Managing pregnancy and anaesthetics in patients with skeletal muscle channelopathies.

The skeletal muscle channelopathies are a group of rare diseases and include non-dystrophic myotonia and periodic paralysis. Given their rarity, little has been published on the management of anaesthesia and pregnancy in this cohort despite being important aspects of care. We have conducted a large study of over 70 patients who underwent anaesthesia and 87 pregnancies to investigate the problems encountered following anaesthesia or during pregnancy. This was performed via patient surveys sent out to genetically confirmed channelopathy patients seen at the National Hospital for Neurology and Neurosurgery. Most significantly in our cohort, patients frequently experienced a worsening or precipitation of symptoms during pregnancy (75%) or following anaesthetic (31%). None of our patients developed malignant hyperthermia, although there are confirmed reports of this in patients with periodic paralysis and mutations in RYR1. There was a significantly higher number of miscarriages compared to the normal population. There was no significant difference in antenatal or delivery complications compared to the general population. However, three neonates did have complications, all of whom were found to carry mutations in SCN4A. This study highlights the importance of counselling patients and clinicians for the possibility of worsening symptoms during pregnancy or anaesthesia and the careful management of neonates following delivery.

Central Role of Subthreshold Currents in Myotonia.

It is generally thought that muscle excitability is almost exclusively controlled by currents responsible for generation of action potentials. We propose that smaller ion channel currents that contribute to setting the resting potential and to subthreshold fluctuations in membrane potential can also modulate excitability in important ways. These channels open at voltages more negative than the action potential threshold and are thus termed subthreshold currents. As subthreshold currents are orders of magnitude smaller than the currents responsible for the action potential, they are hard to identify and easily overlooked. Discovery of their importance in regulation of excitability opens new avenues for improved therapy for muscle channelopathies and diseases of the neuromuscular junction. ANN NEUROL 2020;87:175-183.

A zebrafish model of nondystrophic myotonia with sodium channelopathy.

Nondystrophic myotonias are disorders of Na+ (Nav1.4 or SCN4A) and Cl- (CLCN1) channels in skeletal muscles, and frequently show phenotype heterogeneity. The molecular mechanism underlying their pathophysiology and phenotype heterogeneity remains unclear. As zebrafish models have been recently exploited for studies of the pathophysiology and phenotype heterogeneity of various human genetic diseases, a zebrafish model may be useful for delineating nondystrophic myotonias. Here, we generated transgenic zebrafish expressing a human mutant allele of SCN4A, referred to as Tg(mylpfa:N440K), and needle electromyography revealed increased number of myotonic discharges and positive sharp waves in the muscles of Tg(mylpfa:N440K) than in controls. In addition, forced exercise test at a water temperature of 24 °C showed a decrease in the distance moved, time spent in and number of visits to the zone with stronger swimming resistance. Finally, a forced exercise test at a water temperature of 18 °C exhibited a higher number of dive-bombing periods and drifting-down behavior than in controls. These findings indicate that Tg(mylpfa:N440K) is a good vertebrate model of exercise- and cold-induced human nondystrophic myotonias. This zebrafish model may contribute to provide insight into the pathophysiology of myotonia in sodium channelopathy and could be used to explore a new therapeutic avenue.

Patient-reported study of the impact of pediatric-onset myotonic dystrophy.

INTRODUCTION: The prevalence and impact of symptoms affecting individuals with pediatric forms of myotonic dystrophy type-1 (DM1) are not well understood. METHODS: Patients from the United States, Canada, and Sweden completed a survey that investigated 20 themes associated with pediatric-onset DM1. Participants reported the prevalence and importance of each theme affecting their lives. Surveys from participants were matched with surveys from their caregivers for additional analysis. RESULTS: The most prevalent symptomatic themes included problems with hands or fingers (79%) and gastrointestinal issues (75%). Problems with urinary/bowel control and gastrointestinal issues were reported to have the greatest impact on patients' lives. Responses from participants and their caregivers had varying levels of agreement among symptomatic themes. DISCUSSION: Many symptoms have meaningful impact on disease burden. The highest levels of agreement between caregivers and individuals with pediatric forms of myotonic dystrophy were found for physical activity themes.

Myasthenic congenital myopathy from recessive mutations at a single residue in NaV1.4.

