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The nondystrophic myotonias are rare muscle hyperexcitability disorders caused by gain-of-function mutations in the SCN4A gene or loss-of-function mutations in the CLCN1 gene. Clinically, they are characterized by myotonia, defined as delayed muscle relaxation after voluntary contraction, which leads to symptoms of muscle stiffness, pain, fatigue, and weakness. Diagnosis is based on history and examination findings, the presence of electrical myotonia on electromyography, and genetic confirmation. In the absence of genetic confirmation, the diagnosis is supported by detailed electrophysiological testing, exclusion of other related disorders, and analysis of a variant of uncertain significance if present. Symptomatic treatment with a sodium channel blocker, such as mexiletine, is usually the first step in management, as well as educating patients about potential anesthetic complications., (© 2020 Wiley Periodicals, Inc.)

Skeletal muscle channelopathies are rare genetic neuromuscular conditions that include the nondystrophic myotonias and periodic paralyses. They cause disabling muscle symptoms and can limit educational potential, work opportunities, socialization, and quality of life. Effective therapy is available, making it essential to recognize and treat this group of disorders. Here, the authors highlight important aspects regarding diagnosis and management using illustrative case reports., Competing Interests: Disclosures Part of this work was undertaken at University College London Hospitals/University College London, which received a proportion of funding from the Department of Health’s National Institute for Health Research Biomedical Research Centres funding scheme. M.G. Hanna receives research funds from the Medical Research Council and the UCLH Biomedical Research Centre. E. Matthews receives research funds from Wellcome., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Introduction: Mutations of the voltage-gated sodium channel gene (SCN4A), which encodes Nav1.4, cause nondystrophic myotonia that occasionally is associated with severe apnea and laryngospasm. There are case reports of nondystrophic myotonia due to mutations in the C-terminal tail (CTerm) of Nav1.4, but the functional analysis is scarce., Methods: We present two families with nondystrophic myotonia harboring a novel heterozygous mutation (E1702del) and a known heterozygous mutation (E1702K)., Results: The proband with E1702K exhibited repeated rhabdomyolysis, and the daughter showed laryngospasm and cyanosis. Functional analysis of the two mutations as well as another known heterozygous mutation (T1700_E1703del), all located on EF hand-like motif in CTerm, was conducted with whole-cell recording of heterologously expressed channel. All mutations displayed impaired fast inactivation., Discussion: The CTerm of Nav1.4 is vital for regulating fast inactivation. The study highlights the importance of accumulating pathological mutations of Nav1.4 and their functional analysis data., (© 2020 Wiley Periodicals, Inc.)

Nondystrophic myotonias are disorders of Na + (Na v 1.4 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., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Background: Paramyotonia congenita (PC; OMIM 168300) is a non-dystrophic myotonia caused by mutations in the SCN4A gene. Transient muscle stiffness, usually induced by exposure to cold and aggravated by exercise, is the predominant clinical symptom, and interictal persistent weakness is uncommon., Case Report: We report a family with a history of PC accompanied by persistent hand muscle weakness with masticatory muscle involvement. Persistent weakness was exacerbated with age, and MR analysis showed marked atrophy of temporal, masseter, and finger flexor muscles with fatty replacement. The PC causative mutation T1313M in the SCN4A gene was prevalent in the family. Administration of acetazolamide chloride improved clinical symptoms and the results of cold and short exercise tests. Phenotypic variation within the family was remarkable, as the two younger affected patients did not present with persistent weakness or muscle atrophy., Conclusions: PC associated with the T1313M mutation is a possible cause of persistent distal hand weakness.

Although myopathies and neuromuscular junction disorders are typically distinct, their coexistence has been reported in several inherited and acquired conditions. Affected individuals have variable clinical phenotypes but typically display both a decrement on repetitive nerve stimulation and myopathic findings on muscle biopsy. Inherited causes include myopathies related to mutations in BIN1, DES, DNM2, GMPPB, MTM1, or PLEC and congenital myasthenic syndromes due to mutations in ALG2, ALG14, COL13A1, DOK7, DPAGT1, or GFPT1. Additionally, a decrement due to muscle fiber inexcitability is observed in certain myotonic disorders. The identification of a defect of neuromuscular transmission in an inherited myopathy may assist in establishing a molecular diagnosis and in selecting patients who would benefit from pharmacological correction of this defect. Acquired cases meanwhile stem from the co-occurrence of myasthenia gravis or Lambert-Eaton myasthenic syndrome with an immune-mediated myopathy, which may be due to paraneoplastic disorders or exposure to immune checkpoint inhibitors., (© 2019 Wiley Periodicals, Inc.)

