Dyskinesia due to mexiletine overdose: a rare presentation.

BACKGROUND: Mexiletine, a class IB antiarrhythmic, is a structural analog of lidocaine. Our knowledge of mexiletine overdose is based on lidocaine overdose reports. Only a few cases of mexiletine overdose have been reported, including fatal overdoses. Mexiletine toxicity primarily affects the central nervous, cardiovascular, and gastrointestinal systems. CASE: A 16-year-old female was brought to our hospital by ambulance after taking an unknown dose of mexiletine in a suicide attempt. Ventricular fibrillation developed while in the ambulance; cardiopulmonary resuscitation was started and spontaneous circulation returned within 1 min. The patient had been taking oral mexiletine for 1 month to treat primary erythromelalgia. Her vital signs were normal, but she was unconscious. Following gastric lavage she was transferred to the pediatric intensive care unit. Midazolam and levetiracetam were required due to uncontrolled seizures. During the first hour of hospitalization, severe dyskinesia characterized by abnormal involuntary large hyperkinetic movements in all 4 extremities was observed and successfully treated with 2 doses of intravenous biperiden. The patient was discharged on day 6 of hospitalization. CONCLUSIONS: Mexiletine overdose can be life-threatening. In addition to rapid and effective resuscitation, rapid identification and management of cardiovascular and central nervous system manifestations are key to preventing morbidity and mortality. The presented case had severe dyskinesia that was successfully treated with repeated doses of biperiden. Biperiden did not cause arrhythmia. Based on the presented case, we think biperiden should be considered for the treatment of movement disorders in cases of mexiletine overdose.

Effectiveness and safety of mexiletine in patients at risk for (recurrent) ventricular arrhythmias: a systematic review.

AIMS: While mexiletine has been used for over 40 years for prevention of (recurrent) ventricular arrhythmias and for myotonia, patient access has recently been critically endangered. Here we aim to demonstrate the effectiveness and safety of mexiletine in the treatment of patients with (recurrent) ventricular arrhythmias, emphasizing the absolute necessity of its accessibility. METHODS AND RESULTS: Studies were included in this systematic review (PROSPERO, CRD42020213434) if the efficacy or safety of mexiletine in any dose was evaluated in patients at risk for (recurrent) ventricular arrhythmias with or without comparison with alternative treatments (e.g. placebo). A systematic search was performed in Ovid MEDLINE, Embase, and in the clinical trial registry databases ClinicalTrials.gov and ICTRP. Risk of bias were assessed and tailored to the different study designs. Large heterogeneity in study designs and outcome measures prompted a narrative synthesis approach. In total, 221 studies were included reporting on 8970 patients treated with mexiletine. Age ranged from 0 to 88 years. A decrease in ventricular arrhythmias of >50% was observed in 72% of the studies for pre-mature ventricular complexes, 64% for ventricular tachycardia, and 33% for ventricular fibrillation. Electrocardiographic effects of mexiletine were small; only in a subset of patients with primary arrhythmia syndromes, a relative (desired) QTc decrease was reproducibly observed. As for adverse events, gastrointestinal complaints were most frequently observed (33% of the patients). CONCLUSIONS: In this systematic review, we present all the currently available knowledge of mexiletine in patients at risk for (recurrent) ventricular arrhythmias and show that mexiletine is both effective and safe.

Mexiletine for recurrent ventricular tachycardia in adult patients with structural heart disease and implantable cardioverter defibrillator: an EHRA systematic review.

