Drug treatment for myotonia

A Detailed Clinical Approach to Non-dystrophic Myotonia: A Case Report of Two Brothers With Myotonia Congenita

Non-dystrophic myotonia (NDM) is a group of rare mono-genetic muscle disorders caused by skeletal muscle sodium or chloride channelopathies. These disorders are characterized by high muscle tone and the inability of the muscles to relax spontaneously after voluntary contraction. Myotonia congenita refers to a form of NDM that typically manifests during the later stages of childhood. It occurs as a result of genetic mutations affecting the chloride channels found in the sarcolemma membrane of skeletal muscles. Here, we present a case series of two male siblings born out of third-degree consanguineous union ages 10 and eight years, respectively, who presented with proximal muscle weakness and the characteristic "Herculean body" appearance. They demonstrated characteristic clinical diagnostic signs of myotonia. The diagnosis of myotonia congenita was confirmed through distinctive electromyography (EMG) findings, which were further supported by genetic testing revealing a homozygous mutation c.1445G>A in exon 13 of the CLCN1 gene, indicating autosomal recessive inheritance. This uncommon condition exhibits characteristic clinical manifestations and classical EMG findings, which are difficult to disregard once encountered. Genetic tests serve as a valuable tool to validate the diagnosis.

The Clinical, Myopathological, and Genetic Analysis of 20 Patients With Non-dystrophic Myotonia

Introduction

Non-dystrophic myotonias (NDMs) are skeletal muscle ion channelopathies caused by CLCN1 or SCN4A mutations. This study aimed to describe the clinical, myopathological, and genetic analysis of NDM in a large Chinese cohort.

Methods

We reviewed the clinical manifestations, laboratory results, electrocardiogram, electromyography, muscle biopsy, genetic analysis, treatment, and follow-up of 20 patients (from 18 families) with NDM.

Results

Cases included myotonia congenita (MC, 17/20) and paramyotonia congenita (PMC, 3/20). Muscle stiffness and hypertrophy, grip and percussion myotonia, and the warm-up phenomenon were frequently observed in MC and PMC patients. Facial stiffness, eye closure myotonia, and cold sensitivity were more common in PMC patients and could be accompanied by permanent weakness. Nine MC patients and two PMC patients had cardiac abnormalities, mainly manifested as cardiac arrhythmia, and the father of one patient died of sudden cardiac arrest. Myotonic runs in electromyography were found in all patients, and seven MC patients had mild myopathic changes. There was no difference in muscle pathology between MC and PMC patients, most of whom had abnormal muscle fiber type distribution or selective muscle fiber atrophy. Nineteen CLCN1 variants were found in 17 MC patients, among which c.795T>G (p.D265E) was a new variant, and two SCN4A variants were found in three PMC patients. The patients were treated with mexiletine and/or carbamazepine, and the symptoms of myotonia were partially improved.

Conclusions

MC and PMC have considerable phenotypic overlap. Genetic investigation contributes to identifying the subtype of NDM. The muscle pathology of NDM lacks specific changes.

Drug Repurposing for Rare Diseases: A Role for Academia

Background: The European Commission highlights in its Pharmaceutical Strategy the role of academic researchers in drug repurposing, especially in the development of orphan medicinal products (OMPs). This study summarizes the contribution of academia over the last 5 years to registered repurposed OMPs and describes barriers to success, based upon three real world cases.

Methods: OMPs granted marketing authorization between January 2016 and December 2020 were reviewed for repurposing and whether the idea originated from academia or industry. Three cases of drug repurposing were selected from different therapeutic areas and stages of development to identify obstacles to success.

Results: Thirteen of the 68 OMPs were the result of drug repurposing. In three OMPs, there were two developments such as both a new indication and a modified application. In total, twelve developments originated from academia and four from industry. The three cases showed as barriers to success: lack of outlook for sufficient return of investments (abatacept), lack of regulatory alignment and timing of interaction between healthcare professionals and regulators (etidronate), failure to register an old drug for a fair price, resulting in commercialization as a high priced orphan drug (mexiletine).

Conclusion: While the majority of repurposed OMPs originates in academia, a gap exists between healthcare professionals, regulators and industry. Future strategies should aim to overcome these hurdles leading to more patient benefit through sustainable access of repurposed drugs. Potential solutions include improved regulatory and reimbursement knowledge by academia and the right for regulators to integrate new effectiveness data into product labels.

Cost-Based Price Calculation of Mexiletine for Nondystrophic Myotonia

OBJECTIVES

Mexiletine is a long-known drug used for the treatment of arrhythmias and repurposed in the 1980s for patients with nondystrophic myotonia (NDM). Recently, the price of mexiletine in Europe increased significantly after registration as an orphan drug for NDM. This led to international discussions on affordability and willingness to reimburse mexiletine in the absence of background information that would justify such a price. Our objective was to calculate a cost-based price for mexiletine for adult patients with NDM based on detailed information on development costs.

METHODS

We calculated a fair price based on a cost-based pricing model for commercial mexiletine to treat adults with NDM using a recent European drug-pricing model as a framework to include actual costs incurred. Three scenarios were applied: 1 with minimum estimated costs, 1 with maximum estimated costs, and 1 with costs as if mexiletine was innovative.

RESULTS

The calculated fair price of mexiletine per patient per year (PPPY) is €452 for the minimum scenario and €1996 for the maximum scenario. By using hypothetical R&D costs used for innovative drugs, the price would be €6685 PPPY. In Europe, the list price of mexiletine ranges from €30 707-60 730 PPPY, based on 600 mg daily.

CONCLUSIONS

The current list price for mexiletine in Europe is manifold higher than any scenario of the cost-based models. Accounting for the reduced costs for clinical development in a repurposing scenario, the cost-based pricing model provides a fair commercial price range, which can be used as benchmark for pricing negotiations and/or reimbursement decisions.