Background: Paramyotonia congenita is a non-dystrophic myotonia, in which muscle relaxation is delayed after voluntary or evoked contraction. This condition cannot be distinguished on the basis of symptoms and signs alone. It requires consideration of genetics as more than 100 mutations in the CLCN1 gene and at least 20 mutations in the SCN4A gene are associated with the clinical features of the non-dystrophic myotonias. Only a few families with the described features but no genetic testing have been reported in Slovakia. This prompted us to investigate genetic mutations in the SCN4A gene in 3 Slovak families clinically diagnosed with paramyotonia., Subjects and Methods: Genomic DNA of the family members was extracted from peripheral blood and amplified by polymerase chain reaction. SCN4A variants were screened by Sanger sequencing., Results: Our results revealed 2 potential disease-causing mutations present in the probands and affected family members - mutations c.3938C > T (p.T1313M) in two families and mutation c.2111C>T (p. T704M) in one family., Conclusion: Our results may help to identify genetic determinants as well as clarify genotype-phenotype relationships in patients with paramyotonia in Slovakia.

Some patients with multiple sclerosis (MS) complain of symptoms, such as myokymia, myotonia, spasms, and stiffness, which have been demonstrated to be due to a concurrent non-dystrophic myotonia, i.e. myotonia congenita or paramyotonia congenita. Beyond the known casual association between MS and non-dystrophic myotonia, a channelopathy representing a primary trait of MS rather than an epiphenomenon of demyelization (i.e., an acquired channelopathy) may exist. Indeed, the finding of MS patients with no genetic evidence of non-dystrophic myotonia but showing a clinical picture resembling this condition would support this hypothesis. Thirty patients with MS and no concurrent diagnosis of myotonia congenita or paramyotonia congenita were submitted to the Fournier protocol. Some of these MS patients presented abnormal muscle excitability with scarce myotonic discharges, but only a few of them had clinical features compatible with myotonia congenita or paramyotonia congenita syndromes. Even though the low number of recruited patients did not allow a robust statistical analysis, our data seemed to indicate the presence of an ion channel dysfunction that is independent of the acquired channelopathies and likely represents a common pathophysiological mechanism underlying a unique channelopathy simultaneously involving the peripheral and the central nervous system in individuals with MS. Confirming the presence of such a primary channelopathy in MS patients is of non-negligible importance, since dysfunction of ion channels may represent a suitable therapeutic target in MS.

Introduction: The gain-of-function mutations that underlie sodium channel myotonia (SCM) and paramyotonia congenital (PMC) produce differing clinical phenotypes. We used muscle velocity recovery cycles (MVRCs) to investigate membrane properties., Methods: MVRCs and responses to trains of stimuli were compared in patients with SCM (n = 9), PMC (n = 8), and normal controls (n = 26)., Results: The muscle relative refractory period was reduced in SCM, consistent with faster recovery of the mutant sodium channels from inactivation. Both SCM and PMC showed an increased early supernormality and increased mean supernormality following multiple conditioning stimuli, consistent with slowed sodium channel inactivation. Trains of fast impulses caused a loss of amplitude in PMC, after which only half of the muscle fibers recovered, suggesting that the remainder stayed depolarized by persistent sodium currents., Discussion: The differing effects of mutations on sodium channel function can be demonstrated in human subjects in vivo using this technique. Muscle Nerve 57: 586-594, 2018., (© 2017 Wiley Periodicals, Inc.)

