How is Myasthenia Gravis (MG) Diagnosed?

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.

Reviewed by a Cleveland Clinic medical professional.

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