Taurine Augments Telomerase Activity and Promotes Chondrogenesis in Dental Pulp Stem Cells

Background: Stem cell therapy has become an advanced and state-of-the-art procedure to regenerate lost tissues of the human body. Cartilage repair is a challenging task in which stem cells find potential application. One of the important biologic modifiers that can cause chondrogenic differentiation of stem cells is taurine. However, taurine has not been investigated for its effects on dental pulp derived stem cell (DPSC) chondrogenic differentiation. Objective: The objective of the study was to investigate if taurine administration to DPSCs heralds chondrogenic differentiation as ascertained by expression of SOX9, COL2A1, ACAN, ELN, and COMP. The study also investigated if the differentiated cells synthesized glycosaminoglycans, a marker of cartilage formation. The study also aimed to assess proliferative activity of the cells after taurine administration by measuring the hTERT gene and protein expression. Materials and methods: DPSCs were obtained from a molecular biology laboratory and characterization of stem cell markers was done by flow cytometry. The cells were subjected to a MTT assay using various concentrations of taurine. Following this, hTERT gene and protein estimation was done in the control, telomerase inhibitor treated DPSC (TI-III), 10 μM taurine treated DPSC, and TI-III + 10 μM taurine treated DPSCs. A polymerase chain reaction was done to assess gene expression of SOX9, COL2A1, ACAN, ELN, and COMP genes and glycosaminoglycans were estimated in control cells, Induced DPSCs, induced and TI-III treated DPSCs, and 10 μM taurine treated DPSCs. Results: DPSCs expressed CD73, CD90, and CD105 and did not express CD34, CD45, and HLA-DR, which demonstrated that they were mesenchymal stem cells. The MTT assay revealed that various concentrations of taurine did not affect the cell viability of DPSCs. A concentration of 10 μM of taurine was used for further assays. With regard to the hTERT gene and protein expression, the taurine treated cells expressed the highest levels that were statistically significant compared to the other groups. Taurine was also found to restore hTERT expression in telomerase inhibitor treated cells. With regard to chondrogenesis related genes, taurine administration significantly increased the expression of SOX9, COL2A1, ACAN, and ELN genes in DPSCs and caused a significant increase in glycosaminoglycan production by the cells. Conclusions: Taurine can be regarded a biologic modifier that can significantly augment chondrogenic differentiation of DPSCs and can find potential applications in regenerative medicine in the area of cartilage regeneration.

Muscle Ultrasound Shear Wave Elastography as a Non-Invasive Biomarker in Myotonia

Myotonia, i.e., delayed muscle relaxation in certain hereditary muscle disorders, can be assessed quantitatively using different techniques ranging from force measurements to electrodiagnostics. Ultrasound shear wave elastography (SWE) has been proposed as a novel tool in biomechanics and neuromuscular medicine for the non-invasive estimation of muscle elasticity and, indirectly, muscle force. The aim of this study is to provide ‘proof-of-principle’ that SWE allows a quantitative measurement of the duration of delayed muscle relaxation in myotonia in a simple clinical setting. In six myotonic muscle disorder patients and six healthy volunteers, shear wave velocities (SWV) parallel to the fiber orientation in the flexor digitorum superficialis muscle in the forearm were recorded with a temporal resolution of one per second during fist-clenching and subsequent relaxation; the relaxation time to 10% of normalized shear wave velocity (RT_0.1) was calculated. Forty-six SWE imaging sequences were acquired, yielding a mean RT_0.1 of 7.38 s in myotonic muscle disorder patients, significantly higher than in healthy volunteers (1.36 s), which is comparable to data obtained by mechanical dynamometry. SWV measurements during the baseline relaxation and voluntary contraction phases did not differ significantly between groups. We conclude that SWE is a promising, non-invasive, widely available tool for the quantitative assessment of myotonia to aid in diagnosis and therapeutic monitoring.

Targeted Therapies for Skeletal Muscle Ion Channelopathies: Systematic Review and Steps Towards Precision Medicine

BACKGROUND

Skeletal muscle ion channelopathies include non-dystrophic myotonias (NDM), periodic paralyses (PP), congenital myasthenic syndrome, and recently identified congenital myopathies. The treatment of these diseases is mainly symptomatic, aimed at reducing muscle excitability in NDM or modifying triggers of attacks in PP.

OBJECTIVE

This systematic review collected the evidences regarding effects of pharmacological treatment on muscle ion channelopathies, focusing on the possible link between treatments and genetic background.

METHODS

We searched databases for randomized clinical trials (RCT) and other human studies reporting pharmacological treatments. Preclinical studies were considered to gain further information regarding mutation-dependent drug effects. All steps were performed by two independent investigators, while two others critically reviewed the entire process.

RESULTS

For NMD, RCT showed therapeutic benefits of mexiletine and lamotrigine, while other human studies suggest some efficacy of various sodium channel blockers and of the carbonic anhydrase inhibitor (CAI) acetazolamide. Preclinical studies suggest that mutations may alter sensitivity of the channel to sodium channel blockers in vitro, which has been translated to humans in some cases. For hyperkalemic and hypokalemic PP, RCT showed efficacy of the CAI dichlorphenamide in preventing paralysis. However, hypokalemic PP patients carrying sodium channel mutations may have fewer benefits from CAI compared to those carrying calcium channel mutations. Few data are available for treatment of congenital myopathies.

CONCLUSIONS

These studies provided limited information about the response to treatments of individual mutations or groups of mutations. A major effort is needed to perform human studies for designing a mutation-driven precision medicine in muscle ion channelopathies.

