DEAR DOCTOR: I was diagnosed with myasthenia gravis in 2014 after years of trying to find an answer to my symptoms. The disease is so difficult to diagnose that I'm afraid others have been searching as well, but without answers. Could you shed some light on this disease?
DEAR READER: To understand myasthenia gravis, you have to first understand how the nerves of your body make your muscles contract. Nerves that control muscle contraction attach to the muscle in an area called the neuromuscular junction. There, a nerve releases the neurotransmitter acetylcholine, which then binds to a receptor in the muscle. The binding to the receptor causes a cascade of changes that leads to the contraction of a muscle.
In the majority of people with myasthenia gravis, antibodies attack the acetylcholine receptor at the neuromuscular junction. The attack damages, and can even destroy, these receptors to such an extent that nerve impulses can't cause muscular contraction. Some people have other antibodies that impair the actual formation of the receptor. The thymus gland, a lymphatic gland behind the breastbone, is often the source of the antibodies that lead to myasthenia.
The telltale sign of myasthenia gravis is muscle weakness and muscle fatigue with recurrent muscle use. Patients can have no signs of muscle weakness in the morning, but with repetitive contraction of muscles during the day, the muscles get fatigued and weak. About 50 percent of the time, the muscles of the eye are affected, leading to drooping of the upper eyelid and sometimes double vision. Myasthenia gravis can also cause weakness of the jaw, leading to difficulty chewing or closing the mouth, and weakness of the facial muscles, interfering with facial expressions.
Elsewhere in the body, the disease can affect the muscles of the neck, making it difficult for people to hold up their head, and the upper arms and legs (arms are more often affected than legs). Sometimes, the disease also affects movements of the wrists, fingers and ankles. When severe, myasthenia gravis can impact the respiratory muscles, causing difficulty breathing, and even the muscles involved with swallowing.
Sudden severe weakness from the disease, called myasthenic crisis, can be precipitated by infections, antibiotics and some heart medications. In these situations, breathing difficulties lead to hospitalization and sometimes ventilator use.
Myasthenia is a rare disease, which may be why some doctors don't recognize the symptoms. Each year, myasthenia is diagnosed in eight to 10 out of 1 million people; within the United States, about one out of every 5,000 people are affected. The disease can be diagnosed by blood tests that detect antibodies to the acetylcholine receptor or antibodies to the enzyme that helps form the receptor, called MuSK. Another way to diagnose myasthenia is through repetitive nerve stimulation tests that evaluate whether muscle strength declines.
Muscle weakness can be treated with a medication that increases the amount of acetylcholine at the receptor or through drugs that dampen the immune system in order to limit the receptor-attacking antibodies. In younger patients with enlarged thymus glands, removal of the gland can cure myasthenia. One comforting fact is that in the majority of patients dealing with myasthenia, the symptoms go away with time.
(Robert Ashley, M.D., is an internist and assistant professor of medicine at the University of California, Los Angeles.)
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