The aim of the study was to systematically review evidence on the effectiveness and safety of oral mexiletine administered in monotherapy or in combination with other antiarrhythmic drugs for recurrent ventricular arrhythmia (ventricular tachycardia/ventricular fibrillation, VT/VF) in adult patients with structural heart disease (SHD) and implantable cardioverter defibrillators (ICDs). We systematically searched MEDLINE, Embase, and CENTRAL databases from inception to 27 August 2021 for prospective and retrospective studies investigating mexiletine in the target population. The main outcome was the reduction of ICD therapy. The main safety outcome was the presence of any serious adverse events (SAEs) leading to mexiletine discontinuation. Study quality was assessed using the Cochrane risk of bias tool or the Newcastle-Ottawa scale. Four studies comprising 86 mexiletine recipients were included in the review. We also obtained individual data of 50 patients from two studies. Ischaemic cardiomyopathy (ICM) was present in 86% of patients. The quality of included studies was moderate/low. A narrative review was undertaken as studies varied widely in terms of study population and treatment. Across studies, mexiletine treatment (with or without amiodarone) seemed to consistently reduce the number of ICD therapies especially in a population where catheter ablation (CA) was unsuccessful or contraindicated. In ICM patients deemed eligible for CA, mexiletine seemed to be inferior to CA. Mexiletine was discontinued in 14% of cases, mainly for gastrointestinal or neurological SAE. Mexiletine seems to be an option for the long-term treatment of recurrent VT/VF in adult patients with SHD, especially ICM, and ICD in whom CA was unsuccessful or not suitable.

Human-induced pluripotent stem cell-derived cardiomyocytes: Cardiovascular properties and metabolism and pharmacokinetics of deuterated mexiletine analogs.

Prolongation of the cardiac action potential (AP) and early after depolarizations (EADs) are electrical anomalies of cardiomyocytes that can lead to lethal arrhythmias and are potential liabilities for existing drugs and drug candidates in development. For example, long QT syndrome-3 (LQTS3) is caused by mutations in the Nav 1.5 sodium channel that debilitate channel inactivation and cause arrhythmias. We tested the hypothesis that a useful drug (i.e., mexiletine) with potential liabilities (i.e., potassium channel inhibition and adverse reactions) could be re-engineered by dynamic medicinal chemistry to afford a new drug candidate with greater efficacy and less toxicity. Human cardiomyocytes were generated from LQTS3 patient-derived induced pluripotent stem cells (hIPSCs) and normal hIPSCs to determine beneficial (on-target) and detrimental effects (off-target) of mexiletine and synthetic analogs, respectively. The approach combined "drug discovery" and "hit to lead" refinement and showed that iterations of medicinal chemistry and physiological testing afforded optimized compound 22. Compared to mexiletine, compound 22 showed a 1.85-fold greater AUC and no detectable CNS toxicity at 100 mg/kg. In vitro hepatic metabolism studies showed that 22 was metabolized via cytochrome P-450, as previously shown, and by the flavin-containing monooxygenase (FMO). Deuterated-22 showed decreased metabolism and showed acceptable cardiovascular and physicochemical properties.

Characterisation of mexiletine's translational therapeutic index for suppression of ischaemia-induced ventricular fibrillation in the rat isolated heart.

The 'translational therapeutic index' (TTI) is a drug's ratio of nonclinical threshold dose (or concentration) for significant benefit versus threshold for adversity. In early nonclinical research, discovery and safety studies are normally undertaken separately. Our aim was to evaluate a novel integrated approach for generating a TTI for drugs intended for prevention of ischaemia-induced ventricular fibrillation (VF). We templated the current best available class 1b antiarrhythmic, mexiletine, using the rat Langendorff preparation. Mexiletine's beneficial effects on the incidence of VF caused by 120 min regional ischaemia were contrasted with its concurrent adverse effects (on several variables) in the same hearts, to generate a TTI. Mexiletine 0.1 and 0.5 µM had no adverse effects, but did not reduce VF incidence. Mexiletine 1 µM reduced VF incidence to 0% but had adverse effects on atrioventricular conduction and ventricular repolarization. Separate studies undertaken using an intraventricular balloon revealed no detrimental effects of mexiletine (1 and 5 µM) on mechanical function, or any benefit against reperfusion-related dysfunction. Mexiletine's TTI was found to be less than two, which accords with its clinical therapeutic index. Although non-cardiac adversity, identifiable from additional in vivo studies, may reduce the TTI further, it cannot increase it. Our experimental approach represents a useful early-stage integrated risk/benefit method that, when TTI is found to be low, would eliminate unsuitable class 1b drugs prior to next stage in vivo work, with mexiletine's TTI defining the gold standard that would need to be bettered.

