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Myasthenia gravis (MG)

Myasthenia gravis (MG) is a chronic autoimmune neuromuscular disease marked by varying degrees of weakness of the skeletal (voluntary) muscles of the body. The name myasthenia gravis, which is Latin and Greek in origin, literally means "grave muscle weakness." With current therapies, however, most cases of myasthenia gravis are not as grave as the name implies. In fact, most individuals with myasthenia gravis have a normal life expectancy.

MG 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 from 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. These antibodies are produced by the body's own immune system. Myasthenia gravis is an autoimmune disease in which the immune system, which normally protects the body from foreign organisms, mistakenly attacks itself.

Myasthenia gravis occurs in all ethnic groups and both genders. It most commonly affects young adult women (under 40) and older men (over 60), but it can occur at any age. MG is not directly inherited nor is it contagious. Occasionally, the disease may occur in more than one member of the same family.

Symptoms

The hallmark of myasthenia gravis is muscle weakness that increases during periods of activity and improves after periods of rest. Although myasthenia gravis may affect any voluntary muscle, muscles that control eye and eyelid movement, facial expression and swallowing are most frequently affected. The onset of the disorder may be sudden and symptoms often are not immediately recognized as myasthenia gravis.

The degree of muscle weakness involved in myasthenia gravis varies greatly among individuals, ranging from a localized form limited to eye muscles (ocular myasthenia), to a severe or generalized form in which many muscles – sometimes including those that control breathing – are affected. Symptoms, which vary in type and severity, may include a drooping of one or both eyelids, blurred or double vision due to weakness of the muscles that control eye movements, unstable or waddling gait, a change in facial expression, difficulty swallowing, shortness of breath, impaired speech and weakness is the arms, hands, fingers, legs and neck.

Diagnosis

Your doctor will review your medical history and perform physical and neurological examinations, looking for impairment of eye movements or muscle weakness without any changes in your ability to feel things.

If your doctor suspects myasthenia gravis, several tests can confirm the diagnosis. A special blood test can detect the presence of immune molecules or acetylcholine receptor antibodies. Most patients with myasthenia gravis have abnormally elevated levels of these antibodies.

Recently, a second antibody, called the anti-MuSK antibody, has been found in about 30 to 40 percent of individuals with myasthenia gravis who do not have acetylcholine receptor antibodies. This antibody can also be tested for in the blood.

The edrophonium test uses intravenous administration of edrophonium chloride to very briefly relieve weakness in people with myasthenia gravis. The drug blocks the breakdown of acetylcholine and temporarily increases the levels of acetylcholine at the neuromuscular junction.

Other methods to confirm the diagnosis include a type of nerve conduction study that tests for specific muscle fatigue by repetitive nerve stimulation.

Single fiber electromyography (EMG) can also detect impaired transmission of signals from the nerves to muscles. EMG measures the electrical potential of muscle cells when single muscle fibers are stimulated by electrical impulses.

Treatment

  • Anticholinesterase medications
  • Immunosuppressive drugs
  • Thymectomy when indicated
  • Plasmapheresis


Today, myasthenia gravis can usually be controlled. 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, azathioprine, cyclosporin, mycophenolate mofetil and tacrolimus may also be used. These medications improve muscle strength by suppressing the production of abnormal antibodies.

Thymectomy, the surgical removal of the thymus gland, which often is abnormal in individuals with myasthenia gravis, reduces symptoms in some individuals without thymoma (tumors of the thymus gland) and may cure some people, possibly by rebalancing the immune system. Thymectomy is recommended for individuals with thymoma.

Other therapies used to treat myasthenia gravis include plasmapheresis, a procedure in which serum containing the abnormal antibodies is removed from the blood while cells are replaced, and high-dose intravenous immune globulin, which temporarily modifies the immune system by adding antibodies from donated blood. These therapies may be used to help during especially difficult periods of weakness. Your neurologist will determine which treatment option is best for you depending on the severity of the weakness, the muscles affected and your age and other medical problems.

Source: National Institute of Neurological Disorders and Stroke


For More Information

Myasthenia Gravis Foundation of America
American Autoimmune Related Diseases Association, Inc.
Muscular Dystrophy Association
National Institute of Neurological Disorders and Stroke
MUMS National Parent-to-Parent Network
New Horizons Un-Limited, Inc.
Genetic and Rare Diseases (GARD) Information Center
European Society for Immunodeficiencies
Autoimmunity Community

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