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KMID : 0860920070090010030
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Journal of the Korean Association EMG-Electrodiagnostic Medicine
2007 Volume.9 No. 1 p.30 ~ p.35
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Measurement of Decrement in the Area of CMAP to Repetitive Nerve Stimulation is More Sensitive than the Amplitude in the Diagnosis of Myasthenia Gravis
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Sohn Jong-Hee
Choi Hui-Chul
Lee Sang-Moo
Kwon Ki-Han
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Abstract
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Objectives: The measurement of the decremental muscle response to repetitive nerve stimulation (RNS) has low sensitivity for the diagnosis of neuromuscular transmission defect compared with SFEMG. But repetitive nerve stimulation test (RNST) is a simple, easy and widely utilized test which provide object for evaluation of neuromuscular transmission defect. Thus, we evaluated the use of decrements in CMAP area in comparison with conventional decrements in CMAP amplitude for detecting disorders of neuro- muscular transmission.
Methods: We analyzed RNS data from 37 patients with myasthenia gravis and 34 normal control, retro- spectively. Myasthenia patients were divided into five groups based on clinical type (Osserman classifica- tion). We compared decrement in area and amplitude between the first and fourth CMAP of muscle response at stimulus rate of 3 Hz and 5 Hz in orbicularis oculi, trapezius and abductor digiti minimi (ADQ) muscles among control and MG group.
Results: There were no significant statistical differences in CMAP area and amplitude in all tested mus- cles (orbicularis oculi, trapezius, ADQ) in normal control group, whereas in patients with myasthenia gravis, there were statistically significant decremental responses in CMAP area in only orbicularis oculi muscle with lower-rate stimulation. The sensitivity of RNST was higher with CMAP area than with CMAP amplitude (67.6% vs 54.1%). Especially, clear distinction was seen in sensitivity of RNS with ocular myasthenia (Osserman type I) between CMAP area and amplitude, the sensitivity of RNS was two times higher with area decrement than with amplitude decrement (38.9% vs 16.7%).
Conclusion: Measurement of decrements in area of response to RNS is useful and sensitive method in the evaluation of patients with suspected deficits of neuromuscular transmission. Especially it was helpful if there was a suspicion of ocular myasthenia gravis or a borderline decremental response with lower-rate stimulation.
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KEYWORD
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Repetitive nerve stimulation test, Myasthenia gravis, CMAP area
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