Background: The central nervous system involvement, in terms of a maladaptive sensory-motor plasticity, is well known in patients with dystrophic myotonias (DMs). To date, there are no data suggesting a central nervous system involvement in non-dystrophic myotonias (NDMs)., Objective: To investigate sensory-motor plasticity in patients with Myotonia Congenita (MC) and Paramyotonia Congenita (PMC) with or without mexiletine., Methods: Twelve patients with a clinical, genetic, and electromyographic evidence of MC, fifteen with PMC, and 25 healthy controls (HC) were included in the study. TMS on both primary motor cortices (M1) and a rapid paired associative stimulation (rPAS) paradigm were carried out to assess M1 excitability and sensory-motor plasticity., Results: patients showed a higher cortical excitability and a deterioration of the topographic specificity of rPAS aftereffects, as compared to HCs. There was no correlation among neurophysiological and clinical-demographic characteristics. Noteworthy, the patients who were under mexiletine showed a minor impairment of the topographic specificity of rPAS aftereffects as compared to those who did not take the drug., Conclusion: our findings could suggest the deterioration of cortical sensory-motor plasticity in patients with NDMs as a trait of the disease.

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., (© 2016 S. Karger AG, Basel.)

We report a patient with paramyotonia congenita/hyperkalemic periodic paralysis due to Nav1.4 I693T mutation who had worsening of myotonia and muscle weakness in the setting of hypomagnesemia and hypocalcemia with marked recovery after magnesium administration. Computer simulations of the effects of the I693T mutation were introduced in the muscle fiber model by both hyperpolarizing shifts in the Nav1.4 channel activation and a faster recovery from slow channel inactivation. A further shift in the Nav1.4 channel activation in the hyperpolarizing direction as expected with low divalent cations resulted in myotonia that progressed to membrane inexcitability. Shifting the channel activation in the depolarizing direction as would be anticipated from magnesium supplementation abolished the myotonia. These observations provide clinical and biophysical evidence that the muscle symptoms in sodium channelopathy are sensitive to divalent cations. Exploration of the role of magnesium administration in therapy or prophylaxis is warranted with a randomized clinical trial., (Published by Elsevier B.V.)

Non-dystrophic myotonic syndromes represent a heterogeneous group of clinically quite similar diseases sharing the feature of myotonia. These syndromes can be separated into chloride and sodium channelopathies, with gene-defects in chloride or sodium channel proteins of the sarcolemmal membrane. Myotonia has its basis in an electrical instability of the sarcolemmal membrane. In the present study we examine the discriminative power of the resulting myotonic discharges for these disorders. Needle electromyography was performed by an electromyographer blinded for genetic diagnosis in 66 non-dystrophic myotonia patients (32 chloride and 34 sodium channelopathy). Five muscles in each patient were examined. Individual trains of myotonic discharges were extracted and analyzed with respect to firing characteristics. Myotonic discharge characteristics in the rectus femoris muscle almost perfectly discriminated chloride from sodium channelopathy patients. The first interdischarge interval as a single variable was longer than 30 ms in all but one of the chloride channelopathy patients and shorter than 30 ms in all of the sodium channelopathy patients. This resulted in a detection rate of over 95%. Myotonic discharges of a single muscle can be used to better guide toward a molecular diagnosis in non-dystrophic myotonic syndromes., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Genetic testing is considered the only reliable diagnostic approach in myotonic dystrophy. However it has recently been reported that a considerable number of patients with genetically proven types of the disease have unusual phenotypic presentation. The aim of our study was to evaluate motor unit reorganization reflected by various electrophysiological abnormalities in myotonic dystrophies and to compare findings between type 1 (DM 1) and type 2 myotonic dystrophy (DM2). Quantitative electromyography (EMG) recordings in 63 patients (33 with DM1 and 30 with DM2) from the biceps brachii (BB), rectus femoris (RF), first dorsal interosseus (FDI), and tibialis anterior (TA) muscles were analyzed. Mean amplitude and size index (SI) of motor unit potentials recorded in TA and RF muscles, mean potential duration in TA, and mean SI and the number of outliers with amplitude above the normal range in BB were significantly increased in DM2 as compared to DM1. Myotonic discharges were recorded more frequently in DM1 than in DM2. EMG findings significantly differ between DM1 and DM2. The presence of high amplitude potentials in lower limb muscles in DM2 patients, atypical for myogenic muscle lesions, could be explained by muscle fiber hypertrophy observed in muscle biopsies., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Skeletal muscle channelopathies are rare heterogeneous diseases with marked genotypic and phenotypic variability. Despite advances in understanding of the molecular pathology of these disorders, the diverse phenotypic manifestations remain a challenge in diagnosis and therapeutics. These disorders can cause lifetime disability and affect quality of life. There is no treatment of these disorders approved by the US Food and Drug Administration at this time. Recognition and treatment of symptoms might reduce morbidity and improve quality of life. This article summarizes the clinical manifestations, diagnostic studies, pathophysiology, and treatment options in nondystrophic myotonia, congenital myasthenic syndrome, and periodic paralyses., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Myotonic dystrophy is the commonest adult muscular dystrophy. Myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (DM2) are often discussed jointly, and although they share many clinical and molecular features, differences do exist. Historically, more is known about DM1 than about DM2. The literature in the field of myotonic dystrophy is broad, with advances in our understanding of DM2. This article reviews recent developments in DM2 with respect to diagnosis, systemic features, and molecular mechanisms of the disease.

