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    • br Introduction Myasthenia gravis MG is

    2024-03-28


    Introduction Myasthenia gravis (MG) is an autoimmune disease in which antibodies against targets on the postsynaptic muscle membrane cause neuromuscular transmission failure. About 85% of patients with MG have autoantibodies against Dorsomorphin receptor (AChR) [1]. In AChR negative MG patients antibodies to muscle specific tyrosine kinase (MuSK) have been identified in 40–50% of cases [2]. MG associated with anti-MuSK antibodies is considered to be a distinct clinical entity [3] and it differs in many aspects from the typical presentation of AChR positive MG (AChr MG). Unlike AChR MG patients, who experience weakness and fatigue in electrophysiologically normal muscles, myopathic changes have been described in patients with MuSK positive MG [4], [5], as well as visible wasting of the tongue and facial muscles [3], [6]. Moreover, MRI studies demonstrated fatty degeneration in the cranial muscles of MuSK positive MG (MuSK MG) patients, similar to patients with proven muscle disease such as myotonic dystrophy, while such changes were not present in patients with AChR MG [6]. Therefore, the muscle atrophy observed in the tongue and facial muscles of MuSK MG patients has been thought to be secondary to the myopathic process [7]. There is also evidence of significant muscle ultrastructural changes in MuSK MG patients. Myopathic and mitochondrial abnormalities are more prominent in MuSK MG with giant, swollen, and degenerated mitochondria with fragmented cristae. The most common changes in AChR MG muscles are fiber atrophy, myofibrillar disarray, and Z-line streaming, consistent with mild neurogenic abnormalities [8]. Since the origin of myopathic electromyography (EMG) changes in MuSK MG patients is still unclear, we aimed to further address this issue. Quantitative EMG methods, including turns/amplitude (T/A) and multi-motor unit potential (MUP) analysis, are more sensitive than regular EMG, which only qualitatively analyzes interference pattern and depends on subjective interpretation of the findings. The aim of this study was to explore the importance of quantitative EMG techniques in the detection of myopathic EMG changes in MuSK MG patients.
    Methods
    Results In our previously published paper [10], EMG examination revealed the presence of myopathic EMG changes in around one-third of MuSK MG patients in the extremity muscles and in more than 80% in the facial muscles. Conversely, myopathic EMG changes were registered in around one-third of AChR MG patients in the facial muscles and were extremely rare in the extremity muscles. T/A analysis was performed in the EDC, deltoid and OO muscles. In one AChR MG patient photoperiodism examination could not be performed in the EDC and deltoid muscles due to severe weakness of these muscles. For the same reason, T/A analysis could not be performed in the OO muscle in two MuSK MG patients. The results of T/A analysis are presented in Table 1. T/A analysis registered similar results between MuSK MG and AChR MG patients. Number of MUP turns was the only registered parameter which was lower in the MuSK MG patients compared to the AChR MG patients (p=0.043). Multi-MUP analysis was performed in the deltoid and OO muscles and the results are presented in Table 2. Values of all analyzed MUP parameters were similar between MuSK and AChR MG patients (p>0.05). When the results of our MG patients were compared to the published reference values for MUP parameters in the OO [11] and deltoid [12] muscles, a myopathic lesion was confirmed in the OO muscle in 26 (83.9%) MuSK MG and 24 (85.7%) AChR MG patients, while in the deltoid muscle myopathic findings were recorded in 26 (83.9%) MuSK MG and in 21 (75%) AChR MG patients, mainly in the form of short duration MUP. Neither of these differences were statistically significant. In the MuSK MG patients, lower values of MUP amplitude in the OO muscle were recorded in patients with longer disease duration (p=0.031), but such a correlation was not found in the deltoid muscle (p=0.919). In AChR MG patients, there was no correlation between the disease duration and the MUP amplitude, neither in the OO (p=0.936) or the deltoid muscle (p=0.695).