OBJECTIVE: To identify the genetic and physiologic basis for recessive myasthenic congenital myopathy in 2 families, suggestive of a channelopathy involving the sodium channel gene, SCN4A. METHODS: A combination of whole exome sequencing and targeted mutation analysis, followed by voltage-clamp studies of mutant sodium channels expressed in fibroblasts (HEK cells) and Xenopus oocytes. RESULTS: Missense mutations of the same residue in the skeletal muscle sodium channel, R1460 of NaV1.4, were identified in a family and a single patient of Finnish origin (p.R1460Q) and a proband in the United States (p.R1460W). Congenital hypotonia, breathing difficulties, bulbar weakness, and fatigability had recessive inheritance (homozygous p.R1460W or compound heterozygous p.R1460Q and p.R1059X), whereas carriers were either asymptomatic (p.R1460W) or had myotonia (p.R1460Q). Sodium currents conducted by mutant channels showed unusual mixed defects with both loss-of-function (reduced amplitude, hyperpolarized shift of inactivation) and gain-of-function (slower entry and faster recovery from inactivation) changes. CONCLUSIONS: Novel mutations in families with myasthenic congenital myopathy have been identified at p.R1460 of the sodium channel. Recessive inheritance, with experimentally established loss-of-function, is a consistent feature of sodium channel based myasthenia, whereas the mixed gain of function for p.R1460 may also cause susceptibility to myotonia.

Quantitative myotonia assessment with a commercially available dynamometer in myotonic dystrophy types 1 and 2.

INTRODUCTION: The objective of this study was to develop a simple method for quantitative assessment of myotonia in patients with myotonic dystrophy type 1 (DM1) and DM2, to compare the myotonia severity, and to correlate this objective outcome with a subjective scale, the Myotonia Behaviour Scale (MBS). METHODS: A commercially available dynamometer was used for all measurements. The relaxation time after voluntary contraction was measured in 20 patients with DM1, 25 patients with DM2, and 35 healthy controls. RESULTS: The average relaxation time was 0.17 s in controls, 2.96 s in patients with DM1, and 0.4 s in patients with DM2. The correlation between relaxation time and MBS score was significant, 0.627 in patients with DM1 and 0.581 in patients with DM2. DISCUSSION: Our method provides a valid and reliable quantitative measure of grip myotonia suitable as an outcome measure in clinical trials and as part of routine examinations to gather data on the natural history of myotonic disorders. Muscle Nerve 59:431-435, 2019.

[Dystrophic and non-dystrophic myotonias]. / Dystrophische und nicht-dystrophische Myotonien.

Myotonic syndromes are rare neuromuscular diseases characterized by the clinical or neurophysiological detection of myotonia. The genetic defects involve primarily or secondarily the muscular isoforms of the ion channels. The channel dysfunction consecutively leads to a hyper-excitability of the muscle membrane and the clinical symptom myotonia. Two forms of dystrophic myotonic diseases are currently known: the myotonic dystrophy type 1 (DM1) and the myotonic dystrophy type 2 (DM2). They are multisystemic diseases clinically characterized by a combination of myotonia and other muscular symptoms (muscle weakness, wasting and myalgia) together with the involvement of other organs and systems (cataract, diabetes, heart diseases, hormone dysfunctions). The non-dystrophic myotonic diseases are caused by mutations affecting either the chloride ion channels or the sodium ion channels. The clinical picture is dominated by the presence of myotonia and other minor muscular complaints as mild episodic weakness and muscle hypertrophy. The differential diagnosis among the myotonic syndromes is extremely challenging leading to a significant diagnostic delay. This review will update on the main clinical, diagnostic and therapeutic aspects of myotonic syndromes to guide general neurologists through an earlier diagnosis and better management.

A Mixed Periodic Paralysis & Myotonia Mutant, P1158S, Imparts pH-Sensitivity in Skeletal Muscle Voltage-gated Sodium Channels.

Skeletal muscle channelopathies, many of which are inherited as autosomal dominant mutations, include myotonia and periodic paralysis. Myotonia is defined by a delayed relaxation after muscular contraction, whereas periodic paralysis is defined by episodic attacks of weakness. One sub-type of periodic paralysis, known as hypokalemic periodic paralysis (hypoPP), is associated with low potassium levels. Interestingly, the P1158S missense mutant, located in the third domain S4-S5 linker of the "skeletal muscle", Nav1.4, has been implicated in causing both myotonia and hypoPP. A common trigger for these conditions is physical activity. We previously reported that Nav1.4 is relatively insensitive to changes in extracellular pH compared to Nav1.2 and Nav1.5. Given that intense exercise is often accompanied by blood acidosis, we decided to test whether changes in pH would push gating in P1158S towards either phenotype. Our results suggest that, unlike in WT-Nav1.4, low pH depolarizes the voltage-dependence of activation and steady-state fast inactivation, decreases current density, and increases late currents in P1185S. Thus, P1185S turns the normally pH-insensitive Nav1.4 into a proton-sensitive channel. Using action potential modeling we predict a pH-to-phenotype correlation in patients with P1158S. We conclude that activities which alter blood pH may trigger the noted phenotypes in P1158S patients.