Introduction: Standard electromyography (EMG) is useful in the diagnosis of myotonic dystrophy type 1 (DM1) and type 2 (DM2), but it does not differentiate between them. The aim of this study was to estimate the utility of the short exercise test (SET) and short exercise test with cooling (SETC) in differentiating between DM1 and DM2., Methods: SET and SETC were performed in 32 patients with DM1 (mean age 35.8 ± 12.7 years) and 28 patients with DM2 (mean age 44.5 ± 12.5 years)., Results: We observed a significant decline in compound motor action potential (CMAP) amplitude in DM1 with both SET and SETC immediately after effort. In DM2, there was no marked change in CMAP amplitude with either SET or SETC., Conclusions: SET and SETC may serve as useful tools for clinical differentiation between DM1 and DM2, and they may be used as a guide for molecular testing., (Copyright © 2013 Wiley Periodicals, Inc.)

Objective: To develop RNA splicing biomarkers of disease severity and therapeutic response in myotonic dystrophy type 1 (DM1) and type 2 (DM2)., Methods: In a discovery cohort, we used microarrays to perform global analysis of alternative splicing in DM1 and DM2. The newly identified splicing changes were combined with previous data to create a panel of 50 putative splicing defects. In a validation cohort of 50 DM1 subjects, we measured the strength of ankle dorsiflexion (ADF) and then obtained a needle biopsy of tibialis anterior (TA) to analyze splice events in muscle RNA. The specificity of DM-associated splicing defects was assessed in disease controls. The CTG expansion size in muscle tissue was determined by Southern blot. The reversibility of splicing defects was assessed in transgenic mice by using antisense oligonucleotides to reduce levels of toxic RNA., Results: Forty-two splicing defects were confirmed in TA muscle in the validation cohort. Among these, 20 events showed graded changes that correlated with ADF weakness. Five other splice events were strongly affected in DM1 subjects with normal ADF strength. Comparison to disease controls and mouse models indicated that splicing changes were DM-specific, mainly attributable to MBNL1 sequestration, and reversible in mice by targeted knockdown of toxic RNA. Splicing defects and weakness were not correlated with CTG expansion size in muscle tissue., Interpretation: Alternative splicing changes in skeletal muscle may serve as biomarkers of disease severity and therapeutic response in myotonic dystrophy., (© 2013 American Neurological Association.)

Background: Frequency and severity of cardiac involvement in DM2 are still controversial. The aims of our study were to determine the frequency and progression of cardiac and muscle involvement in a relatively large cohort of patients with DM2 throughout Italy and Germany and to provide long-term outcomes in this disorder., Methods: 104 DM2 and 117 DM1 patients underwent baseline and follow-up assessments of, ECG, 24h Holter monitoring, 2D echocardiography and electrophysiological study (EPS) when appropriate, and manual muscle strength testing (mean follow-up: 7.4 ± 4.1 for DM2 and 5.7 ± 4 years for DM1)., Results: Overall, 10% of DM2 patients vs 31% of DM1 patients had PR ≥ 200 ms and 17% of DM2 patients vs 48% of DM1 patients had QRSD ≥ 100 ms. Six patients with DM2 vs 28 patients with DM1 required PM/ICD implantations. DM2 patients were stronger than DM1 patients at baseline, but muscle strength worsened significantly over time (p<0.0001), just as in DM1, although at a slower annual rate., Conclusion: Our data demonstrate that the frequency and severity of cardiac involvement and of muscle weakness are reduced in DM2 compared to DM1 and that progression is slower and less severe. Nonetheless, careful cardiac evaluation is recommended in this patient population to identify patients at risk for potential major cardiac arrhythmias., (Copyright © 2012 Elsevier Ireland Ltd. All rights reserved.)

