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Myasthenia gravis is a chronic disorder 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.

The disorder affects only the function of your muscles, and the muscle weakness you experience improves when you rest. Myasthenia gravis may cause double vision, drooping eyelids, difficulties with speech, chewing, swallowing and breathing, as well as weakness of your limbs.

Myasthenia gravis can affect people of any age, but it's more common in women younger than 40 or older than 70, and in men older than 50. There's no cure for myasthenia gravis, but treatments are available to help control the signs and symptoms of the condition.

Myasthenia gravis can affect any of the muscles that you control voluntarily. It most commonly affects certain muscles, including those of your face, eyes, arms and legs, and those muscles involved in chewing, swallowing and talking. Muscles that control breathing and the movement of your head, arms and legs also can be involved. Signs and symptoms may include:

• Facial muscle weakness, including drooping eyelids
• Double vision
• Difficulty in breathing, talking, chewing or swallowing
• Muscle weakness in your arms or legs
• Fatigue brought on by repetitive motions

The more often a muscle action is repeated, the worse the weakness becomes. In myasthenia gravis, good days may alternate with bad days. Temporary remissions may occur. In rare cases, breathing or swallowing problems worsen markedly, requiring emergency medical care.

When your neuromuscular system functions normally, the chemical acetylcholine transmits nerve impulses to your muscles. At specialized areas of your muscles, called neuromuscular junctions, receptor sites receive nerve impulses and signal your muscles to contract, such as when you raise a spoon to your mouth.

In myasthenia gravis, there's a breakdown in communication between your nerves and your muscles. The culprit is your immune system. For unknown reasons, myasthenia gravis causes your immune system to produce antibodies that block or destroy many of the receptor sites for acetylcholine in your muscles. 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 (benign).

Some factors can make myasthenia gravis worse, including fatigue, illness, stress, extreme heat, and some medications, such as beta blockers, calcium channel blockers, quinine and some antibiotics. Check with your doctor before taking any new medications, even over-the-counter drugs.

If you experience muscle weakness or have difficulty controlling the muscles of your eyes, face and mouth, have trouble breathing or experience fluctuating weakness in your arms and legs, see your doctor. These signs and symptoms could be indications of myasthenia gravis.

Although there's no cure for myasthenia gravis, the outlook for managing its signs and symptoms is good. The earlier you see your doctor, the sooner treatments can be initiated to help you improve your muscle strength, and the sooner you can learn about strategies to help you use your energy in the most efficient ways.

The key sign that points to the possibility of myasthenia gravis is muscle weakness that improves with rest. Tests to confirm the diagnosis may include:

• Neurological examination. This may include testing of your reflexes, muscle strength, muscle tone, senses of touch and sight, gait, posture, coordination, balance and mental skills.
• Blood analysis. A blood test may reveal the presence of abnormal antibodies that disrupt the receptor sites where nerve impulses signal your muscles to move.
• Edrophonium test. 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.
• Nerve conduction studies and single-fiber electromyography. During the first part of this test, a small electrical impulse is applied to your skin, stimulating your nerves in order to test the strength of your muscle contraction. In the second part, a thin-needle electrode inserted into one of your muscles helps measure patterns of electrical activity in your muscle at rest and with slight muscle contraction.

Doctors use a variety of treatments, alone or in combination, to relieve symptoms of myasthenia gravis:

• Medications. Drugs called cholinesterase inhibitors, such as pyridostigmine (Mestinon) and neostigmine (Prostigmin), enhance communication between nerves and muscles. These drugs don't treat the underlying problem, but they do improve muscle contraction and muscle strength.

  Corticosteroids inhibit the immune system, limiting antibody production. Prolonged use of corticosteroids, however, can lead to serious side effects, such as bone thinning, weight gain, diabetes, increased risk of some infections and a redistribution of body fat.

  Your doctor may also prescribe other medications that alter your immune system, such as azathioprine (Imuran), mycophenolate mofetil (CellCept), cyclophosphamide (Cytoxan) or cyclosporine (Sandimmune, Neoral).

• Surgery. It's generally been believed that removal of the thymus gland (thymectomy) brings relief to the majority of people with myasthenia gravis. But only about 25 percent of those who have the surgery go into remission within a year, and significant improvement is often delayed for years. The surgery has generally been recommended for people younger than 60.

  Recent research has cast some doubt on the benefit of thymectomy in people with a tumor in the thymus (thymoma). A large international trial is being conducted to assess whether thymectomy should be routinely recommended for people who have myasthenia gravis, but don't have a thymoma.

• Plasmapheresis (plaz-muh-fuh-RE-sis). This procedure can remedy life-threatening stages of myasthenia gravis. Plasmapheresis involves removal of antibodies from your blood that block transmission of signals from your nerve endings to your muscles' receptor sites. Blood is taken from your body, passed through a filter that removes antibodies and then returned to your body.

  This approach is expensive and time-consuming. Also, other forms of therapy are necessary for long-term restoration of muscle strength. Otherwise, the immune system soon makes new antibodies to replace those that have been removed.

• Intravenous immune globulin. This therapy provides your body with normal antibodies, which alters your immune system response. It has a lower risk of side effects than do plasmapheresis and immune-suppressing therapy, but it can take a week or two to start working and lasts only several weeks to months.

  This therapy is expensive, and it's not clear that intravenous immune globulin offers clear advantages over other available therapies. For those reasons, it's often reserved for people who are severely ill.

As part of your treatment, your doctor may suggest physical therapy and occupational therapy to help you adjust to tasks you need to do around the house and in your job.

Be sure not to take any medications without checking with your doctor. Some medications, such as some heart medications and botulinum toxin, can worsen myasthenia gravis.

Supplementing your medical care with these approaches may help you make the most of your energy and cope with the symptoms of myasthenia gravis:

• Adjust your eating routine. Try to eat when you have good muscle strength. A good time is after taking a cholinesterase inhibitor, such as pyridostigmine (Mestinon) and neostigmine (Prostigmin). Also, take your time eating and rest between bites. More frequent, smaller meals may be easier to handle. Also, try soft foods and avoid sticky foods that require lots of chewing.
• Use safety precautions at home. Install grab bars or railings in places where you may need support, such as next to the bathtub. Keep the floors and halls in your house clear of clutter, cords and loose rugs. Outside your home, keep the steps, sidewalk and path to your car clear.
• Use electric appliances and power tools. Save your energy in the bathroom, in the kitchen or at the workbench by using electric appliances, such as toothbrushes, can openers and screwdrivers.
• Wear an eye patch. If you have double vision, using an eye patch can help relieve this problem. Wear the patch while you read or watch television. To avoid eyestrain, periodically switch the patch from one eye to the other.
• Plan. If you have a chore to do around the house, shopping to do or an errand to run, plan the activity to coincide with the time at which your medication provides your peak energy level. If you're working on a project at home, gather everything you need for the job at one time, to eliminate extra trips that may drain your energy.
• Ask for help. Depending on your energy level, you may not be able to do everything you have planned around the house or run every errand that you need to. Ask family members and friends to lend a hand.
• Manage stress. Because emotional stress can make myasthenia gravis worse, look for ways to reduce stress. These may include relaxation techniques such as biofeedback and meditation.