Paramyotonia congenita is an autosomal dominant muscle disease that is characterized by cold-induced and exercise-induced myotonia. 4 Paramyotonia congenita is one of a group of muscle diseases that are due to missense mutations in the gene that codes for the skeletal muscle sodium channel α-subunit (SCN4A). 1, 3–5 All of these so-called muscle “sodium channelopathies” share an abnormality of muscle membrane excitability that can be variably expressed as myotonia or weakness. Three of these allelic disorders characteristically exhibit myotonia and have been named the “sodium channel myotonias.” 1, 3, 4, 12 Currently, the known sodium channel myotonias are paramyotonia congenita, hyperkalemic periodic paralysis, and potassium-aggravated myotonia. 1, 3, 4, 12 To avoid misunderstanding, it is worth pointing out that neither myotonic dystrophy nor myotonia congenita is due to mutations of muscle sodium channels. Myotonic dystrophy is due to an expanded CTG repeat in the 3 untranslated region of the gene encoding the putative serine–threonine protein kinase on chromosome 19q13. 3, and myotonia congenita is due to mutations of the gene coding for the skeletal muscle chloride channel (CLCN1). 1, 4 All the mutations of the skeletal muscle sodium channel that cause paramyotonia congenita, hyperkalemic periodic paralysis, and potassium-aggravated myotonia result in gain-of-function defects, whereby the mutant channels pass more Na+ current than normal. In most cases, this is due to an impairment of fast inactivation of the mutant sodium channels. 1, 4, 5, 12 Normally, voltage-gated sodium channels in muscle membrane open briefly during depolarization and then close to a fast inactivated state, thereby limiting the duration of action potentials and initiating repolarization. During the period when sodium channels are inactivated, the muscle membrane is inexcitable. This “refractory period” lasts until the sodium channels recover from inactivation. Thus, inactivation of sodium channels limits the excitability and, consequently, the firing rate of action potentials along the muscle membrane. In the sodium channel diseases of skeletal muscle, the impaired inactivation of mutant sodium channels results in an increased Na+ influx through the muscle membrane, causing either slight depolarization with hyperexcitability (myotonia) or sustained depolarization with inexcitability (paresis). 1, 4, 5, 12 In most cases of paramyotonia congenita, the mutant sodium channels exhibit abnormalities of channel gating that involve not only a slowing of the rate of fast inactivation but also an acceleration of the rate of recovery from inactivation. 1, 4, 5, 12 Thus, the mutant sodium channels spend less time in the inactivated state, resulting in a persistent inward Na+ current that promotes excessive membrane depolarization and abnormal trains of myotonic potentials. Anesthesia and surgery pose special risks for patients with sodium channelopathies of skeletal muscle. 2, 8, 11 Most complications during surgery are related directly to the precipitation of myotonia, which may complicate the course of anesthesia and be difficult to abolish. 2, 8, 11 Many patients who experience these complications display extreme sensitivity to some drugs and abnormal reactions to others. Severe and even life-threatening complications have been reported in patients with muscle sodium channelopathies during induction and maintenance of anesthesia. One of the most striking complications that can occur during induction of anesthesia is associated with the use of depolarizing neuromuscular blocking agents such as succinylcholine. 8, 11 Because of their direct depolarizing …