Summary: Myotonic dystrophy is classified as one of the myotonic syndromes although myotonia is only a minor characteristic of it. It is, in fact, also a multisystem disease with cardiac, digestive, ocular, and endocrine abnormalities. Two subgroups are currently identified with many similarities: DM1 refers to classic dystrophia myotonica (Steinert disease), while DM2, formerly called proximal myotonic myopathy has a later onset. The congenital form is present only in DM1. The genetic causes of DM1 and 2 are different but end up in a similar way of altering RNAm processing and splicing of other genes. The anesthetic risk is increased in case of DM1 type. This review summarizes current knowledge concerning the pathophysiology and anesthetic management of this disease in children and adults., (© 2013 John Wiley & Sons Ltd.)

Objectives: Although sleep disturbances are common in myotonic dystrophy type 1 (DM1), sleep disturbances in myotonic dystrophy type 2 (DM2) have not been well-characterized. We aimed to determine the frequency of sleep disturbances in DM2., Methods: We conducted a case-control study of 54 genetically confirmed DM2 subjects and 104 medical controls without DM1 or DM2, and surveyed common sleep disturbances, including symptoms of probable restless legs syndrome (RLS), excessive daytime sleepiness (EDS), sleep quality, fatigue, obstructive sleep apnea (OSA), probable REM sleep behavior disorder (pRBD), and pain. Thirty patients with DM2 and 43 controls responded to the survey. Group comparisons with parametric statistical tests and multiple linear and logistic regression analyses were conducted for the dependent variables of EDS and poor sleep quality., Results: The mean ages of patients with DM2 and controls were 63.8 and 64.5 years, respectively. Significant sleep disturbances in patients with DM2 compared to controls included probable RLS (60.0% vs 14.0%, p < 0.0001), EDS (p < 0.001), sleep quality (p = 0.02), and fatigue (p < 0.0001). EDS and fatigue symptoms were independently associated with DM2 diagnosis (p < 0.01) after controlling for age, sex, RLS, and pain scores. There were no group differences in OSA (p = 0.87) or pRBD (p = 0.12) scores., Conclusions: RLS, EDS, and fatigue are frequent sleep disturbances in patients with DM2, while OSA and pRBD symptoms are not. EDS was independently associated with DM2 diagnosis, suggesting possible primary CNS hypersomnia mechanisms. Further studies utilizing objective sleep measures are needed to better characterize sleep comorbidities in DM2.

A patient presenting with paramyotonia congenita (Eulenburg's paramyotonia) was seen at the preanaesthetic visit during pregnancy. The underlying disease was known for years. Analysis of the literature and advice taken from specialists emphasized the safe use of regional anaesthesia and analgesia which was indeed used for labour and delivery without any complication. By contrast, the limited information available on the use of general anaesthesia suggests the risks associated with the use of succinylcholine and possibly with halogenated agents. Additional and useful factors that may limit the occurrence of myotonic crises such as maintenance of normal temperature and plasma potassium concentration, should be undertaken simultaneously., (Copyright © 2013. Published by Elsevier SAS.)

Myotonia is a heritable disorder in which patients are unable to willfully relax their muscles. The physiological basis for myotonia lies in well-established deficiencies of skeletal muscle chloride and sodium conductances. What is unclear is how normal muscle function can temporarily return with repeated movement, the so-called "warm-up" phenomenon. Electrophysiological analyses of the skeletal muscle voltage-gated sodium channel Nav 1.4 (gene name SCN4A), a key player in myotonia, have revealed several parallels between the Nav 1.4 biophysical signature, specifically slow-inactivation, and myotonic warm-up, which suggest that Nav 1.4 is critical not only in producing the myotonic reaction, but also in mediating the warm-up., (Copyright © 2012 Wiley Periodicals, Inc.)