Advances in assessing myotonia: Can sensor-engineered glove have a role?

Non-dystrophic (NDMs) and Dystrophic Myotonias (DMs) are diseases characterized by the presence of myotonia with or without muscle weakness. A standardized myotonia assessment is important to more objectively quantify the handgrip myotonia. We screened 10 patients affected by NDM and 10 patients with DM, using the sensor-engineered glove (SEG). The time required to perform a complete finger extension (grip myotonia time, GMT) at maximum velocity (MV) after maximum voluntary contraction (MVC) was evaluated through an ad hoc protocol including rest, exercise, and ice effects on handgrip myotonia. We observed a general trend to GMT increase when applying the ice block and a GMT decrease when repeating GM movements, at individual level in both NDM and DM patients. SEG is an automated, non-invasive, quick, and easy technique for evaluating handgrip myotonia in NDM and DM patients. SEG could, therefore, be considered a promising tool to evaluate myotonia and monitor treatment efficacy for clinical trials.

Aerobic training in myotonia congenita: Effect on myotonia and fitness.

INTRODUCTION: Exercise has not been investigated in myotonia congenita (MC). We investigated whether regular aerobic training can reduce myotonia and improve fitness. METHODS: Untrained patients with MC (age: 24-62 years; n = 6) completed 28 ± 3 sessions of 30-minute cycle ergometer training at 75% of maximal capacity for 11 ± 1 weeks. Fitness was evaluated by maximal oxygen uptake. The level of myotonia was assessed by the Myotonia Behavior Scale, 14 step stair test, timed up and go test, and hand and eye closure-open tests. RESULTS: Training increased fitness by 9% (95% confidence interval [CI], 1-17%; P = 0.02) and maximal workload by 10% (95% CI, 3-18%; P = 0.03). None of the myotonia tests changed in a clinically meaningful way. CONCLUSIONS: Regular endurance training improves fitness and maximal workload performance in patients with MC. The lack of creatine kinase elevations indicates that muscle damage did not occur. Improved fitness, however, did not change myotonic symptoms in this small cohort. Muscle Nerve 56: 696-699, 2017.

Increased sodium channel use-dependent inhibition by a new potent analogue of tocainide greatly enhances in vivo antimyotonic activity.

Although the sodium channel blocker, mexiletine, is the first choice drug in myotonia, some myotonic patients remain unsatisfied due to contraindications, lack of tolerability, or incomplete response. More therapeutic options are thus needed for myotonic patients, which require clinical trials based on solid preclinical data. In previous structure-activity relationship studies, we identified two newly-synthesized derivatives of tocainide, To040 and To042, with greatly enhanced potency and use-dependent behavior in inhibiting sodium currents in frog skeletal muscle fibers. The current study was performed to verify their potential as antimyotonic agents. Patch-clamp experiments show that both compounds, especially To042, are greatly more potent and use-dependent blockers of human skeletal muscle hNav1.4 channels compared to tocainide and mexiletine. Reduced effects on F1586C hNav1.4 mutant suggest that the compounds bind to the local anesthetic receptor, but that the increased hindrance and lipophilia of the N-substituent may further strengthen drug-receptor interaction and use-dependence. Compared to mexiletine, To042 was 120 times more potent to block hNav1.4 channels in a myotonia-like cellular condition and 100 times more potent to improve muscle stiffness in vivo in a previously-validated rat model of myotonia. To explore toxicological profile, To042 was tested on hERG potassium currents, motor coordination using rotarod, and C2C12 cell line for cytotoxicity. All these experiments suggest a satisfactory therapeutic index for To042. This study shows that, owing to a huge use-dependent block of sodium channels, To042 is a promising candidate drug for myotonia and possibly other membrane excitability disorders, warranting further preclinical and human studies.

A case of non-dystrophic myotonia with concomitant mutations in the SCN4A and CLCN1 genes.

