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Myasthenia gravis can be difficult to diagnose because weakness is a common symptom of many disorders. Add to this the fact that symptoms may be vague, fluctuate or only affect certain muscles. And MG doesn’t "perform" on demand; the eyelid that droops at 7 p.m. may not show for a 9 a.m. doctor appointment. Identification may be complicated further when more than one autoimmune disorder is involved. It’s not unusual for a diagnosis of MG to be delayed.

These steps may be taken to confirm a diagnosis of MG:

  • Neurological exam: This may include testing your reflexes, muscle strength, muscle tone, senses of touch and sight, gait, posture, coordination, balance and mental skills. Impaired eye movement or muscle weakness may prompt a doctor to evaluate further.
  • Blood tests to measure myasthenic antibodies in the blood: About 80% of all MG patients have elevated levels of AChR antibodies that block or destroy acetylcholine receptor sites on the muscles. However, blood tests may not be conclusive, particularly in patients with mild cases of MG or with ocular MG.
  • Tensilon® Test: A short-acting drug called edrophonium chloride (Tensilon®) is given intravenously. If weakness, especially in the eye muscles, briefly and temporarily improves, it indicates you may have myasthenia gravis. The drug does this by blocking an enzyme that breaks down acetylcholine, the chemical that transmits signals from nerve endings to muscles. A trial use of oral pyridostigmine bromide (Mestinon) is an alternative approach.
  • Nerve conduction studies/repetitive nerve stimulation: A small electrical impulse is applied to your skin. This stimulates your nerves to test the strength of your muscle contraction. Muscle fibers in patients with MG fatigue easily and don’t bounce back from repeated stimulation as well as those of a healthy person.
  • Single fiber electromyography (EMG): A thin-needle electrode is inserted into one of your muscles to help measure patterns of electrical activity in your muscle at rest and with slight muscle contraction. The test can detect defective nerve-to-muscle communication.
  • Imaging: Computed tomography (CT) or magnetic resonance imaging (MRI) are often done to identify an abnormal thymus gland or a thymus gland tumor (thymoma).

There is no one foolproof method of diagnosis for MG. Sometimes test results are inconclusive, even when your examination and story seem to point to a diagnosis of the disease. If this happens, it may be appropriate to track symptoms over time until your diagnosis can be clarified. On occasion, some testing is done to rule out other disorders.

Reviewed by the MGF of Illinois Medical Advisory Board, September 2008.

Unless otherwise stated, the information provided here is of a general nature, composed by non-medical personnel. It is meant to be accurate and helpful advice for MG patients. It is not intended to be medical opinion, nor is it a substitute for personal professional medical care.