Myotonic dystrophy is the most common type of muscular dystrophy in adults and is characterised by progressive myopathy, myotonia, and multiorgan involvement. Two genetically distinct entities have been identified. Myotonic dystrophy type 1 (also known as Steinert's disease) was first described more than 100 years ago, whereas myotonic dystrophy type 2 was identified only 18 years ago, after genetic testing for type 1 disease could be applied. Both diseases are caused by autosomal dominant nucleotide repeat expansions. In patients with myotonic dystrophy type 1, a (CTG)(n) expansion is present in DMPK, whereas in patients with type 2 disease, there is a (CCTG)(n) expansion in CNBP. When transcribed into CUG-containing RNA, mutant transcripts aggregate as nuclear foci that sequester RNA-binding proteins, resulting in a spliceopathy of downstream effector genes. The prevailing paradigm therefore is that both disorders are toxic RNA diseases. However, research indicates several additional pathogenic effects take place with respect to protein translation and turnover. Despite clinical and genetic similarities, myotonic dystrophy type 1 and type 2 are distinct disorders requiring different diagnostic and management strategies., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Objective: To electrophysiologically characterize the Na(v)1.4 mutant N440K found in a Korean family with a syndrome combining symptoms of paramyotonia congenita, hyperkalemic periodic paralysis, and potassium-aggravated myotonia., Methods: We characterized transiently expressed wild-type and mutant Na(v)1.4 using whole-cell voltage-clamp analysis., Results: N440K produced a significant depolarizing shift in the voltage dependence of fast inactivation and increased persistent current and acceleration in fast inactivation recovery, which gave rise to a 2-fold elevation in the dynamic availability of the mutant channels. In addition, the mutant channels required substantially longer and stronger depolarization to enter the slow-inactivated state., Conclusions: N440K causes a gain of function consistent with skeletal muscle hyperexcitability as observed in individuals with the mutation. How the same mutation results in distinct phenotypes in the 2 kindreds remains to be determined.

This study determines the presence and extent of muscle changes in 31 myotonic dystrophy type 2 (DM2) patients detected by muscle ultrasound. Results were compared to 31 adult-onset myotonic dystrophy type 1 patients (DM1) and healthy controls. Furthermore, we tested the hypothesis that structural muscle changes correlate with age, quantitative muscle force and serum creatine kinase in both disorders. In DM2 all seven examined muscles (right masseter muscle, right and left biceps brachii, right and left forearm flexors, right rectus femoris, and left tibialis anterior muscle) showed increased mean echo intensities (p ≤ 0.001). Atrophy of the masseter muscle and rectus femoris were both found in 23% of DM2 patients. Muscle thickness was significantly more decreased in the elbow flexors in DM2 compared to DM1. Echo intensity sum score correlated positively with age in DM2 (r=0.57, p=0.001) and negatively with muscle force (r=0.36, p=0.048). We conclude that all tested muscles are affected and structurally abnormal in DM2 patients. Proximal arm muscles are more affected in DM2 compared to DM1, which corresponds to clinical findings., (Copyright © 2012. Published by Elsevier B.V.)

Myotonic dystrophy type 1 and type 2 (DM1 and DM2) are genetic diseases in which mutant transcripts containing expanded CUG or CCUG repeats cause cellular dysfunction by altering the processing or metabolism of specific mRNAs and miRNAs. The toxic effects of mutant RNA are mediated partly through effects on proteins that regulate alternative splicing. Here we show that alternative splicing of exon 29 (E29) of Ca(V)1.1, a calcium channel that controls skeletal muscle excitation-contraction coupling, is markedly repressed in DM1 and DM2. The extent of E29 skipping correlated with severity of weakness in tibialis anterior muscle of DM1 patients. Two splicing factors previously implicated in DM1, MBNL1 and CUGBP1, participated in the regulation of E29 splicing. In muscle fibers of wild-type mice, the Ca(V)1.1 channel conductance and voltage sensitivity were increased by splice-shifting oligonucleotides that induce E29 skipping. In contrast to human DM1, expression of CUG-expanded RNA caused only a modest increase in E29 skipping in mice. However, forced skipping of E29 in these mice, to levels approaching those observed in human DM1, aggravated the muscle pathology as evidenced by increased central nucleation. Together, these results indicate that DM-associated splicing defects alter Ca(V)1.1 function, with potential for exacerbation of myopathy.

Perturbations to body temperature affect almost all cellular processes and, within certain limits, results in minimal effects on overall physiology. Genetic mutations to ion channels, or channelopathies, can shift the fine homeostatic balance resulting in a decreased threshold to temperature induced disturbances. This review summarizes the functional consequences of currently identified voltage-gated sodium (NaV) channelopathies that lead to disorders with a temperature sensitive phenotype. A comprehensive knowledge of the relationships between genotype and environment is not only important for understanding the etiology of disease, but also for developing safe and effective treatment paradigms.