A case of paramyotonia congenita in pregnancy

Paramyotonia congenita is a rare autosomal dominant non-dystrophic myopathy caused by mutations in the SNC4A gene, which encodes for the voltage-gated sodium channel in skeletal muscle. Symptom onset is typically during early childhood and is characterised by myotonia followed by flaccid paralysis or weakness, usually exacerbated by repeated muscle contractions or cold temperatures. Pregnancy has been reported to increase symptoms of myotonia; however, there is limited information in the literature regarding the possible effects of paramyotonia congenita on pregnancy and labour. We present a successful case of a 20-year-old primigravida with confirmed paramyotonia congenita and review the literature regarding paramyotonia congenita during pregnancy.

Prescription Benzodiazepine Use in Privately Insured U.S. Children and Adolescents

INTRODUCTION

Benzodiazepines are commonly prescribed in the U.S. but entail safety concerns, including dependency. In pediatrics, many indications lack trial data. Authors aimed to describe youth initiating prescription benzodiazepine treatment, identify potential indications and prescribing concerns, estimate the duration of treatment by potential indication, and identify factors that predict long-term use.

METHODS

The study cohort included children (aged 3-12 years) and adolescents (aged 13-17 years) initiating prescription benzodiazepine treatment (≥3 days' supply) from January 2010 to September 2015 in a U.S. commercial claims database. Potential indications included selected ICD-9-CM diagnoses (≤30 days prior). Long-term (≥6 months) benzodiazepine treatment was estimated with Kaplan-Meier estimation and modified Poisson regression identified independent predictors of long-term benzodiazepine treatment (analysis completed in 2018).

RESULTS

Of 24,504 children and 61,046 adolescents initiating benzodiazepines, 62% of the children and 68% of the adolescents had a potential indication. Anxiety disorders were the most common indication, with mental health indications more common among adolescents (45%) than children (23%) and epilepsy and movement disorders higher in children. Recent opioid prescriptions were common before benzodiazepine initiation (children, 22%; adolescents, 21%). Six percent of the initiators became long-term benzodiazepine users. Potential indication, provider contact, psychotropic medication, and chronic conditions independently predicted long-term benzodiazepine treatment in adolescents and children.

CONCLUSIONS

U.S. children and adolescents are prescribed benzodiazepines for various mental health and other medical conditions, many lacking evidence of pediatric efficacy. Long-term benzodiazepine treatment, concurrent opioid prescriptions, psychotropic use, and prior substance use disorder diagnoses suggest safety risks among some youth prescribed benzodiazepines.

Taurine: A Potential Ergogenic Aid for Preventing Muscle Damage and Protein Catabolism and Decreasing Oxidative Stress Produced by Endurance Exercise

The aim of this study was to evaluate the effects of taurine and chocolate milk supplementation on oxidative stress and protein metabolism markers, and aerobic parameters in triathletes.

Methods: A double-blind, crossover study was conducted with 10 male triathletes, aged 30.9 ± 1.3 year, height 1.79 ± 0.01 m and body weight 77.45 ± 2.4 kg. Three grams of taurine and 400 ml of chocolate milk (TAUchoc), or a placebo (chocolate milk) (CHOC) was ingested post exercise for 8 weeks. Oxidative stress marker levels, and 24 h urinary nitrogen, creatinine, and urea excretion were measured before and after 8 weeks of training and supplementation with TAUchoc or CHOC. A maximal incremental running test on a treadmill was performed in order to evaluate aerobic parameters: V_max, heart rate (HR) and rate of perceived exertion (RPE).

Results: TAUchoc treatment during the 8 weeks resulted in increased taurine plasma levels (PRE 201.32 ± 29.03 μmol/L and POST 234.36 ± 35.51 μmol/L, p = 0.01), decreased malondialdehyde levels (19.4%, p = 0.03) and urinary nitrogen excretion (−33%, p = 0.03), and promoted positive nitrogen balance (p = 0.01). There were no changes in reduced glutathione (TAUchoc PRE 0.72 ± 0.08 mmol/L and POST 0.83 ± 0.08 mmol/L; CHOC PRE 0.69 ± 0.08 mmol/L and POST 0.81 ± 0.06 mmol/L), vitamin E plasma levels (TAUchoc PRE 33.99 ± 2.52 μmol/L and 35.95 ± 2.80 μmol/L and CHOC PRE 31.48 ± 2.12 μmol/L and POST 33.77 ± 3.64 μmol/L), or aerobic parameters, which were obtained in the last phase of the maximal incremental running test (V_max TAUchoc PRE 13 ± 1.4 km/h and POST 13.22 ± 1.34 km/h; CHOC PRE 13.11 ± 2.34 km/h and POST 13.11 ± 2.72 km/h), the heart rate values were TAUchoc PRE 181.89 ± 24.18 bpm and POST 168.89 ± 46.56 bpm; CHOC PRE 181.56 ± 2.14 bpm and POST 179.78 ± 3.4 bpm, and the RPE were TAUchoc PRE 8.33 ± 2.4 AU and POST 9.1 ± 2.1 AU; CHOC PRE 8.11 ± 4.94 AU and POST 8.78 ± 2.78 AU).

Conclusion: Taurine supplementation did not improve aerobic parameters, but was effective in increasing taurine plasma levels and decreasing oxidative stress markers, which suggests that taurine may prevent oxidative stress in triathletes.

A review of the use of mexiletine in patients with myotonic dystrophy and non-dystrophic myotonia

Introduction

Myotonia is found in a number of muscle diseases, including myotonic dystrophy and non-dystrophic myotonia. The resulting symptoms of myotonia can interfere with daily activities such as walking or climbing the stairs. Due to the rarity of both these conditions, pharmacological treatment of myotonia is largely anecdotal and is led by specialist clinicians who tend to favour the use of mexiletine, a class 1b antiarrhythmic sodium antagonist.

