What is it?
Myasthenia gravis is characterized by weakness and rapid fatigue of any of the muscles under your voluntary control. The cause of myasthenia gravis is a breakdown in the normal communication between nerves and muscles.
There is no cure for myasthenia gravis, but treatment can help relieve signs and symptoms — such as weakness of arm or leg muscles, double vision, drooping eyelids, and difficulties with speech, chewing, swallowing and breathing.
While myasthenia gravis can affect people of any age, it's more common in women younger than 40 and in men older than 60. The disorder occurs in one or two people per 10,000.
Muscle weakness caused by myasthenia gravis worsens as the affected muscle is used repeatedly. Since symptoms typically improve with rest, your muscle weakness may come and go. While myasthenia gravis can affect any of the muscles that you control voluntarily, certain muscle groups are more commonly affected than others.
In more than half the people who develop myasthenia gravis, their first signs and symptoms involve eye problems, such as:
- Drooping of one or both eyelids (ptosis)
- Double vision (diplopia), which may be horizontal or vertical
- Blurred vision, which may come and go
Face and throat muscles
In about 15 percent of people with myasthenia gravis, the first symptoms involve face and throat muscles, which can cause difficulties with:
- Speaking. Your speech may be very soft or sound nasal, depending upon which muscles have been affected.
- Swallowing. You may choke very easily, which makes it difficult to eat, drink or take pills. In some cases, liquids you're trying to swallow may come out your nose.
- Chewing. The muscles used for chewing may wear out halfway through a meal, particularly if you've been eating something hard to chew, such as steak.
- Facial expressions. Family members may note that you've "lost your smile" if the muscles that control your facial expressions are affected.
Arm and leg muscles
Myasthenia gravis can cause weakness in your arms and legs, but this usually happens in conjunction with muscle weakness in other parts of your body — such as your eyes, face or throat. The disorder usually affects arms more often than legs. However, if it affects your legs, you may waddle when you walk.
Your nerves communicate with your muscles by releasing chemicals, called neurotransmitters, which fit precisely into receptor sites on the muscle cells. In myasthenia gravis, your immune system produces antibodies that block or destroy many of your muscles' receptor sites for a neurotransmitter called acetylcholine. With fewer receptor sites available, your muscles receive fewer nerve signals, resulting in weakness.
It's believed that the thymus gland, a part of your immune system located in the upper chest beneath the breastbone, may trigger or maintain the production of these antibodies. Large in infancy, the thymus is small in healthy adults. But, in some adults with myasthenia gravis, the thymus is abnormally large. Some people also have tumors of the thymus. Usually, thymus gland tumors are noncancerous.
Some factors can make myasthenia gravis worse, including:
- Extreme heat
- Some medications — such as beta blockers, calcium channel blockers, quinine and some antibiotics
Complications of myasthenia gravis are treatable, but some can be life-threatening.
Myasthenic crisis is a life-threatening condition, which occurs when the muscles that control breathing become too weak to do their jobs. Emergency treatment is needed to provide mechanical assistance with breathing. Medications and blood-filtering therapies help people recover from myasthenic crisis, so they can again breathe on their own.
About 15 percent of the people who have myasthenia gravis have a tumor in their thymus, a gland under the breastbone that is involved with the immune system. Most of these tumors are noncancerous.
People who have myasthenia gravis are also more likely to have the following problems:
- Underactive or overactive thyroid. The thyroid gland, located in the neck, secretes hormones that regulate your metabolism. If your thyroid is underactive, your body uses energy more slowly. An overactive thyroid makes your body use energy too quickly.
- Lupus. Lupus is a disease in which your immune system attacks certain parts of your body. Common symptoms include painful or swollen joints, hair loss, extreme fatigue and a red rash on the face.
- Rheumatoid arthritis. This type of arthritis is caused by problems with your immune system. It is most conspicuous in the wrists and fingers, and can result in joint deformities that make it difficult to use your hands.
The key sign that points to the possibility of myasthenia gravis is muscle weakness that improves with rest. Tests to help confirm the diagnosis may include:
Injection of the chemical edrophonium (Tensilon) may result in a sudden, although temporary, improvement in your muscle strength — an indication that you may have myasthenia gravis. Edrophonium acts to block an enzyme that breaks down acetylcholine, the chemical that transmits signals from your nerve endings to your muscle receptor sites.
A blood test may reveal the presence of abnormal antibodies that disrupt the receptor sites where nerve impulses signal your muscles to move.
Repetitive nerve stimulation
This is a type of nerve conduction study, in which electrodes are attached to your skin over the muscles to be tested. Small pulses of electricity are sent through the electrodes to measure the nerve's ability to send a signal to your muscle. To diagnose myasthenia gravis, the nerve will be tested many times to see if its ability to send signals worsens with fatigue.
Single-fiber electromyography (EMG)
Electromyography (EMG) measures the electrical activity traveling between your brain and your muscle. It involves inserting a very fine wire electrode through your skin and into a muscle. In single-fiber EMGs, a single muscle fiber is tested. Most people find this test to be somewhat uncomfortable.
Your doctor may order a CT scan or an MRI to see if there's a tumor or other abnormality in your thymus.