The nondystrophic myotonias and primary periodic paralyses are an important group of genetic muscle diseases characterized by dysfunction of ion channels that regulate membrane excitability. Clinical manifestations vary and include myotonia, hyperkalemic and hypokalemic periodic paralysis, progressive myopathy, and cardiac arrhythmias. The severity of myotonia ranges from severe neonatal presentation causing respiratory compromise through to mild later-onset disease. It remains unclear why the frequency of attacks of paralysis varies greatly or why many patients develop a severe permanent fixed myopathy. Recent detailed characterizations of human genetic mutations in voltage-gated muscle sodium (gene: SCN4A), chloride (gene: CLCN1), calcium (gene: CACNA1S), and inward rectifier potassium (genes: KCNJ2, KCNJ18) channels have resulted in new insights into disease mechanisms, clinical phenotypic variation, and therapeutic options.

Myotonic dystrophies (DMs) are autosomal dominant disorders with multisystemic clinical features. DMs are categorized as DM1, caused by a (CTG)n expansion mutation in 19q13, and DM2, caused by a (CCTG)n expansion mutation in 3q21. Clinical feature of DM2 are diffuse and proximal dominant weakness, wasting, myotonia, cardiac problems, cataracts, insulin-resistance. DM2 is considered to milder form than DM1. Here We compared clinical feature in both DMs. We identified a Japanese patient with DM2 and showed clinical features same as a past report. But DM2 is clinically variable, further investigation of Japanese patients is needed in order to confirm these findings in Japan.

Sleep disorders in myotonic dystrophy type 1 (DM1) are common and include sleep-disordered breathing, hypersomnia, and fatigue. Little is known regarding the occurrence of sleep disturbance in myotonic dystrophy type 2 (DM2). We hypothesized that DM2 patients may frequently harbor sleep disorders. We reviewed medical records of all genetically confirmed cases of DM2 seen at our sleep center between 1997 and 2010 for demographic, laboratory, overnight oximetry, and polysomnography (PSG) data. Eight patients (5 women, 3 men) with DM2 were identified. Excessive daytime sleepiness was seen in 6 patients (75%), insomnia in 5 (62.5%), and excessive fatigue in 4 (50%). Obstructive sleep apnea was diagnosed in 3 of 5 patients (60%) studied with PSG. Respiratory muscle weakness was present in all 6 patients (100%) who received pulmonary function testing. Four of 8 (50%) met criteria for diagnosis of restless legs syndrome. The clinical spectrum of DM2 may include a wide range of sleep disturbances. Although respiratory muscle weakness was frequent, sustained sleep-related hypoxia suggestive of hypoventilation was not seen in our patients. Further prospective studies are needed to examine the frequency and scope of sleep disturbances in DM2., (Copyright © 2012 S. Karger AG, Basel.)

Background: Myotonic dystrophy type 2 (DM2) is an autosomal dominant, multisystem disorder caused by a CCTG tetranucleotide repeat expansion located in intron 1 of the zinc finger protein 9 gene (ZNF9 gene) on chromosome 3q 21.3., Objectives: To describe the clinical, electrophysiologic and pathologic findings in patients with myotonic dystrophy 2., Methods: We evaluated 10 patients genetically, clinically and electrophysiologically during the years 2007 to 2008., Results: All patients were of Jewish European ancestry. Among affected individuals, eight patients had symptoms of proximal muscle weakness, two had muscle pain, and two exhibited myotonia. On physical examination six patients had severe weakness of hip flexor muscles. Seven individuals underwent cataract surgery, and cardiac involvement was seen in one case. On the initial electromyographic (EMG) examination five patients demonstrated myotonic discharges; repeated studies showed these discharges in nine cases. Six muscle biopsies showed non-specific pathological changes. Seven patients had an affected first-degree relative with either a diagnosed or an undiagnosed muscular disorder consistent with an autosomal dominant trait., Conclusions: DM2 may often present with proximal muscle weakness without myotonia. EMG may initially fail to show myotonic discharges, but these discharges may eventually show in most cases on repeated EMG. Thus, DM2 may be underdiagnosed and should be included in the differential diagnosis of adult patients of Jewish European ancestry presenting with proximal lower limb weakness.

