pulmonary function testing - which measures breathing strength - helps to predict whether respiration may fail and lead to a myasthenic crisis may also be used[7]. A myasthenic crisis occurs when the muscles that control breathing weaken to the point that ventilation is inadequate, creating a medical emergency and requiring a respirator for assisted ventilation. In patients whose respiratory muscles are weak, crises - which generally call for immediate medical attention - may be triggered by infection, fever, or an adverse reaction to medication.During pulmonary function testing (spirometry) the forced vital capacity may be monitored at intervals in order not to miss a gradual worsening of muscular weakness. Severe myasthenia may cause respiratory failure due to exhaustion of the respiratory muscles.

Chest X-ray for Myasthenia Gravis Diagnosis

A chest X-ray is frequently performed; it may point towards alternative diagnoses (e.g. Lambert-Eaton due to a lung tumor) and comorbidity. It may also identify widening of the mediastinum suggestive of thymoma, but a computed tomography (CT) may be used to identify an abnormal thymus gland or the presence of a thymoma.  A CT is a more sensitive chest x-ray test that may be used to identify thymomas, and are generally done for this reason [6].

Muscle Biopsies for Myasthenia Gravis Diagnosis

Muscle biopsy is only performed if the Myasthenia Gravis diagnosis is in doubt and a muscular condition is suspected [3].

Myasthenia Gravis Treatment

Myasthenia gravis is a treatable disease and can be controlled. There are several myasthenia gravis therapies available to help reduce and improve muscle weakness.

Treatment is by medication and/or surgery. Medication consists mainly of cholinesterase inhibitors to directly improve muscle function and immunosuppressant drugs to reduce the autoimmune process. Thymectomy is a surgical method to treat MG. For emergency treatment, plasmapheresis or IVIG can be used as a temporary measure to remove antibodies from the blood circulation.

Medications used to treat the disorder include anticholinesterase agents such as neostigmine and pyridostigmine, which help improve neuromuscular transmission and increase muscle strength. Immunosuppressive drugs such as prednisone, cyclosporine, and azathioprine may also be used. These medications improve muscle strength by suppressing the production of abnormal antibodies. They must be used with careful medical follow up because they may cause major side effects. Some cases of myasthenia gravis may go into remission temporarily and muscle weakness may disappear completely so that medications can be discontinued.

Thymectomy, the surgical removal of the thymus gland (which often is abnormal in myasthenia gravis patients), reduces symptoms in more than 70 percent of patients without thymoma and may cure some individuals, possibly by re-balancing the immune system. . Stable, long-lasting complete remissions are the goal of thymectomy. Other therapies used to treat myasthenia gravis include plasmapheresis, a procedure in which abnormal antibodies are removed from the blood, and high-dose intravenous immune globulin, which temporarily modifies the immune system and provides the body with normal antibodies from donated blood. These therapies may be used to help individuals during especially difficult periods of weakness. A neurologist will determine which treatment option is best for each individual depending on the severity of the weakness, which muscles are affected, and the individual's age and other associated medical problems.

Myasthenia Gravis Prognosis

It should be noted that myasthenia gravis is not usually a progressive disease.Myasthenia gravis may seem to involve only certain muscles, especially the muscles that move the eye and lift the lid. It has been felt that when myasthenia gravis fails to cause general muscle weakness over 2 years or more that the chances of developing weakness in other non-ocular muscles in the future is low. This is probably true but not a guarantee that systemic weakness will not occur. The symptoms may come and go, but the symptoms usually do not get worse as the patient ages. For some, the symptoms decrease after a span of 3-5 years[8]. It remains very important, that if problems with breathing or swallowing ever develop, it needs to be brought to the attention of your physician immediately.

Quality of life can vary depending on the myasthenia gravis severity and the cause of the disease With myasthenia gravis treatment, myasthenia gravis patients may have a normal life expectancy ( except for those with a malignant thymoma whose lesser life expectancy is on account of the thymoma itself and is otherwise unrelated to the myasthenia).

The drugs used to control myasthenia gravis either diminish in effectiveness over time (cholinesterase inhibitors) or cause severe side effects of their own (immunosuppressants), therefore life expectancy and/or quality of life may be affected.

Myasthenia Gravis Organizations and Resources

Myasthenia Gravis Foundation of America, Inc.
355 Lexington Avenue
New York, NY   10017-6603
This e-mail address is being protected from spam bots, you need JavaScript enabled to view it
http://www.myasthenia.org
Tel: 800-541-5454 212-297-2156
Fax: 212-307-9047

Muscular Dystrophy Association
3300 East Sunrise Drive
Tucson, AZ   85718-3208
This e-mail address is being protected from spam bots, you need JavaScript enabled to view it
http://www.mda.org
Tel: 520-529-2000 800-344-4863
Fax: 520-529-5300

Myasthenia Gravis Publications and Information

  • Myasthenia Gravis Fact Sheet. The Myasthenia Gravis information sheet compiled by the National Institute of Neurological Disorders and Stroke (NINDS).

Myasthenia Gravis References

  1. Conti-Fine BM, Milani M, Kaminski HJ (2006). "Myasthenia gravis: past, present, and future". J. Clin. Invest. 116 (11): 2843-54. doi:10.1172/JCI29894. PMID 17080188. 
  2. Scherer K, Bedlack RS, Simel DL. (2005). "Does this patient have myasthenia gravis?". JAMA 293 (15): 1906-14. doi:10.1001/jama.293.15.1906. PMID 15840866. 
  3. Losen M, Stassen MH, Martínez-Martínez P, et al. (2005). "Increased expression of rapsyn in muscles prevents acetylcholine receptor loss in experimental autoimmune myasthenia gravis". Brain 128: 2327-37. PMID 16150851. 
  4. Baets, M.H.; H.J.G.H. Oosterhuis (1993). Myasthenia gravis. DRD Press. pp. 158. ISBN 3805547366. 
  5. Seybold ME (1986). "The office Tensilon test for ocular myasthenia gravis". Arch Neurol43: 842-3. PMID 3729766. 
  6. de Kraker M, Kluin J, Renken N, Maat AP, Bogers AJ (2005). "CT and myasthenia gravis: correlation between mediastinal imaging and histopathological findings". Interact Cardiovasc Thorac Surg 4: 267-71. PMID 17670406. 
  7. Thieben MJ, Blacker DJ, Liu PY, Harper CM Jr, Wijdicks EF (2005). "Pulmonary function tests and blood gases in worsening myasthenia gravis". Muscle Nerve 32: 664-667. PMID 16025526
  8. Juel VC. (2004). "Myasthenia gravis: management of myasthenic crisis and perioperative care.". Semin Neurol 24 (1): 75-81. PMID 15229794
  9. Calhoun R, et al. (1999). "Results of transcervical thymectomy for myasthenia gravis in 100 consecutive patients.". Annals of Surgery 230 (4): 555-561. doi:10.1097/00000658-199910000-00011. PMID 10522725
 
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