New Challenges Resulting From the Loss of Function of Nav1.4 in Neuromuscular Diseases

The voltage-gated sodium channel Nav1.4 is a major actor in the excitability of skeletal myofibers, driving the muscle force in response to nerve stimulation. Supporting further this key role, mutations in SCN4A, the gene encoding the pore-forming α subunit of Nav1.4, are responsible for a clinical spectrum of human diseases ranging from muscle stiffness (sodium channel myotonia, SCM) to muscle weakness. For years, only dominantly-inherited diseases resulting from Nav1.4 gain of function (GoF) were known, i.e., non-dystrophic myotonia (delayed muscle relaxation due to myofiber hyperexcitability), paramyotonia congenita and hyperkalemic or hypokalemic periodic paralyses (episodic flaccid muscle weakness due to transient myofiber hypoexcitability). These last 5 years, SCN4A mutations inducing Nav1.4 loss of function (LoF) were identified as the cause of dominantly and recessively-inherited disorders with muscle weakness: periodic paralyses with hypokalemic attacks, congenital myasthenic syndromes and congenital myopathies. We propose to name this clinical spectrum sodium channel weakness (SCW) as the mirror of SCM. Nav1.4 LoF as a cause of permanent muscle weakness was quite unexpected as the Na+ current density in the sarcolemma is large, securing the ability to generate and propagate muscle action potentials. The properties of SCN4A LoF mutations are well documented at the channel level in cellular electrophysiological studies However, much less is known about the functional consequences of Nav1.4 LoF in skeletal myofibers with no available pertinent cell or animal models. Regarding the therapeutic issues for Nav1.4 channelopathies, former efforts were aimed at developing subtype-selective Nav channel antagonists to block myofiber hyperexcitability. Non-selective, Nav channel blockers are clinically efficient in SCM and paramyotonia congenita, whereas patient education and carbonic anhydrase inhibitors are helpful to prevent attacks in periodic paralyses. Developing therapeutic tools able to counteract Nav1.4 LoF in skeletal muscles is then a new challenge in the field of Nav channelopathies. Here, we review the current knowledge regarding Nav1.4 LoF and discuss the possible therapeutic strategies to be developed in order to improve muscle force in SCW.

Anaesthesia challenges of a parturient with paramyotonia congenita and terminal filum lipoma presenting for labour and caesarean section under epidural anaesthesia – a case report

Background Paramyotonia congenita is a rare autosomal dominant myopathy which presents with periodic weakness due to cold and exercise. It is caused by mutations of the SCN4 gene which encodes the sodium channel in skeletal muscles. Case presentation We report a full term obstetric patient with both paramyotonia congenita and terminal filum lipoma who presents for induction of labour followed by an emergency caesarean section performed under epidural anesthesia. Her recovery is subsequently complicated by a 3-day history of postpartum paraparesis attributed to hypokalemic periodic paralysis. Conclusion We describe the perioperative anesthesia considerations and challenges in this case with a review of the current literature. This case report highlights the importance of early proactive and collaborative multidisciplinary approach, maintaining normal temperature and electrolytes with a heightened vigilance for muscle-related perioperative complications.

Identification of Novel Mutations of the CLCN-1 and SCN4A Genes in Non-dystrophic Myotonia in China