Objective

To identify and review randomised controlled trials in order to assess the efficacy and safety of use of mexiletine in myotonic dystrophy and non-dystrophic myotonia for two different patient cases.

Search methods

The literature search was conducted using MEDLINE, EMBASE and The Cochrane Library (from January 1990 to December 2014). Specialist neurology centres were also contacted.

Selection criteria

All randomised controlled trials between January 1990 and December 2014 which compared the use of mexiletine for the treatment of myotonia in patients who suffer from myotonic dystrophy and non-dystrophic were included in this review. Primary outcome: reduction of clinical myotonia.

Results

Two randomised controlled trials were included for review. Both studies are underpowered; however, there is evidence to support the use of mexiletine for the improvement of clinical myotonia.

Conclusions

Larger randomised controlled trials are required, which look at the functional effect of myotonia as a primary outcome (ie, stair test) and the long-term use of mexiletine. This is needed to establish the ongoing efficacy and safety of the long-term use of mexiletine in the management of myotonia.

Myotonic disorders: A review article

The myotonic disorders are a heterogeneous group of genetically determined diseases that are unified by the presence of myotonia, which is defined as failure of muscle relaxation after activation. The presentation of these disorders can range from asymptomatic electrical myotonia, as seen in some forms of myotonia congenita (MC), to severe disability with muscle weakness, cardiac conduction defects, and other systemic features as in myotonic dystrophy type I (DM1). In this review, we describe the clinical features and pathophysiology of the different myotonic disorders, their laboratory and electrophysiologic findings and briefly review the currently available treatments.

Congenital and childhood myotonic dystrophy: Current aspects of disease and future directions

Myotonic dystrophy type 1 (DM1) is multisystem disease arising from mutant CTG expansion in the non-translating region of the dystrophia myotonica protein kinase gene. While DM1 is the most common adult muscular dystrophy, with a worldwide prevalence of one in eight thousand, age of onset varies from before birth to adulthood. There is a broad spectrum of clinical severity, ranging from mild to severe, which correlates with number of DNA repeats. Importantly, the early clinical manifestations and management in congenital and childhood DM1 differ from classic adult DM1. In neonates and children, DM1 predominantly affects muscle strength, cognition, respiratory, central nervous and gastrointestinal systems. Sleep disorders are often under recognised yet a significant morbidity. No effective disease modifying treatment is currently available and neonates and children with DM1 may experience severe physical and intellectual disability, which may be life limiting in the most severe forms. Management is currently supportive, incorporating regular surveillance and treatment of manifestations. Novel therapies, which target the gene and the pathogenic mechanism of abnormal splicing are emerging. Genetic counselling is critical in this autosomal dominant genetic disease with variable penetrance and potential maternal anticipation, as is assisting with family planning and undertaking cascade testing to instigate health surveillance in affected family members. This review incorporates discussion of the clinical manifestations and management of congenital and childhood DM1, with a particular focus on hypersomnolence and sleep disorders. In addition, the molecular genetics, mechanisms of disease pathogenesis and development of novel treatment strategies in DM1 will be summarised.

Taurine: the appeal of a safe amino acid for skeletal muscle disorders

Taurine is a natural amino acid present as free form in many mammalian tissues and in particular in skeletal muscle. Taurine exerts many physiological functions, including membrane stabilization, osmoregulation and cytoprotective effects, antioxidant and anti-inflammatory actions as well as modulation of intracellular calcium concentration and ion channel function. In addition taurine may control muscle metabolism and gene expression, through yet unclear mechanisms. This review summarizes the effects of taurine on specific muscle targets and pathways as well as its therapeutic potential to restore skeletal muscle function and performance in various pathological conditions. Evidences support the link between alteration of intracellular taurine level in skeletal muscle and different pathophysiological conditions, such as disuse-induced muscle atrophy, muscular dystrophy and/or senescence, reinforcing the interest towards its exogenous supplementation. In addition, taurine treatment can be beneficial to reduce sarcolemmal hyper-excitability in myotonia-related syndromes. Although further studies are necessary to fill the gaps between animals and humans, the benefit of the amino acid appears to be due to its multiple actions on cellular functions while toxicity seems relatively low. Human clinical trials using taurine in various pathologies such as diabetes, cardiovascular and neurological disorders have been performed and may represent a guide-line for designing specific studies in patients of neuromuscular diseases.

Combined N-of-1 trials to investigate mexiletine in non-dystrophic myotonia using a Bayesian approach; study rationale and protocol

Background

To obtain evidence for the clinical and cost-effectiveness of treatments for patients with rare diseases is a challenge. Non-dystrophic myotonia (NDM) is a group of inherited, rare muscle diseases characterized by muscle stiffness. The reimbursement of mexiletine, the expert opinion drug for NDM, has been discontinued in some countries due to a lack of independent randomized controlled trials (RCTs). It remains unclear however, which concessions can be accepted towards the level 1 evidence needed for coverage decisions, in rare diseases. Considering the large number of rare diseases with a lack of treatment evidence, more experience with innovative trial designs is needed. Both NDM and mexiletine are well suited for an N-of-1 trial design. A Bayesian approach allows for the combination of N-of-1 trials, which enables the assessment of outcomes on the patient and group level simultaneously.

Methods/Design

We will combine 30 individual, double-blind, randomized, placebo-controlled N-of-1 trials of mexiletine (600 mg daily) vs. placebo in genetically confirmed NDM patients using hierarchical Bayesian modeling. Our results will be compared and combined with the main results of an international cross-over RCT (mexiletine vs. placebo in NDM) published in 2012 that will be used as an informative prior. Similar criteria of eligibility, treatment regimen, end-points and measurement instruments are employed as used in the international cross-over RCT.

