Electromyography

Repetitive Nerve Stimulation (RNS)
The amplitude of the compound muscle action potential (CMAP) elicited by repetitive nerve stimulation is normal or only slightly reduced in patients without MG. The amplitude of the fourth or fifth response to a train of low frequency nerve stimuli falls at least 10% from the initial value in myasthenic patients. This decrementing response to RNS is seen more often in proximal muscles, such as the facial muscles, biceps, deltoid, and trapezius than in hand muscles. A significant decrement to RNS in either a hand or shoulder muscle is found in about 60% of patients with myasthenia gravis.

Single Fiber EMG (SFEMG)

Voluntary SFEMG is done with the patient making minor contraction of the muscle with the physician using either a standard single fiber electrode or a concentric needle EMG electrode with the smallest recording surface. The third technique, axonal micro-stimulation, requires the terminal nerve branch to be activate with a small amount of electrical current while recording with the electrode. All of the techniques are technically demanding. Each have specific normative values to which the patient’s study can be compared if the same methodology is used. The latter technique is very useful in sedated infants and children.

Comparison of Diagnostic Techniques

Intravenous edrophonium chloride is often diagnostic in patients with ptosis or ophthalmoparesis, but is less useful when other muscles are weak. Elevated serum concentrations of AChR binding and probably MuSK antibodies virtually assures the diagnosis of myasthenia gravis, but normal concentrations do not exclude the diagnosis. Repetitive nerve stimulation confirms impaired neuromuscular transmission but is not specific to myasthenia gravis and is frequently normal in patients with mild or purely ocular disease. The measurement of jitter by SFEMG is the most sensitive clinical test of neuromuscular transmission and is abnormal in almost all patients with myasthenia gravis. A normal test in a weak muscle excludes the diagnosis of myasthenia gravis, but an abnormal test can occur when other motor unit disorders cause defects in neuromuscular transmission.

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