Pouget J, Serratrice G
Rev Neurol (Paris). 1983;139(11):665-72.
A case of generalized myotonia with autosomal recessive transmission related to Becker's type is reported. A muscular weakness improved by exercise was combined to myotonia. In addition to abundant myotonic discharges, electromyography showed alteration of the voluntary pattern during sustained contraction. The repetitive stimulation showed a marked decrement of the motor potential amplitude with high frequency stimulation. Carbamazepine (800 mg/day) and diphenylhydantoin (300 mg/day) were without effect while myotonia and muscular weakness were considerably and quickly improved by mexiletine (400 mg/day). Tests on ergometric bicycle allowed to quantify this improvement. The physiopathological mechanism of muscular weakness is considered as a depolarization block due to the cumulative depolarization which occurs during myotonic discharge. Most antimyotonic drugs belong to the group of local anesthetics. One of their characteristics is a blocking potency depending on the frequency of membrane activity. Their action is predominant on fibers with repetitive discharges; thus they block the myotonic discharge and prevent the membrane block due to cumulative depolarization. All local anesthetics do not have the same antimyotonic effect. The low molecular weight of mexiletine entails a faster time constant for the block recovery of the sodium channel. It follows that the drug block mainly depends on the frequency of membrane activity and this characteristic could explain the remarkable antimyotonic effect of mexiletine.
报告了一例与贝克尔型相关的常染色体隐性遗传的全身性肌强直病例。运动可改善的肌无力与肌强直并存。除大量肌强直放电外，肌电图显示持续收缩时随意运动模式改变。重复刺激显示高频刺激时运动电位幅度显著递减。卡马西平（800毫克/天）和苯妥英（300毫克/天）无效，而美西律（400毫克/天）可使肌强直和肌无力迅速得到明显改善。通过测力计自行车测试对这种改善进行了量化。肌无力的生理病理机制被认为是由于肌强直放电期间发生的累积去极化导致的去极化阻滞。大多数抗肌强直药物属于局部麻醉剂类。它们的一个特点是阻断效力取决于膜活动频率。它们对重复放电的纤维作用占主导；因此它们阻断肌强直放电并防止因累积去极化导致的膜阻滞。并非所有局部麻醉剂都有相同的抗肌强直作用。美西律的低分子量使得钠通道阻滞恢复的时间常数更快。由此可见，该药物的阻滞主要取决于膜活动频率，这一特性可以解释美西律显著的抗肌强直作用。