Discussion

The treatment of patients with NDM with mexiletine offers a unique opportunity to compare outcomes and efficiency of novel N-of-1 trial-based designs and conventional approaches in producing evidence of clinical and cost-effectiveness of treatments for patients with rare diseases.

Trial registration

ClinicalTrials.gov Identifier: NCT02045667

Treatment and management of neuromuscular channelopathies

OPINION STATEMENT

Neuromuscular channelopathies are heterogeneous disorders with marked phenotypic and genotypic variability. These include non-dystrophic myotonia (NDM), periodic paralysis (PP), and congenital myasthenic syndrome (CMS). Their diverse clinical manifestations remain a challenge in diagnosis and management to this date. These disorders impact quality of life and cause lifelong disabling symptoms. Treatment options are few and not FDA-approved. This is largely due to a paucity of large, randomized clinical trials in these rare diseases. Challenges of conducting such trials include the rarity of these disorders and the genetic heterogeneity. Physicians rely on off-label use of drugs to treat muscle channelopathies to reduce morbidity and improve quality of life. Besides pharmacological treatment, dietary modifications, lifestyle changes, awareness of triggers, and genetic counseling also play an important role in long-term disease management. This article reviews the current management strategies for neuromuscular channelopathies.

Myotonic dystrophy

Myotonic dystrophy (dystrophia myotonica, DM) is one of the most common lethal monogenic disorders in populations of European descent. DM type 1 was first described over a century ago. More recently, a second form of the disease, DM type 2 was recognized, which results from repeat expansion in a different gene. Both disorders have autosomal dominant inheritance and multisystem features, including myotonic myopathy, cataract, and cardiac conduction disease. This article reviews the clinical presentation and pathophysiology of DM and discusses current management and future potential for developing targeted therapies.

Effect of mexiletine on transitory depression of compound motor action potential in recessive myotonia congenita

OBJECTIVE

We aim to demonstrate the effect of mexiletine on the compound muscle action potential (CMAP) amplitude transitory depression (TD) in a cohort of patients with recessive myotonia congenita.

METHODS

We evaluated 21 patients with recessive myotonia congenita referred to our institute from 1990 to 2013 and treated with mexiletine chlorhydrate. All patients underwent prolonged 3 Hz repetitive nerve stimulation (3 Hz-PLRS) before and after the beginning of treatment.

RESULTS

We observed in all subjects a reduction of CMAP amplitude TD after the beginning of treatment. The mean value of the TD nadir before starting mexiletine treatment was -62.0% and reduced to -28.8% after the therapy was started (51.6% reduction, p<0.001).

CONCLUSIONS

The 3 Hz-PLRS is configured as a neurophysiological test able to indirectly detect and quantify, through the measurement of TD, the clinical phenomenon of the transitory weakness that occurs in myotonic syndromes due to CLCN1 mutations.

SIGNIFICANCE

This neurophysiological test might be considered a helpful tool to assess the effect of anti-myotonic drugs, as mexiletine, in recessive myotonia congenita.

Nondystrophic myotonia: challenges and future directions

Non-dystrophic myotonias are rare diseases caused by mutations in skeletal muscle chloride and sodium ion channels with considerable phenotypic overlap between diseases. Common symptoms include muscle stiffness, transitory weakness, fatigue, and pain. Although seldom life-shortening, these myotonias cause life-time disability and affected individuals cannot perform many daily activities. A notable feature of the recessive form of chloride channelopathies is the presence of transient weakness. While there has been considerable progress in skeletal muscle channelopathies with regards to identifying biophysical abnormalities, the mechanism of transient weakness remains unclear. A recent study published in Experimental Neurology (Desaphy et al., 2013) explored this question further by comparing the biophysical properties of 3 chloride channel mutations associated with recessive myotonia congenita, with varying susceptibility to transient weakness. The authors identified a variety of functional defects in channel behavior among the 3 mutations, suggesting that this variability contributes to the differing phenotypes among chloride channelopathies. This commentary discusses nondystrophic myotonias, the results of Desaphy et al., and the treatment challenges in this rare disease.

Mexiletine for symptoms and signs of myotonia in nondystrophic myotonia: a randomized controlled trial

CONTEXT

Nondystrophic myotonias (NDMs) are rare diseases caused by mutations in skeletal muscle ion channels. Patients experience delayed muscle relaxation causing functionally limiting stiffness and pain. Mexiletine-induced sodium channel blockade reduced myotonia in small studies; however, as is common in rare diseases, larger studies of safety and efficacy have not previously been considered feasible.

OBJECTIVE

To determine the effects of mexiletine for symptoms and signs of myotonia in patients with NDMs.

DESIGN, SETTING, AND PARTICIPANTS

A randomized, double-blind, placebo-controlled 2-period crossover study at 7 neuromuscular referral centers in 4 countries of 59 patients with NDMs conducted between December 23, 2008, and March 30, 2011, as part of the National Institutes of Health-funded Rare Disease Clinical Research Network.

INTERVENTION

Oral 200-mg mexiletine or placebo capsules 3 times daily for 4 weeks, followed by the opposite intervention for 4 weeks, with 1-week washout in between.

MAIN OUTCOME MEASURES

Patient-reported severity score of stiffness recorded on an interactive voice response (IVR) diary (scale of 1 = minimal to 9 = worst ever experienced). Secondary end points included IVR-reported changes in pain, weakness, and tiredness; clinical myotonia assessment; quantitative measure of handgrip myotonia; and Individualized Neuromuscular Quality of Life summary quality of life score (INQOL-QOL, percentage of maximal detrimental impact).

