Myasthenia gravis (MG) is a chronic disease characterized by weakness and rapid fatigue of the voluntary muscles. The weakness is due to a breakdown in communication between a nerve ending and its adjoining muscle fiber. The onset of MG can be sudden, with severe and generalized muscle weakness, but more often its symptoms in the early stages are subtle and variable, making it difficult to diagnose correctly. MG affects people of all ages and either sex, but it more affects young women and old men.
MG is a disorder of the neuromuscular junction - the space between the nerve and muscle. Neurotransmitters are released from the nerve in response to an electrical signal and cross this space to activate muscle movements. MG is an autoimmune disorder, meaning the body’s immune system actually attacks itself. As a result, in MG, the receptors to these neurotransmitters (in particular, acetylcholine) are reduced in number and function. Thus, the muscle never receives appropriate signals and patients naturally experience muscle weakness. ‘Generalized’ MG can affect muscles of the eyes (resulting in double vision or drooping of the eyelid), pharynx (difficulty swallowing and speaking clearly), chest wall (shortness of breath), and arms and legs (weakness and easy fatigability). A more focal form of MG known as ‘ocular MG’, can present with visual symptoms alone. However, a significant percentage of patients with ocular MG can progress to the generalized form. MG tends to have a fluctuating course, but can be worsened by exertions, a variety of infections, certain medications, co-existent illnesses, and pregnancy. MG may be associated with other autoimmune disorders, most commonly of the thyroid and gastrointestinal tract. It may also be associated with a tumor of the thymus gland (called a thymoma).
The first noticeable symptom of MG is frequently weakness of the eye muscles. The disease may remain confined to this area of the body, or it may progress to muscles involving swallowing, chewing, speech or limb movement. Symptoms vary among patients, but can include:
- A drooping of one or both the eyelids (ptosis)
- Blurred or double vision
- Unstable or "waddling" gait
- Weakness in the arms or legs
- Difficulty swallowing
- Difficulty breathing
- Difficulty speaking clearly
Once symptoms begin, muscle weakness may worsen over days or weeks. Symptoms may remain constant, may progress or may fluctuate from hour to hour or day to day. Weakness tends to worsen with exercise and at the end of the day. When MG affects a patient’s ability to swallow or breathe, the disease can become life-threatening if not treated immediately. However, the disease is seldom fatal if managed properly.
MG develops when antibodies are produced against the region of the muscle that connects with a nerve ending. For muscle to contract, the neurotransmitter chemical acetylcholine must attach to the muscle membrane after its release from the nerve ending. But this is prevented by the antibody’s attack on the muscle’s acetylcholine receptors. The result is muscle weakness.
Doctors have found that the thymus gland appears to trigger this autoimmune response by encouraging production of acetylcholine receptor antibodies.
Located behind the breastbone, the thymus gland is part of the body’s immune system, producing white blood cells called lymphocytes, which are responsible for immune responses to bacteria, toxins and viruses. Normally large in infancy, the thymus gland shrinks in size so that in the normal adult it is hardly functional. For patients who have MG, however, the thymus gland can have an excess number of cells (hyperplasia), which may contribute to the autoimmune response.
The diagnosis of MG heavily depends on the history and examination. When MG is suspected, the doctor will perform a thorough physical exam, looking for muscle weakness or fatigue. Several tests may be performed.
- Tensilon test: Tensilon is a drug that blocks the breakdown of acetylcholine, helping the neurotransmitter activate the muscle. For patients who do not have MG, the drug will do nothing to improve tired muscles. However, for patients with MG, the test is said to be positive when muscle strength improves. This improvement in muscle strength lasts only several minutes, however.
- Electromyogram (EMG): An EMG procedure assesses nerve and muscle function electrically and can support the diagnosis of MG.
- Blood tests: For patients with possible MG, a blood test may be ordered to measure the presence of acetylcholine receptor antibodies. About 75 percent of patients with MG show an elevated amount of these antibodies in the blood. In one-third of MG patients with generalized weakness and without elevated acetylcholine receptor antibodies, MuSK antibody is increased, which can help with the diagnosis. The test for MuSK antibody is also commercially available.
If diagnosed promptly, some patients may be cured of MG by removal of the thymus gland (thymectomy) or aggressive immunosuppressant therapy. The effectiveness of treating MG depends on many factors, such as the severity of the disease, the duration of the disease, the patient’s age and the patient’s overall health. For the most part, however, MG can be well-controlled with the following treatment approaches:
- Cholinesterase inhibitors: These are commonly used drugs to treat MG, but they are most useful in mild forms of the condition. These drugs work by preventing the breakdown of acetylcholine, thus increasing the muscle’s ability to contract. The most commonly prescribed form of this drug is pyridostigmine (Mestinon).
- Immunosuppressants: Corticosteroids and other drugs such as cyclosporine and azathioprine help patients with MG by suppressing the activity of the immune system. These treatments are generally for more severely ill patients. Although they are often very effective, these drugs can have serious associated side effects. The patient should consult with his or her doctor regarding the potential value of these drugs in treating MG.
- Thymectomy: A thymectomy is a surgical removal of the thymus gland. There is evidence that thymectomy can lead to remission or reduced drug dependency, but it is more likely to work if undertaken within 6-12 months of the first onset of symptoms. A controlled clinical study of the efficacy of thymectomy in MG is currently underway.
- Plasmapheresis: Plasmapheresis is a blood plasma exchange process. It "filters" the blood of acetylcholine receptor antibodies by replacing the patient’s plasma with donor plasma. Plasmapheresis can be a life-saving tool in the treatment of MG; however, it is expensive, time-consuming, and can be associated with side effects such as low blood pressure, infection, and blood clots.
- IVIg: Intravenous infusion of immunoglobulin can modulate the immune system and reduce the effects of causative autoantibodies in MG. It is used to treat MG with acute worsening, especially with breathing involvement.
For patients with MG, a healthy lifestyle and education about one’s disease can maximize management. Some of the best ways to accomplish this include:
- Eating foods that are high in potassium such as bananas, tomatoes, apricots, and broccoli
- Avoiding overexertion
- Advising physicians of the diagnosis prior to taking prescribed drugs that may aggravate MG
- Avoiding excessive heat