Non-dystrophic myotonias are caused by mutations of either the skeletal muscle chloride (CLCN1) or sodium channel (SCN4A) gene. They exhibit several distinct phenotypes, including myotonia congenita, paramyotonia congenita and sodium channel myotonia, and a genotype-phenotype correlation has been established. However, there are atypical cases that do not fit with the standard classification. We report a case of 27-year-old male who had non-dystrophic myotonia with periodic paralysis and two heterozygous mutations, E950K in CLCN1 and F1290L in SCN4A. His mother, who exhibited myotonia without paralytic attack, only harbored E950K, and no mutations were identified in his asymptomatic father. Therefore, the E950K mutation was presumed to be pathogenic, although it was reported as an extremely rare genetic variant. The proband experienced paralytic attacks that lasted for weeks and were less likely to be caused by CLCN1 mutation alone. Functional analysis of the F1290L mutant channel heterologously expressed in cultured cells revealed enhanced activation inducing membrane hyperexcitability. We therefore propose that the two mutations had additive effects on membrane excitability that resulted in more prominent myotonia in the proband. Our case stresses the value of performing genetic analysis of both CLCN1 and SCN4A genes for myotonic patients with an atypical phenotype.

Influences of Pregnancy on Different Genetic Subtypes of Non-Dystrophic Myotonia and Periodic Paralysis.

BACKGROUND: There are only few reports of pregnancy and delivery in non-dystrophic myotonia or periodic paralysis caused by CLCN1 or SCN4A gene mutations. METHODS: We report the medical histories and personal attitudes of 5 unrelated German patients, 2 following autosomal recessive inheritance (case 1; most likely and case 2; confirmed Becker disease) and 3 following autosomal dominant inheritance (case 3; CLCN1 mutation, cases 4-5; SCN4A mutations), who delivered a total of 9 children. RESULTS: Apart from case 5 with periodic paralysis, who had 5 early miscarriages and pre-eclampsia resulting in cesarean delivery, there was no evidence of increased obstetric complication rates, and neonatal outcome was favorable. In all patients, there was aggravation of myotonia or weakness in pregnancy, followed by a short-term improvement after delivery in cases 2 and 3. Mexiletine medication improved the clinical features significantly in case 2 but was unable to control pregnancy-related deterioration. In case 4 (and her sister) and case 5, there was a clear disease aggravation in pregnancy resulting in hospitalization or repeated neurological examinations. CONCLUSION: Pregnancy can be regarded as a strong triggering factor in inherited non-dystrophic myotonias and periodic paralysis, regardless of the underlying gene defect.

Clinical and Electrophysiological Findings in Hereditary Inclusion Body Myopathy Compared With Sporadic Inclusion Body Myositis.

OBJECTIVE: To compare the clinical and electrophysiological findings in hereditary inclusion body myopathy (hIBM) and sporadic inclusion body myositis (sIBM) patients. METHODS: We retrospectively identified 8 genetically proven hIBM patients and 1 DNAJB6 myopathy with pathological features of hIBM, and compared their clinical, electromyographic, and serological data with a group of 51 pathologically proven sIBM patients. RESULTS: hIBM patients had a younger mean age of onset (36 vs. 60 years, P = 0.0001). Diagnostic delay was shorter in sIBM (6 vs. 15 years, P = 0.0003). Wrist flexors (P = 0.02), digit flexors (P = 0.01), digit extensors (P = 0.02), and quadriceps (P = 0.008) muscles were more frequently affected in sIBM. Fibrillation potentials were more common in sIBM patients (P = 0.03). Electrical myotonia was found in 4 hIBM patients, not significantly different from sIBM patients (P = 0.45). Creatinine kinase was higher in sIBM patients (799 vs 232, P = 0.03). CONCLUSIONS: sIBM and hIBM seem to have similar electromyographic changes. The combination of clinical, serological, and histopathological findings can guide genetic testing to the final diagnosis.

Muscleblind-Like 1 and Muscleblind-Like 3 Depletion Synergistically Enhances Myotonia by Altering Clc-1 RNA Translation.