RESULTS

Mexiletine significantly improved patient-reported severity score stiffness on the IVR diary. Because of a statistically significant interaction between treatment and period for this outcome, primary end point is presented by period (period 1 means were 2.53 for mexiletine and 4.21 for placebo; difference, -1.68; 95% CI, -2.66 to -0.706; P < .001; period 2 means were 1.60 for mexiletine and 5.27 for placebo; difference, -3.68; 95% CI, -3.85 to -0.139; P = .04). Mexiletine improved the INQOL-QOL score (mexiletine, 14.0 vs placebo, 16.7; difference, -2.69; 95% CI, -4.07 to -1.30; P < .001) and decreased handgrip myotonia on clinical examination (mexiletine, 0.164 seconds vs placebo, 0.494 seconds; difference, -0.330; 95% CI, -0.633 to -0.142; P < .001). The most common adverse effect was gastrointestinal (9 mexiletine and 1 placebo). Two participants experienced transient cardiac effects that did not require stopping the study (1 in each group). One serious adverse event was determined to be not study related.

CONCLUSION

In this preliminary study of patients with NDMs, the use of mexiletine compared with placebo resulted in improved patient-reported stiffness over 4 weeks of treatment, despite some concern about the maintenance of blinding.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT00832000.

In vivo evaluation of antimyotonic efficacy of β-adrenergic drugs in a rat model of myotonia

The sodium channel blocker mexiletine is considered the first-line drug in myotonic syndromes, a group of muscle disorders characterized by membrane over-excitability. We previously showed that the β-adrenoceptor modulators, clenbuterol and propranolol, block voltage-gated sodium channels in a manner reminiscent to mexiletine, whereas salbutamol and nadolol do not. We now developed a pharmacological rat model of myotonia congenita to perform in vivo preclinical test of antimyotonic drugs. Myotonia was induced by i.p. injection of 30 mg/kg of anthracene-9-carboxylic acid (9-AC), a muscle chloride channel blocker, and evaluated by measuring the time of righting reflex (TRR). The TRR was prolonged from <0.5 s in control conditions to a maximum of ∼4 s, thirty minutes after 9-AC injection, then gradually recovered in a few hours. Oral administration of mexiletine twenty minutes after 9-AC injection significantly hampered the TRR prolongation, with an half-maximum efficient dose (ED(50)) of 12 mg/kg. Both propranolol and clenbuterol produced a dose-dependent antimyotonic effect similar to mexiletine, with ED(50) values close to 20 mg/kg. Antimyotonic effects of 40 mg/kg mexiletine and propranolol lasted for 2 h. We also demonstrated, using patch-clamp methods, that both propranolol enantiomers exerted a similar block of skeletal muscle hNav1.4 channels expressed in HEK293 cells. The two enantiomers (15 mg/kg) also showed a similar antimyotonic activity in vivo in the myotonic rat. Among the drugs tested, the R(+)-enantiomer of propranolol may merit further investigation in humans, because it exerts antimyotonic effect in the rat model, while lacking of significant activity on the β-adrenergic pathway. This study provides a new and useful in vivo preclinical model of myotonia congenita in order to individuate the most promising antimyotonic drugs to be tested in humans.

Skeletal muscle na channel disorders

Five inherited human disorders affecting skeletal muscle contraction have been traced to mutations in the gene encoding the voltage-gated sodium channel Na_v1.4. The main symptoms of these disorders are myotonia or periodic paralysis caused by changes in skeletal muscle fiber excitability. Symptoms of these disorders vary from mild or latent disease to incapacitating or even death in severe cases. As new human sodium channel mutations corresponding to disease states become discovered, the importance of understanding the role of the sodium channel in skeletal muscle function and disease state grows.

An interactive voice response diary for patients with non-dystrophic myotonia

INTRODUCTION

Non-dystrophic myotonia (NDM) is caused by mutations in muscle chloride and sodium channels. Currently, there is no standardized instrument for documenting symptom frequency and severity in NDM.

METHODS

Subjects used an automated, interactive, telephone-based voice response diary (IVR) to record frequency and severity of stiffness, weakness, pain, and tiredness once a week for 8 weeks, after their baseline visits.

RESULTS

We describe the IVR and report data on 76 subjects for a total of 385 person-weeks. Overall there were 5.1 calls per subject. Forty-eight subjects called in 5 or more times, and 14 called in 8 times. Stiffness was both the most frequent and severe symptom. Warm-up and handgrip myotonia were associated with higher severity scores for stiffness.

CONCLUSIONS

IVR is a convenient technology to allow patient reporting of repeated and real-time symptom frequency and severity, and it is presently being used in a trial of mexiletine in NDM.

Myotonia congenita and myotonic dystrophy: surveillance and management

OPINION STATEMENT

Myotonia can be treated both pharmacologically and by lifestyle modifications. Cell membrane stabilizers are the medications most commonly used for symptomatic treatment of myotonia. Most patients do not require treatment for the myotonia itself, unless it is severe, but physicians must be aware of anesthesia risks in both myotonia congenita and myotonic dystrophy. A mainstay of management of myotonic dystrophy is the surveillance and treatment of its various systemic complications.

Therapeutic potential of splice-switching oligonucleotides

Alternative splicing enables a single pre-messenger RNA transcript to yield multiple protein isoforms, making it a major contributor to the diversity of the proteome. While this process is essential for normal development, aberrations in alternative splicing are the cause of a multitude of human diseases. Methods for manipulating alternative splicing would thus be of therapeutic value. Chemically modified antisense oligonucleotides that alter alternative splicing by directing splice site selection have been developed to achieve this end. These splice-switching oligonucleotides (SSOs) have been applied to correct aberrant splicing, induce expression of a therapeutic splice variant, or induce expression of a novel therapeutic splice variant in a number of disease-relevant genes. Recently, in vivo efficacy of SSOs has been reported using animal disease models, as well as in results from the first clinical trial.

Mexiletine is an effective antimyotonia treatment in myotonic dystrophy type 1

OBJECTIVE

To determine if mexiletine is safe and effective in reducing myotonia in myotonic dystrophy type 1 (DM1).

