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    • br Discussion Myopathic EMG changes are frequently detected

    2023-08-25


    Discussion Myopathic EMG changes are frequently detected in patients with MuSK MG, unlike in AChR MG patients [10]. However, these EMG changes are not always accompanied by myopathic histopathological findings. It has been shown that mitochondrial abnormalities seem to be more prominent in MuSK MG, whereas neurogenic atrophy is observed in AChR MG patients [8]. Since the origin of myopathy in MuSK MG patients is still unclear and there are discrepancies between EMG and histopathological findings, we wanted to further analyze this issue. In order to be more sensitive and to quantitate myopathic changes in different forms of MG, we studied multi-MUP analysis, the most accepted method of analysis of individual MUP, and T/A analysis, the best studied method of automatic interference pattern analysis. Since multi-MUP analysis is performed under weak muscle contraction and examines only 95 9 activated at weak effort, it is sampling a higher percentage of type I fiber activity, while abnormalities of motor units activated at greater effort cannot be revealed. T/A analysis can be performed under different degrees of muscle activation. As the force of contraction increases, more and larger motor units are recruited and the electrical potentials they generate are summated. Because of this, the mean change in amplitude per turn of the interference pattern increases with force of contraction to the maximum possible, whereas the number of turns per second initially rises and then plateaus at higher force levels. As the maximum force of voluntary contraction varies between individuals and disease states, it is difficult to make comparisons between different patients. Therefore, Stalberg et al. proposed T/A analysis modification [13], which allows quantitative EMG interference pattern analysis disregarding estimation of muscle force. Since this method analyzes the T/A at various forces, this analysis samples a higher percentage of type II muscle fiber activity. A number of MUP studies on limb muscles of MG patients have shown a reduction in MUP duration [14], [15], [16], as occurred in our study. Nevertheless, pathohistological examination of muscle biopsies of MG patients who had myopathic features on MUP analysis revealed no abnormalities in most cases [17], [18]. Therefore, it was suggested that this discrepancy was most likely due to the functional shortening of MUP due to neuromuscular transmission dysfunction rather than anatomical changes. Contrary to this explanation, Somnier and Trojaborg [15] showed no consistent evidence to support the view that blocking of muscular fibers by itself could account for the reduction of MUP duration. Previously published studies have shown superior sensitivity of T/A analysis to demonstrate myopathic changes as compared to individual MUP analysis [19], [20], [21]. Cruz Martinez et al. [22] studied seven patients with MG and reported that all showed myopathic change with T/A analysis using force monitoring. Lo Monaco et al. [14] found a myopathic pattern in four and neurogenic pattern in two out of 17 MG patients by T/A analysis. Again, muscle biopsy did not support the EMG findings of these patients with neurogenic and myopathic change by T/A analysis. T/A analysis at low force failed to differentiate MG patients from controls whereas ratio of turns to mean amplitude was increased in seven out of eight patients compared to controls at maximum force [14]. These changes were attributed to fatigability as abnormal findings appeared during maintained contraction or after activation. The results of quantitative EMG techniques in subgroups of MG patients, according to their antibody status, have rarely been published. One such study showed similar quantitative EMG results in the facial muscles of MuSK MG patients and patients with primary muscle diseases, such as myotonic dystrophy [7]. In this study, myopathic EMG changes were registered in 50% of MuSK MG patients and in 40% of AChR MG patients. Facial muscles, both in the MuSK MG and AChR MG group, had short-duration MUP, without increased polyphasia, similar to those reported in patients with known myopathic changes secondary to myotonic dystrophy [11]. Since the dominant finding was that of short duration MUP, it led to the hypothesis that the facial muscle atrophy in MuSK MG patients is secondary to a myopathic process, probably as the result of uniform muscle fibre shrinkage or loss of muscle fibers from motor units.