This test evaluates the function of the neuromuscular junction. Two mg edrophonium chloride (cholinesterase inhibitor) are injected IV. If there is no response after 30 seconds, the remaining 8 mg are injected. A positive test reveals an increase in muscle strength (or reduction of eyelid ptosis) within 1 minute. The weakness returns within 5-30 minutes. The test is characteristically positive in myasthenia gravis, but can also be positive in mitochondrial disorders, LEMS and congenital myasthenic syndromes.