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KMID : 0861220010030020176
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Journal of the Korean Society Clinical Neurophysiology
2001 Volume.3 No. 2 p.176 ~ p.186
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Electrodiagnostic tests of neuromuscular junction disorders
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Kim Dae-Seong
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Abstract
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In the normal neuromuscular junction, the nerve action potential(NAP) opens the P-type calcium channels, and resultant influx of calciumions produces the simultaneous exocytosis of 쨿60 presynaptic vesicles containing many acetylcholine(ACh) molecules. Released ACh molecules then traverse the synaptic cleft and bind to the postsynaptic ACh receptors, producing the minature end-plate potentials(MEPPs) which summate to form an end-plate potential(EPP). In normal end-plate, the size of EPPs are big enough to reach the threshold for the generation of single fiber action potentials(SFAPs). Myasthenia gravis(MG) is a relatively common postsynaptic disorder caused by the formation of autoantibodies to the ACh receptors, and its main electrophysiological abnormality is the reduced electrical response to the ACh quanta(reduced quantal response(q)). Lambert-Eaton myasthenic syndrome(LEMS), in contrast, is a rare presynaptic disorder caused by the formation of the autoantibodies to the voltage-gated calcium channel, and are electrophysiologically characterized by the reduced number of synaptic vesicles released in response to the NAP(reduced quantal content(m)). In both conditions, the safety factor is reduced and EPPs often fails to reach the threshold for firing and clinical weakness develops. The single fiber electromyography(SFEMG) is a specialized technique measuring the degree of stability in neuromuscular transmission, and its important parameters include mean jitter, blocking and fiber density. A modified form of SFEMG using the electrical microstimulation(stimulation SFEMG, S-SFEMG) is also available and has some advantages over classic volunatry SFEMG(V-SFEMG). The repetitive nerve stimulation test(RNS) mainly consists of two parts - low-rate of stimulation(LRS) and high-rate of stimulation(HRS). In MG, decremental response in LRS is usual because of the reduced safety factor and progressive depletion of presynaptic vesicle reserve produced by the LRS. In LEMS, HRS induce the accumulation of intracellular calcium ions and then produce progressive increase in quantal content(m), and thus, characteristic incremental response to HRS is observed. In the diagnosis of MG, the RNS is a relatively specific, but not so sensitive test in comparison to the others. Although the SFEMG is the most sensitive test for the diagnosis of MG, its usage is limited due to the non-specific nature of the results, technical difficulties, and time-wasting aspect of the test.
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KEYWORD
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Neuromuscular junction disorders, Sigle fiber EMG(SFEMG), Repetitive nerve stimulation(RNS) test
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