BACKGROUND

Myotonia is an early, prominent symptom in DM1 and contributes to decreased dexterity, gait instability, difficulty with speech/swallowing, and muscle pain. A few preliminary trials have suggested that the antiarrhythmic drug mexiletine is useful, symptomatic treatment for nondystrophic myotonic disorders and DM1.

METHODS

We performed 2 randomized, double-blind, placebo-controlled crossover trials, each involving 20 ambulatory DM1 participants with grip or percussion myotonia on examination. The initial trial compared 150 mg of mexiletine 3 times daily to placebo, and the second trial compared 200 mg of mexiletine 3 times daily to placebo. Treatment periods were 7 weeks in duration separated by a 4- to 8-week washout period. The primary measure of myotonia was time for isometric grip force to relax from 90% to 5% of peak force after a 3-second maximum grip contraction. EKG measurements and adverse events were monitored in both trials.

RESULTS

There was a significant reduction in grip relaxation time with both 150 and 200 mg dosages of mexiletine. Treatment with mexiletine at either dosage was not associated with any serious adverse events, or with prolongation of the PR or QTc intervals or of QRS duration. Mild adverse events were observed with both placebo and mexiletine treatment.

CONCLUSIONS

Mexiletine at dosages of 150 and 200 mg 3 times daily is effective, safe, and well-tolerated over 7 weeks as an antimyotonia treatment in DM1.

CLASSIFICATION OF EVIDENCE

This study provides Class I evidence that mexiletine at dosages of 150 and 200 mg 3 times daily over 7 weeks is well-tolerated and effective in reducing handgrip relaxation time in DM1.

Skeletal muscle channelopathies: new insights into the periodic paralyses and nondystrophic myotonias

PURPOSE OF REVIEW

To summarize advances in our understanding of the clinical phenotypes, genetics, and molecular pathophysiology of the periodic paralyses, the nondystrophic myotonias, and other muscle channelopathies.

RECENT FINDINGS

The number of pathogenic mutations causing periodic paralysis, nondystrophic myotonias, and ryanodinopathies continues to grow with the advent of exon hierarchy analysis strategies for genetic screening and better understanding and recognition of disease phenotypes. Recent studies have expanded and clarified the role of gating pore current in channelopathy pathogenesis. It has been shown that the gating pore current can account for the molecular and phenotypic diseases observed in the muscle sodium channelopathies, and, given that homologous residues are affected in mutations of calcium channels, it is possible that pore leak represents a pathomechanism applicable to many channel diseases. Improvements in treatment of the muscle channelopathies are on the horizon. A randomized controlled trial has been initiated for the study of mexiletine in nondystrophic myotonias. The class IC antiarrhythmia drug flecainide has been shown to depress ventricular ectopy and improve exercise capacity in patients with Andersen-Tawil syndrome.

SUMMARY

Recent studies have expanded our understanding of gating pore current as a disease-causing mechanism in the muscle channelopathies and have allowed new correlations to be drawn between disease genotype and phenotype.

The non-dystrophic myotonias: molecular pathogenesis, diagnosis and treatment

The non-dystrophic myotonias are an important group of skeletal muscle channelopathies electrophysiologically characterized by altered membrane excitability. Many distinct clinical phenotypes are now recognized and range in severity from severe neonatal myotonia with respiratory compromise through to milder late-onset myotonic muscle stiffness. Specific genetic mutations in the major skeletal muscle voltage gated chloride channel gene and in the voltage gated sodium channel gene are causative in most patients. Recent work has allowed more precise correlations between the genotype and the electrophysiological and clinical phenotype. The majority of patients with myotonia have either a primary or secondary loss of membrane chloride conductance predicted to result in reduction of the resting membrane potential. Causative mutations in the sodium channel gene result in an abnormal gain of sodium channel function that may show marked temperature dependence. Despite significant advances in the clinical, genetic and molecular pathophysiological understanding of these disorders, which we review here, there are important unresolved issues we address: (i) recent work suggests that specialized clinical neurophysiology can identify channel specific patterns and aid genetic diagnosis in many cases however, it is not yet clear if such techniques can be refined to predict the causative gene in all cases or even predict the precise genotype; (ii) although clinical experience indicates these patients can have significant progressive morbidity, the detailed natural history and determinants of morbidity have not been specifically studied in a prospective fashion; (iii) some patients develop myopathy, but its frequency, severity and possible response to treatment remains undetermined, furthermore, the pathophysiogical link between ion channel dysfunction and muscle degeneration is unknown; (iv) there is currently insufficient clinical trial evidence to recommend a standard treatment. Limited data suggest that sodium channel blocking agents have some efficacy. However, establishing the effectiveness of a therapy requires completion of multi-centre randomized controlled trials employing accurate outcome measures including reliable quantitation of myotonia. More specific pharmacological approaches are required and could include those which might preferentially reduce persistent muscle sodium currents or enhance the conductance of mutant chloride channels. Alternative strategies may be directed at preventing premature mutant channel degradation or correcting the mis-targeting of the mutant channels.