Background: The aim of our study was to characterize the genetic, pathological and clinical alterations of 17 patients in China presenting with non-dystrophic myotonia (NDM). Methods: We first sequenced the CLCN-1 gene in patients having clinical features and muscle pathology indicative of NDM. If no mutations were detected, we subsequently analyzed the SCN4A, KCNE3 and CACNA1S genes. Results: As determined by needle electromyography, patients may have accompanying atypical myopathy as well as muscle hypertrophy, secondary dystonia and joint contracture All participants in this study were administered mexiletine in combination with carbamazepine and showed significant improvements in their myotonia symptoms. Routine pathological examinations showed mild abnormalities in muscle pathology. Oxidative enzyme activity was decreased in many fibers. ATPase studies of fiber subtypes demonstrated a predominance of type 2A fibers and a complete absence of type 2B muscle fibers in patients with CLCN-1 mutations. CLCN-1 gene mutations were found in 8 cases diagnosed with myotonia congenital by gene screening. The detected mutations included 5 missense, 2 nonsense, 1 deletion and 2 insertions, and these CLCN-1 mutations were concentrated in exons 8 and 12. Further gene analysis showed 4 mutations in the SCN4A gene in patients diagnosed with paramyotonia congenita. One of these mutations was consistent with a previously reported mutation, whereas 3 mutations were novel. All of these novel mutations occurred within “hot spots” of exons 22 and 24. Five patients with NDM lacked any identifiable mutations in CLCN-1, SCN4A, CACNA1S or KCNE3. Conclusions: Myotonia congenita and paramyotonia congenita are the predominant forms of NDM in China. NDM may be best diagnosed using genetic analysis in combination with clinical features. New mutations of the CLCN-1 and SCN4A genes in patients with NDM were detected, we postulate that novel pathogenic genes for NDM occur in China.

Changes in Resurgent Sodium Current Contribute to the Hyperexcitability of Muscles in Patients with Paramyotonia Congenita

Paramyotonia congenita (PMC) is a rare hereditary skeletal muscle disorder. The major symptom, muscle stiffness, is frequently induced by cold exposure and repetitive exercise. Mutations in human SCN4A gene, which encodes the α-subunit of Nav1.4 channel, are responsible for PMC. Mutation screening of SCN4A gene from two PMC families identified two missense mutations, p.T1313M and p.R1448H. To elucidate the electrophysiological abnormalities caused by the mutations, the p.T1313M, p.R1448H, and wild-type (WT) SCN4A genes were transient expressed on Chinese hamster ovary (CHO-K1) cells. The detailed study on the gating defects of the mutant channels using the whole-cell patch clamping technique was performed. The mutant Nav1.4 channels impaired the basic gating properties with increasing sustained and window currents during membrane depolarization and facilitated the genesis of resurgent currents during repolarization. The mutations caused a hyperpolarization shift in the fast inactivation and slightly enhanced the slow inactivation with an increase in half-maximal inactivation voltage. No differences were found in the decay kinetics of the tail current between mutant and WT channels. In addition to generating the larger resurgent sodium current, the time to peak in the mutant channels was longer than that in the WT channels. In conclusion, our results demonstrated that the mutations p.T1313M and p.R1448H in Nav1.4 channels can enhance fast inactivation, slow inactivation, and resurgent current, revealing that subtle changes in gating processes can influence the clinical phenotype.

Case Report: General Anesthetic Management for Laparoscopic Cholecystectomy in Paramyotonia Congenita

Background: Paramytonia congenita is a rare disorder affecting skeletal muscle. Patients with this condition experience intermittent episodes of sustained myotonia, though the disease is non-progressive. Due to the predisposition for prolonged muscle contraction, special attention must be paid to anesthetic management during operative procedures to prevent complications similar to those seen in patients with malignant hyperthermia. To date, however, limited reports of anesthetic management in paramyotonia congenita are available. Case: Therefore, the present report describes successful general anesthetic management in a patient with paramyotonia congenita using propofol and ketamine for induction, non-depolarizing rocuronium for muscle paralysis, and continuous nitric oxide and IV propofol infusion for sedation. The patient remained stable throughout the case without myotonic episodes or other complications. Conclusion: Our report describes successful anesthetic management in a patient with paramyotonia congenita. This provides a potential management plan that may be applied to PC patients undergoing a variety of surgical procedures and eliminates risk associated with succinylcholine and possibly volatile anesthetics. Further research is needed to determine whether this approach is superior to previously reported techniques, and should also aim to identify which agents may be effectively utilized to reverse a myotonic episode in PC patients should one occur intra-operatively.