How is pharmacological testing performed in the evaluation of myasthenia gravis (MG)?

Updated: Aug 27, 2018
  • Author: Abbas A Jowkar, MBBS; Chief Editor: Nicholas Lorenzo, MD, CPE, MHCM, FAAPL  more...
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To perform the test, a butterfly needle is placed in an accessible vein. A 2 mg (0.2 mL) test dose is administered initially, as some patients are extremely sensitive to low doses. If no response and no untoward effects are noted after 30 seconds, remainder of the drug 8 mg (0.8 mL) is injected in 2 mg increments every 10-15 seconds. If the patient has objective improvement (improvement in ptosis or ophthalmoparesis) or a severe side effect, the test is aborted. The test is not considered positive if the patient reports feeling stronger. Sinus bradycardia due to excessive cholinergic stimulation of the heart is a serious complication; consequently, an ampule of atropine should be available at the bedside or in the clinic room while the test is performed.

This test may give both false-negative results and false-positive results. It has a low sensitivity in ocular MG; 50% of patients presenting with eye symptoms will be missed. On the other hand, diseases other than MG, such as amyotrophic lateral sclerosis (ALS) and cavernous sinus lesions, LEMS, botulism, congenital myasthenic syndromes, and GBS can score positive on the test. [38] This test has been combined with electromyography (EMG) and ocular tonography to increase its sensitivity in ocular MG; however, it still produces false-negative and false-positive results. Thus, a positive edrophonium test indicates abnormal neuromuscular transmission and does not specify a disease condition.

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