Myasthenia Gravis (MG) FAQ

by Women's Health Information Center

What is myasthenia gravis (MG)?

Myasthenia gravis is an autoimmune disease that affects the transmission of signals from nerves to muscles. The name myasthenia gravis comes from Greek and Latin words meaning "grave muscle weakness." Today, however, most cases of MG are not as "grave" as the name implies. In fact, most people with MG can expect to live normal or nearly normal lives.

The hallmark of MG is muscle weakness that increases during activity and improves after rest. MG often involves muscles that control eye and eyelid movement, facial expression, chewing, talking, and swallowing. The muscles that control breathing and neck and limb movements may also be affected.

The thymus gland, part of the immune system, is abnormal in most MG cases. Some people with MG have benign (noncancerous) tumors of the thymus gland called thymomas.

Some drugs can trigger or worsen MG symptoms.

What causes MG?

MG is caused by a defect in the transmission of nerve signals to muscles. Normally, nerve endings release a substance called acetylcholine that binds or attaches to receptors on the muscle. This leads to muscle contractions. In MG, the body's own immune system produces antibodies that block this transmission.

Who gets myasthenia gravis?

Estimates of the number of people affected by MG vary, ranging from five to 14 people per 100,000.

MG 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. Children sometimes develop MG.

MG is not directly inherited nor is it contagious. Sometimes the disease may occur in more than one member of the same family. If a woman with MG becomes pregnant, sometimes the baby acquires antibodies from the mother and has MG symptoms for a few weeks or months after birth. This is called neonatal myasthenia, and the symptoms can be treated.

In rare cases, myasthenia is caused by a defective gene and appears in infants born to non-myasthenic mothers. This type is called congenital myasthenia.

What is the role of the thymus gland in MG?

The thymus gland, found in the upper chest area beneath the breastbone, is a part of the body's normal immune system. In most adults with MG, the thymus gland is abnormal. Some people with MG develop thymomas or tumors on the thymus gland. Generally thymomas are benign, but they can become malignant (cancerous). The relationship between the thymus gland and MG is not yet fully understood.

What are the signs and symptoms of MG?

The muscles that control eye and eyelid movement, facial expression, and swallowing are most often affected. The onset of the disorder may be sudden. Symptoms often are not immediately recognized as MG.

In most cases, the first noticeable symptom is weakness of the eye muscles. In others, difficulty in swallowing and slurred speech may be the first signs. While rare, first signs of MG can also include difficulty with breathing. The degree of muscle weakness involved in MG varies greatly among persons with this disease. Symptoms, which vary in type and severity, may include:

• Drooping of one or both eyelids (ptosis)
• Blurred or double vision (diplopia) due to weakness of the muscles that control eye movements
• Unstable or waddling gait
• Weakness in arms, hands, fingers, legs, and neck
• Change in facial expression
• Difficulty in swallowing and shortness of breath
• Impaired speech (dysarthria)
• Shortness of breath

How is MG diagnosed?

Unfortunately, a delay in diagnosis of one or two years is not unusual in cases of MG. Weakness is a common symptom of many other disorders. The diagnosis is often missed in people who have mild weakness or in those whose weakness is restricted to only a few muscles.

The first steps of diagnosing MG include a review of the person's medical history and physical and neurological exams. If the doctor suspects MG, several tests are available to confirm the diagnosis.

Antibody blood test. A special blood test can detect the antibodies that prevent nerves from signaling to muscles. While most people with MG have abnormally high levels of these antibodies, some individuals (about 10 percent) can actually test negative for antibodies. And, antibodies may not be detected if only eye muscles are affected.

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