Myotonic dystrophy: therapeutic strategies for the future

Myotonic dystrophy (DM) is a dominantly inherited neurodegenerative disorder for which there is no cure or effective treatment. Investigation of DM pathogenesis has identified a novel disease mechanism that requires development of innovative therapeutic strategies. It is now clear that DM is not caused by expression of a mutant protein. Instead, DM is the first recognized example of an RNA-mediated disease. Expression of the mutated gene gives rise to an expanded repeat RNA that is directly toxic to cells. The mutant RNA is retained in the nucleus, forming ribonuclear inclusions in affected tissue. A primary consequence of RNA toxicity in DM is dysfunction of two classes of RNA binding proteins, which leads to abnormal regulation of alternative splicing, or spliceopathy, of select genes. Spliceopathy now is known to cause myotonia and insulin resistance in DM. As our understanding of pathogenesis continues to improve, therapy targeted directly at the RNA disease mechanism will begin to replace the supportive care currently available. New pharmacologic approaches to treat myotonia and muscle wasting in DM type 1 are already in early clinical trials, and therapies designed to reverse the RNA toxicity have shown promise in preclinical models by correcting spliceopathy and eliminating myotonia. The well-defined ribonuclear inclusions may serve as convenient therapeutic targets to identify new agents that modify RNA toxicity. Continued development of appropriate model systems will allow testing of additional therapeutic strategies as they become available. Although DM is a decidedly complex disorder, its RNA-mediated disease mechanism may prove to be highly susceptible to therapy.

Correction of ClC-1 splicing eliminates chloride channelopathy and myotonia in mouse models of myotonic dystrophy

In myotonic dystrophy (dystrophia myotonica [DM]), an increase in the excitability of skeletal muscle leads to repetitive action potentials, stiffness, and delayed relaxation. This constellation of features, collectively known as myotonia, is associated with abnormal alternative splicing of the muscle-specific chloride channel (ClC-1) and reduced conductance of chloride ions in the sarcolemma. However, the mechanistic basis of the chloride channelopathy and its relationship to the development of myotonia are uncertain. Here we show that a morpholino antisense oligonucleotide (AON) targeting the 3' splice site of ClC-1 exon 7a reversed the defect of ClC-1 alternative splicing in 2 mouse models of DM. By repressing the inclusion of this exon, the AON restored the full-length reading frame in ClC-1 mRNA, upregulated the level of ClC-1 mRNA, increased the expression of ClC-1 protein in the surface membrane, normalized muscle ClC-1 current density and deactivation kinetics, and eliminated myotonic discharges. These observations indicate that the myotonia and chloride channelopathy observed in DM both result from abnormal alternative splicing of ClC-1 and that antisense-induced exon skipping offers a powerful method for correcting alternative splicing defects in DM.

Risk assessment for the amino acids taurine, L-glutamine and L-arginine

Taurine, glutamine and arginine are examples of amino acids which have become increasingly popular as ingredients in dietary supplements and functional foods and beverages. Animal and human clinical research suggests that oral supplementation of these amino acids provides additional health and/or performance benefits beyond those observed from normal intake of dietary protein. The increased consumer awareness and use of these amino acids as ingredients in dietary supplements and functional foods warrant a comprehensive review of their safety through quantitative risk assessment, and identification of a potential safe upper level of intake. The absence of a systematic pattern of adverse effects in humans in response to orally administered taurine (Tau), l-glutamine (Gln) and l-arginine (Arg) precluded the selection of a no observed adverse effect level (NOAEL) or lowest observed adverse effect level (LOAEL). Therefore, by definition, the usual approach to risk assessment for identification of a tolerable upper level of intake (UL) could not be used. Instead, the newer method described as the Observed Safe Level (OSL) or Highest Observed Intake (HOI) was utilized. The OSL risk assessments indicate that based on the available published human clinical trial data, the evidence for the absence of adverse effects is strong for Tau at supplemental intakes up to 3 g/d, Gln at intakes up to 14 g/d and Arg at intakes up to 20 g/d, and these levels are identified as the respective OSLs for normal healthy adults. Although much higher levels of each of these amino acids have been tested without adverse effects and may be safe, the data for intakes above these levels are not sufficient for a confident conclusion of long-term safety, and therefore these values are not selected as the OSLs.

Myotonia congenita

Myotonia is a symptom of many different acquired and genetic muscular conditions that impair the relaxation phase of muscular contraction. Myotonia congenita is a specific inherited disorder of muscle membrane hyperexcitability caused by reduced sarcolemmal chloride conductance due to mutations in CLCN1, the gene coding for the main skeletal muscle chloride channel ClC-1. The disorder may be transmitted as either an autosomal-dominant or recessive trait with close to 130 currently known mutations. Although this is a rare disorder, elucidation of the pathophysiology underlying myotonia congenita established the importance of sarcolemmal chloride conductance in the control of muscle excitability and demonstrated the first example of human disease associated with the ClC family of chloride transporting proteins.

The potential health benefits of taurine in cardiovascular disease

Taurine (2-aminoethanesulphonic acid), a sulphur-containing amino acid, is found in most mammalian tissues. Although it can be synthesized endogenously, the major source of taurine is from the diet. Taurine was found to exhibit diverse biological actions, including protection against ischemia-reperfusion injury, modulation of intracellular calcium concentration, and antioxidant, antiatherogenic and blood pressure-lowering effects. The present review will address the potential beneficial actions of taurine in congestive heart failure, hypertension, ischemic heart disease, atherosclerosis and diabetic cardiomyopathy. There is a wealth of experimental information and some clinical evidence available in the literature suggesting that taurine could be of benefit in cardiovascular disease of different etiologies. However, double-blind long-term clinical trials need to be conducted before taurine can be unequivocally recommended as a nutritional intervention for the prevention and/or treatment of cardiovascular disease.

Perioperative care of patients with neuromuscular disease and dysfunction

A variety of different pathologies result in disease phenotypes that are summarized as neuromuscular diseases because they share commonalty in their clinical consequences for the patient: a progressive weakening of the skeletal muscles. Distinct caution and appropriate changes to the anesthetic plan are advised when care is provided during the perioperative period. The choice of anesthetic technique, anesthetic drugs, and neuromuscular blockade always depends on the type of neuromuscular disease and the surgical procedure planned. A clear diagnosis of the underlying disease and sufficient knowledge and understanding of the pathophysiology are of paramount importance to the practitioner and guide optimal perioperative management of affected patients.