Myotonic dystrophies (DMs) are autosomal dominant disorders with multisystemic clinical features. DMs are categorized as DM1, caused by a (CTG)n expansion mutation in 19q13, and DM2, caused by a (CCTG)n expansion mutation in 3q21. The clinical features of DM2 are diffuse and proximal dominant weakness, wasting, myotonia, cardiac problems, cataracts, and insulinresistance. DM2 is milder than DM1. We compared the clinical features of both DMs and studied a Japanese patient with DM2 who had expansion mutations different from those usually attributed to the disease and originally discovered by European researchers. The detailed clinical features of the Japanese DM2 patient are described.

Paralysis periodica paramyotonia (PPP) is caused by mutation of the adult skeletal muscle sodium channel gene's alpha (α)-subunit (SCN4A). Here, we report four generations of a Chinese family affected by a remarkably severe form of PPP with progressive myopathy. Routine electromyograms (EMG) showed myotonic discharge and after a long exercise test, compound motor action potential amplitudes were markedly decreased by 40-55%. Muscle biopsy revealed obvious vacuolar changes. Moreover, genetic analysis revealed the Met1592Val mutation in the α-subunit, SCN4A. The patients showed a striking clinical and electrophysiological improvement during treatment with acetazolamide. Thus, our findings showed that mutation of Met1592Val in the SCN4A gene is associated with aggressive development of PPP characterized by severe vacuolar myopathy., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Five hereditary sodium channelopathies of skeletal muscle have been identified. Prominent symptoms are either myotonia or weakness caused by an increase or decrease of muscle fiber excitability. The voltage-gated sodium channel NaV1.4, initiator of the muscle action potential, is mutated in all five disorders. Pathogenetically, both loss and gain of function mutations have been described, the latter being the more frequent mechanism and involving not just the ion-conducting pore, but aberrant pores as well. The type of channel malfunction is decisive for therapy which consists either of exerting a direct effect on the sodium channel, i.e., by blocking the pore, or of restoring skeletal muscle membrane potential to reduce the fraction of inactivated channels.

The purpose of this study was to describe the frequency of absent, unrecognized, or minimal myotonic discharges (MDs) in myotonic dystrophy type 2 (DM2). We performed a retrospective review of needle electromyography (EMG) data prior to genetic diagnosis in 49 DM2 patients at the Mayo Clinic. MDs were not reported on first or repeat EMG studies (n = 8) and not found in archived recordings of 4 patients (8%); archived EMG recordings (n = 4) confirmed the absence of MDs (n = 2), including 1 patient with normal insertional activity in all muscles, and misinterpretation of MDs as slow fibrillation potentials (n = 1) and complex repetitive discharge (CRD) activity (n = 1). Eight (16%) patients had minimal classic MDs with diffusely increased insertional activity, including waning-only MDs in all patients in this group with archived EMG recordings (n = 5). Diffuse MDs were found in 33 (67%) patients. Absent or minimal MDs do not exclude DM2. Over-reliance on diffuse MDs in patients who present with myopathy may lead to delay in genetic diagnosis of DM2.

Paramyotonia congenita (PC), first described in 1886 by Eulenberg, is characterized by cold and exercise-induced muscle stiffness and intermittent flaccid paresis not necessarily related to cold or myotonia. Several authors segregated a pure form of PC, which has no periodic paralysis, even after cold exposure. The existence of this phenotype has been debated in the literature. We describe electrophysiological and molecular genetic features of a patient with PC who had no history of periodic paralysis. Immersion in cold water or potassium load could not induce clinical paralysis. However, repetitive nerve stimulation and exercise test demonstrated a drop in compound muscle action potential amplitude. Genetic analysis revealed the substitution of valine for glycine on the human skeletal muscle sodium channel (SCN4A) gene. The G1306V mutation is rare in the classic form of PC, and moreover might be the first in pure paramyotonia.

Paramyotonia congenita in 22 members of Arab (Omani) family is reported. Four generations were affected. All had early onset around 1 year of age, with myotonia and cold intolerance. Age of onset in the index case was at 3 months of age. Six members with the disease were examined and investigated. To our knowledge, this is the first report of the condition from this region. Childhood presentation and differential diagnosis is discussed.