Loss of Muscleblind-like 1 (Mbnl1) is known to alter Clc-1 splicing to result in myotonia. Mbnl1(ΔE3/ΔE3)/Mbnl3(ΔE2) mice, depleted of Mbnl1 and Mbnl3, demonstrate a profound enhancement of myotonia and an increase in the number of muscle fibers with very low Clc-1 currents, where gClmax values approach ~ 1 mS/cm(2), with the absence of a further enhancement in Clc-1 splice errors, alterations in polyA site selection or Clc-1 localization. Significantly, Mbnl1(ΔE3/ΔE3)/Mbnl3(ΔE2) muscles demonstrate an aberrant accumulation of Clc-1 RNA on monosomes and on the first polysomes. Mbnl1 and Mbnl3 bind Clc-1 RNA and both proteins bind Hsp70 and eEF1A, with these associations being reduced in the presence of RNA. Thus binding of Mbnl1 and Mbnl3 to Clc-1 mRNA engaged with ribosomes can facilitate an increase in the local concentration of Hsp70 and eEF1A to assist Clc-1 translation. Dual depletion of Mbnl1 and Mbnl3 therefore initiates both Clc-1 splice errors and translation defects to synergistically enhance myotonia. As the HSA(LR) model for myotonic dystrophy (DM1) shows similar Clc-1 defects, this study demonstrates that both splice errors and translation defects are required for DM1 pathology to manifest. RESEARCH IN CONTEXT: Research in context: Myotonic Dystrophy type 1 (DM1) is a dominant disorder resulting from the expression of expanded CUG repeat RNA, which aberrantly sequesters and inactivates the muscleblind-like (MBNL) family of proteins. In mice, inactivation of Mbnl1 is known to alter Clc-1 splicing to result in myotonia. We demonstrate that concurrent depletion of Mbnl1 and Mbnl3 results in a synergistic enhancement of myotonia, with an increase in muscle fibers showing low chloride currents. The observed synergism results from the aberrant accumulation of Clc-1 mRNA on monosomes and the first polysomes. This translation error reflects the ability of Mbnl1 and Mbnl3 to act as adaptors that recruit Hsp70 and eEF1A to the Clc-1 mRNA engaged with ribosomes, to facilitate translation. Thus our study demonstrates that Clc-1 RNA translation defects work coordinately with Clc-1 splice errors to synergistically enhance myotonia in mice lacking Mbnl1 and Mbnl3.

Myotonia-like symptoms in a patient with spinal and bulbar muscular atrophy.

We describe the case of a 33-year-old man with a 4-year history of worsening muscle stiffness and weakness in his right hand. He showed elevated serum creatine kinase levels at the onset of muscle stiffness that was characterized by delayed muscle relaxation after voluntary contraction. This symptom often occurred during cold exposure, and was partially attenuated by sodium channel blockade. Electrodiagnostic findings in repetitive nerve stimulation, short-exercise, and cooling tests were normal. Electromyography showed chronic denervation potentials in his cranial, cervical, thoracic, and lumbosacral myotomes without myotonic discharge. He exhibited facial and tongue fasciculations, hypernasality, gynecomastia, neurogenic changes in muscle biopsy, and increased serum testosterone levels. Spinal and bulbar muscular atrophy (SBMA) was diagnosed on the basis of the CAG trinucleotide expansion in the gene coding androgen receptor. A myotonia-like symptom without myotonic discharge may present as an early neurological sign of SBMA, which possibly reflects a sodium channel dysfunction in skeletal muscles.

The course and outcome of pregnancy in women with nondystrophic myotonias.

INTRODUCTION: Nondystrophic myotonias (NDM) are muscle channelopathies characterized by clinical and electrical myotonia. Little is known about pregnancy in NDM. The aim of this work was to assess pregnancy outcomes in women with NDM. METHODS: In this investigation, we utilized a cross-sectional approach and obtained questionnaire data and medical record review information. Pregnancy outcomes were compared with U.S. national data. RESULTS: Twenty-five women completed the study. There were 63 pregnancies with 53 live births, 8 miscarriages, 4 terminations, and 1 stillbirth. The infertility rate was 28% (above the U.S. average). Fetal distress was reported in 11.7% (U.S. average 3.6%) of the patients. NDM symptoms worsened in 62% of pregnancies. In those who worsened, symptoms usually resolved completely (66%) or partially (32%) after delivery in 2 days to 12 months, the majority (76%) within 3 months. CONCLUSIONS: Most pregnancy outcomes were favorable. NDM worsened in nearly two-thirds of pregnancies, but usually improved shortly after delivery. Incidences of infertility and fetal distress were higher than overall U.S. rates.

Use of Clinical and Electrical Myotonia to Differentiate Childhood Myopathies.

We retrospectively reviewed 2030 childhood electromyograms performed over an 11-year period (2004-2014). Twenty children (1%) with myotonic discharges were identified and placed into 2 groups. Group A (electrical and clinical myotonia) comprised 9 children (8 with myotonia congenita and 1 with paramyotonia congenita); all of them had diffuse myotonic discharges without clinical weakness or elevated creatine kinase. Group B (electrical myotonia without clinical myotonia) comprised 11 children (4 with inflammatory myopathy; 3, congenital myopathy, 3, muscular dystrophy; and 1, congenital muscular dystrophy). Clinical weakness was demonstrated in all of them and elevated creatine kinase in 6; all had a myopathic electromyogram and scattered myotonic discharges. We conclude that myotonic discharges are a rare but characteristic spontaneous discharge identified during electrodiagnostic studies in children. The presence of electrical and clinical myotonia provides helpful clues to differentiate between various muscle disorders in children.