An autoimmune disorder causing fluctuating weakness of the muscles, especially of the eyes, face, throat, and limbs
- Most common between the ages of 20 and 40 in females and 50 and 70 in males
- More common in females
- Stress may aggravate the symptoms
- Genetics is not a significant factor
Myasthenia gravis is a rare autoimmune disorder in which the immune system produces abnormal antibodies that block the receptors in muscles that receive nerve impulses. As a result, the affected muscles fail to respond or respond only weakly to nerve impulses. The muscles of the face, throat, and those that control eye and eyelid movements are most often affected, causing speech problems, double vision, and drooping eyelids. However, other muscles may be affected, including those in the arms and legs and, rarely, the respiratory muscles.
Myasthenia gravis is a long-term condition that varies in severity. It is more common in women, especially in those aged 20 to 40 years. In men, the disorder tends to occur between the ages of 50 and 70. In the UK, it affects about 1 in 10,000 people.
What are the causes?
About 3 in 4 affected people have an abnormality of the thymus gland, which is part of the body’s immune system and is located at the base of the neck just underneath the breastbone. In a number of these people, myasthenia gravis is associated with a thymoma, a tumour of the thymus gland. Myasthenia gravis may also be associated with other autoimmune disorders, such as the joint disorder rheumatoid arthritis and pernicious anaemia, a type of megaloblastic anaemia. Certain drugs, such as penicillamine (see Antirheumatic drugs) may worsen the symptoms of myasthenia gravis.
What are the symptoms?
The symptoms of myasthenia gravis usually develop gradually over several weeks or months and tend to fluctuate. In some cases, symptoms appear suddenly. Symptoms may include:
Difficulty chewing and swallowing.
Weakness in the arm and leg muscles.
Symptoms usually improve with rest but worsen when the affected muscles are used. Symptoms may also be made worse by stress and, in women, by menstruation. Rarely, if respiratory muscles are weakened, life-threatening breathing difficulties may develop.
About 1 in 7 babies born to mothers with myasthenia gravis develops symptoms immediately after birth, but these usually disappear over a few weeks.
How is it diagnosed?
Your doctor may suspect myasthenia gravis from your symptoms. To confirm the diagnosis, you will probably have a blood test to look for the antibodies that block the receptors in muscles. In some cases, you may also be given an injection that increases the levels of neurotransmitters, the chemicals responsible for conveying nerve impulses from nerves to muscle receptors. If there is a rapid but only temporary improvement in muscle strength, you probably have myasthenia gravis. You may also have electromyography (see Nerve and muscle electrical tests) to look for muscle weakness by measuring electrical activity. CT scanning of the chest may also be carried out to look for a thymoma.
What is the treatment?
Your doctor may prescribe drugs that raise the levels of neurotransmitters. If your symptoms are mild, this treatment may control them sufficiently. If your symptoms are persistent, you may be prescribed oral corticosteroids or other immunosuppressant drugs, such as azathioprine, to reduce antibody production. Treatment with oral corticosteroids may be started in hospital because the drugs may have an adverse effect initially, and symptoms may become worse.
In severe cases, regular plasmapheresis may be necessary. In this procedure, plasma (the fluid part of the blood) is taken from the body, treated to remove the abnormal antibodies, and replaced. Alternatively, a procedure called IVIg, may be used, which involves infusing into a vein large numbers of normal antibodies. If breathing becomes difficult, mechanical ventilation may be needed (see Intensive therapy unit).
If a tumour of the thymus is found, surgery will be performed to remove the thymus gland. Some people under the age of 45 with severe symptoms but no thymoma may also benefit from surgical removal of the thymus gland.
People with myasthenia gravis should avoid extremes of temperature, infections, stress, and becoming overtired, all of which intensify muscle weakness.
What is the prognosis?
About 8 in 10 people with myasthenia gravis can be cured or their symptoms substantially improved with treatment. In a few people, the symptoms disappear spontaneously after about a year. Rarely, if the respiratory muscles are involved, the disorder may be fatal.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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