Myasthenia Gravis – A Rare Muscle Weakness Disorder
Myasthenia gravis is a rare disorder of muscle weakness, affecting approximately 70,000 individuals in the United States. Many confuse this with multiple sclerosis. Multiple sclerosis is a central nervous system disorder affecting the insulation (myelin) on nerve cells in the brain and spinal cord. In contrast myasthenia gravis is a muscle disease where transmission of electrical impulses to the muscle fail. This results in the muscle not contracting fully, resulting in weakness. This condition can selectively affect the eye muscles, muscles of the head and neck or be generalized affecting all muscle, including the diaphragm. If the diaphragm is involved, patients can have varying degrees of breathing problems, including respiratory failure.
Myasthenia gravis (MG) is the standard model of a neurological autoimmune disorder. It is the the best understood of all autoimmune diseases. In autoimmune diseases, the body’s immune system fails to recognize a particular body system as being part of “itself.” In MG, the immune system does not recognize the transmitter receptors for acetylcholine (the muscle communication transmitter) as being part of the body’s own system. The immune system for antibodies against the receptors and attacks them. With fewer receptors on the muscle membrane, the muscle does not get the full chemical message to contract. This causes affected individuals to have weakness in the muscles affected by MG. These antibodies can be detected by a blood test for acetylcholine receptor antibodies. It is important to note, however, that not all patients with myasthenia will have a positive antibody test, known as a false negative (as high as 25%.) Particularly patients with ocular myasthenia frequently have negative tests. This does not mean that they do not have the condition.
Diagnosis of myasthenia is based on a careful medical history and detailed neurological exam. All patients with MG have some degree of visual symptoms including double vision or eyelid drooping. The presence of variable eyelid drooping, particularly if it shifts from side to side, is virtually diagnostic of myasthenia gravis. Confirmatory tests can include EMG testing and a chemical test known as the Tensilon test. Tensilon (edrophonium) is a chemical that temporarily blocks the enzyme that stops the action of acetylcholine. This has the effect of making the acetylcholine receptors that are “on” stay on longer, thereby combating weakness. Patient’s with suspected myasthenia can be given Tensilon. This short acting drug will temporarily reverse the weakness that myasthenic patients have. Other lab work should be performed to exclude other muscle diseases, thyroid problems or other metabolic problems that could cause muscle weakness.
Once the diagnosis is made, patients can be treated with Mestinon ( pyridostigmine.) Mestinon is a long acting version of Tensilon and works by inhibiting cholinesterase, the chemical that breaks down acetylcholine. If we did not have this enzyme, we could not relax our muscles. When acetylcholine is removed from the muscle receptor the muscle no longer contracts. Mestinon works well for symptomatic relief of the muscle weakness and associated symptoms of MG. Other symptoms associated with weakness can include chewing and swallowing difficulties, weakness of breathing with shortness of breath and generalized weakness. Double vision and drooping eyelids, conditions made worse by being in bright sun light, are frequent complaints.
For patients that have persistent symptoms, despite Mestinon therapy, clinicians will frequently employ the use of immunosuppressant therapy that will dampen the response of the immune system. This has the effect of limiting the destruction of the acetylcholine receptors on the muscle membrane surface. The most commonly used drug for this is Prednisone, a steroid drug, frequently used to treat autoimmune disorders. Although Prednisone works very well, it has its downside. Prednisone side effects can include weight gain, elevated blood sugar, cataracts, accelerated osteoporosis and other serious medical problems. Other immunosuppressants have been used with success. These include Imuran (azathioprine), cyclosporine, Cytoxan (cyclophosphamide) and CellCept (mycophenolate.) While being helpful in treating MG, these powerful immunosuppresants have their own lists of potential side effects. The purpose of immunosuppressant therapy is to cause myasthenia to go into remission. This is not always successful.
For patient’s with a severe worsening of their myasthenia symptoms, hospitalization may be necessary. Higher dosage, intravenous steroids can be given in combination with a blood filtering technique, known as plasmaphersis. The latter is used to filter out the acetylcholine receptor antibodies. A surgical technique, thymectomy, can be used to remove the thymus gland in the chest. This gland is responsible for making the cells that make the acetylcholine receptor antibodies.
Myasthenia gravis is a serious neurological condition and autoimmune disease characterized by muscle weakness that can affect vision, swallowing, breathing and general activities. Serious complications, including respiratory failure, can develop for untreated patients. Affected individuals should seek out care from a board certified neurologist, preferably one specializing in movement disorders or neuromuscular disease.