How is MG diagnosed?

Weakness and fatigue are common complaints with a variety of causes, so it is not surprising that the diagnosis of MG is often missed in people when the weakness is mild or restricted to only a few muscles. Once the possibility occurs to a physician, however, there are three general approaches to confirming the diagnosis.

1. Immunological confirmation completed through measuring myasthenic antibodies in the blood. The most reliable assay is for serum acetylcholine receptor binding antibodies which are elevated in 85% of MG patients. Positive results are less likely in patients with mild or purely ocular forms of MG.  For those patients who lack the serum acetylcholine receptor antibody may be tested for a different antibody at the neuromuscular junction called muscle-specific kinase or MuSK.
2. Electrophysiological confirmation, although less specific, may be used if antibody tests are negative. Repetitive nerve stimulation is a nerve conduction study that observes the change in a muscle’s electrical response after stimulation. Another diagnostic technique, single fiber electromyography, evaluates and records electrical activity to assess the health of muscles and the nerve cells that control them.

Pharmacological confirmation

Pharmacological confirmation involves use of drugs that worsen or improve the muscle weakness associated with MG.  When injected, a drug called Tensilon (edrophonium chloride) can reverse myasthenic symptoms for several minutes.  Patients may also be instructed to take a small amount of a longer-acting medication called Mestinon (pyridostigmine bromide) after meals at home to assess its response over several days.