Myasthenia Gravis
Definition
Myasthenia gravis (MG) is a neuromuscular disorder that affects the junction between nerves and muscles. Its symptoms may grow progressively more severe over time. This is a potentially serious condition that requires care from your doctor. If you suspect you have this condition, contact your doctor immediately.
Causes
The root cause of myasthenia gravis is unknown. MG occurs when the body’s immune system attacks receptors in muscle. Normally, these receptors respond to the neurotransmitter acetylcholine. This allows nerve signals to prompt the muscles to move. However, when the immune system prevents these receptors from working normally, the muscles can no longer respond to nerve signals. This causes muscles to become increasingly weak.
The thymus is an organ behind the breastbone, and it is thought to play a role in causing some cases of myasthenia gravis. Immune proteins called antibodies are produced by the thymus. In MG, some of these antibodies may target the acetylcholine receptors. It is still not clear why the thymus begins to produce these harmful antibodies.
The Thymus Gland
Risk Factors
A risk factor is something that increases your chance of getting a disease or condition.
The following factors increase your chance of developing myasthenia gravis:
- Women are more likely to develop MG between the ages of 20 to 30.
- Men are more likely to develop MG between the ages of 60 to 70.
- People with certain genetic markers, called HLA-B8, DR3, are more likely to develop MG.
- Infants of mothers with MG are more likely to develop a temporary form of the condition, called neonatal MG. Neonatal MG occurs because the mother’s abnormal antibodies have entered the baby’s bloodstream. When the baby is born, he or she may exhibit symptoms of muscle weakness. The abnormal maternal antibodies are usually cleared from the baby’s bloodstream within about two months, ending the baby’s symptoms.
Symptoms
Symptoms include:
- Muscle weakness that increases with muscle use/exercise, and improves after resting those muscles
-
Weakness of muscles that control:
- The eyelids
- Eye movement
- Facial expression
- Chewing
- Swallowing
- Coughing
- Breathing
- Arm and leg movement
- Drooping eyelids
- Double and/or blurred vision
- Difficulty swallowing
- Difficulty speaking
- Difficulty walking
- Weakness of the hand muscles
- Myasthenic crisis—This is a severe flare of symptoms. During an episode, breathing may be significantly impaired.
Diagnosis
Your doctor will ask about your symptoms and medical history, and perform a physical exam. You may need to see a neurologist, a specialist who is an expert in diseases of the nervous system.
Tests may include the following:
- Blood tests—to identify the presence of abnormal antibodies and to look for thyroid disease (more common in patients with MG)
- Electromyography (EMG) tests—to reveal abnormalities in muscle functioning
- Repetitive nerve stimulation tests—may show progressively increasing muscle weakness over the course of the test
- Tensilon test—a dose of a medication called edrophonium chloride is given, which will briefly improve muscle weakness
- CT or MRI scan of the chest—to look for a tumor of the thymus gland (thymoma)
- Pulmonary function testing—to assess how MG may be affecting your ability to breathe
Treatment
Talk with your doctor about the best treatment plan for you. Treatment options include:
Acetylcholinesterase Inhibitors
These are medications that interfere with the breakdown of acetylcholine. This increases the amount of acetylcholine available to stimulate receptors on muscle. These medications include pyridostigmine and neostigmine.
Immunosuppressive Therapy
These medications are given to suppress the immune system. As a result, there are fewer antibodies present to attack the acetylcholine receptors. These medications include:
- Prednisone
- Azathioprine
- Mycophenolate mofetil
- Cyclosporine
Immunoglobulin Therapy
Administration of immunoglobulin intravenously might help reduce symptoms of myasthenia, though the exact mechanism is unclear. This treatment seems to have fewer side effects that plasmapheresis.
Plasmapheresis
This is a procedure which cleans the blood of abnormal antibodies. Because the abnormal antibodies will continue to be produced, plasmapheresis may need to be repeated at certain intervals.
Thymectomy
Removal of the thymus gland is recommended for most patients with myasthenia, as well as when thymoma is present. Thymectomy may improve the symptoms of MG in some patients.
Mechanical Ventilation
Mechanical ventilation may be required if breathing is severely impaired, such as during an episode of myasthenic crisis.
Physical Therapy
Physical therapy and occupational therapy do not generally alter the course of the disease, but may be needed to help the patient cope with changes in muscle strength and coordination. It may also help with learning alternative ways to approach daily activities.
Prevention
There are no known ways to prevent the development of MG. The following steps may improve muscle functioning in people who already have the condition:
-
Avoid medications that may worsen MG, such as:
- Beta-blockers
- Aminoglycoside antibiotics
- Quinolone antibiotics
- Class I medications used for abnormal heart rhythms
- Treat any infections promptly.
- If you have trouble swallowing, please notify your doctor immediately. You may wish to consult a dietitian to help you construct a healthy diet that is easier to swallow.
- If you have trouble speaking, you may wish to consult a speech therapist.
Resources:
Myasthenia Gravis Foundation of America
National Institute of Neurological Disorders and Stroke
Canadian Resources:
Muscualar Dystrophy Canada
Myasthenia Gravis Association of BC
References:
Ferri FF, ed. Ferri’s Clinical Advisor: Instant Diagnosis and Treatment. Philadelphia, PA: Mosby Elsevier; 2004.
Goetz CG, ed. Textbook of clinical Neurology. 2nd ed. Philadelphia, PA: WB Saunders Company; 2006.
Goldman L, Ausiello D, eds. Cecil Textbook of Medicine. 22nd ed. Philadelphia, PA: WB Saunders Company; 2004.
Gronseth GS, Barohn RJ. Practice parameter: thymectomy for autoimmune myasthenia gravis (an evidence-based review): report of the Quality Standards Subcommittee of the American Academy of Neurology. Neurology. 2000;55:7-15.
Myasthenia gravis. Dynamed. Available at: http://dynamed101.ebscohost.com/AlphaBrowse.as...Term=myasthenia+. Accessed January 14, 2008.
Last reviewed January 2008 by Judy Chang, MD, FAASM
Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.