Neurology

Myasthenia Gravis

What is Myasthenia Gravis?

The most common form of myasthenia gravis (MG) is a chronic autoimmune neuromuscular disorder that is characterized by fluctuating weakness of the voluntary muscle groups.
 

Who is impacted?

The prevalence of MG in the United States is estimated to be approximately 0.02%. However, MG is probably under diagnosed and the prevalence may be higher. Myasthenia gravis occurs in all races, both genders, and at any age. MG is not thought to be directly inherited nor is it contagious. It does occasionally occur in more than one member of the same family.
 

How is MG diagnosed?

A diagnosis can be confirmed in several ways, including the following:

• Anti-MuSK Antibody testing----a blood test for the remaining 15% of MG patients who have tested negative for the acetylcholine antibody. These patients have seronegative (SN) MG. About 40% of patients with SNMG test positive for the anti-MuSK antibody. The remaining patients have unidentified antibodies causing their MG.
• Office Tests—Sleep, Ice Pack and Edrophonium tests are examinations performed by specialists to evaluate an improvement in strength that may be consistent with MG.
• Electromyography— (EMG) studies can provide support for the diagnosis of MG when characteristic patterns are present. Repetitive Nerve Stimulation is used to check for a pattern of response that is characteristic of MG.
• Single Fiber EMG— studies can provide support for the diagnosis of MG when characteristic patterns are present. The single fiber EMG and AChR antibody test are primary tests used to confirm a clinical diagnosis of MG.
• Acetylcholine Receptor Antibody— a blood test for the abnormal antibodies can be performed to see if they are present. Approximately 85% of MG patients have this antibody and, when detected with an elevated concentration the AChR antibody test is strongly indicative of MG.

Often times, all of these tests are negative or equivocal in someone with a story and examination still seem to point to a diagnosis of MG. A clinician skilled in recognizing MG and distinguishing MG from other conditions would need to determine if such a patient has MG or another disorder.

How is MG treated?

There are many effective treatments for myasthenia gravis. Spontaneous improvement and even remission (although uncommon) may occur without specific therapy.

• Anti-acetylcholinesterase agents.
• Corticosteroids (e.g., prednisone) and immunosuppressive agents
• Plasmapheresis, or plasma exchange, may be useful in the treatment of MG also. This procedure removes the abnormal antibodies from the plasma of the blood. The improvement in muscle strength may be striking, but is usually short-lived, since production of the abnormal antibodies continues. When plasmapheresis is used, it may require repeated exchanges. Plasmapheresis may be especially useful during severe MG weakness or prior to surgery.
• Intravenous immune globulins (IVIG) are sometimes used to affect the function or production of the abnormal antibodies also.