Sacubitril/Valsartan and Mexiletine: A Proarrhythmic Combination?

The uptake of sacubitril/valsartan since the PARADIGM study confirmed its beneficial effects on outcomes over enalapril in chronic systolic heart failure has inevitably led to potential interactions with co-prescribed medications in real-world patients. We report two cases that raise the possibility of an interaction between sacubitril/valsartan and the class Ib anti-arrhythmic mexiletine resulting in proarrhythmic effects. We discuss the pharmacokinetics of both agents and posit potential mechanistic interactions that suggest caution should be used and careful monitoring for (ventricular) arrhythmias applied in patients receiving sacubitril/valsartan and mexiletine.

Effect of Mexiletine on Muscle Stiffness in Patients With Nondystrophic Myotonia Evaluated Using Aggregated N-of-1 Trials.

Importance: In rare diseases it is difficult to achieve high-quality evidence of treatment efficacy because of small cohorts and clinical heterogeneity. With emerging treatments for rare diseases, innovative trial designs are needed. Objective: To investigate the effectiveness of mexiletine in nondystrophic myotonia using an aggregated N-of-1 trials design and compare results between this innovative design and a previously conducted RCT. Design, Setting, and Participants: A series of aggregated, double-blind, randomized, placebo-controlled N-of-1-trials, performed in a single academic referral center. Thirty Dutch adult patients with genetically confirmed nondystrophic myotonia (38 patients screened) were enrolled between February 2014 and June 2015. Follow-up was completed in September 2016. Interventions: Mexiletine (600 mg daily) vs placebo during multiple treatment periods of 4 weeks. Main Outcomes and Measures: Reduction in daily-reported muscle stiffness on a scale of 1 to 9, with higher scores indicating more impairment. A Bayesian hierarchical model aggregated individual N-of-1 trial data to determine the posterior probability of reaching a clinically meaningful effect of a greater than 0.75-point difference. Results: Among 30 enrolled patients (mean age, 43.4 [SD, 15.24] years; 22% men; 19 CLCN1 and 11 SCN4A genotype), 27 completed the study and 3 dropped out (1 because of a serious adverse event). In 24 of the 27 completers, a clinically meaningful treatment effect was found. In the Bayesian hierarchical model, mexiletine resulted in a 100% posterior probability of reaching a clinically meaningful reduction in self-reported muscle stiffness for the nondystrophic myotonia group overall and the CLCN1 genotype subgroup and 93% posterior probability for the SCN4A genotype subgroup. In the total nondystrophic myotonia group, the median muscle stiffness score was 6.08 (interquartile range, 4.71-6.80) at baseline and was 2.50 (95% credible interval [CrI], 1.77-3.24) during the mexiletine period and 5.56 (95% CrI, 4.73-6.39) during the placebo period; difference in symptom score reduction, 3.06 (95% CrI, 1.96-4.15; n = 27) favoring mexiletine. The most common adverse event was gastrointestinal discomfort (21 mexiletine [70%], 1 placebo [3%]). One serious adverse event occurred (1 mexiletine [3%]; allergic skin reaction). Using frequentist reanalysis, mexiletine compared with placebo resulted in a mean reduction in daily-reported muscle stiffness of 3.12 (95% CI, 2.46-3.78), consistent with the previous RCT treatment effect of 2.69 (95% CI, 2.12-3.26). Conclusions and Relevance: In a series of N-of-1 trials of mexiletine vs placebo in patients with nondystrophic myotonia, there was a reduction in mean daily-reported muscle stiffness that was consistent with the treatment effect in a previous randomized clinical trial. These findings support the efficacy of mexiletine for treatment of nondystrophic myotonia as well as the feasibility of N-of-1 trials for assessing interventions in some chronic rare diseases. Trial Registration: ClinicalTrials.gov Identifier: NCT02045667.

Mexiletine Usage in a Chronic Pain Clinic: Indications, Tolerability, and Side Effects.

BACKGROUND: Intravenous lidocaine has multiple applications in the management of acute and chronic pain. Mexiletine, an oral lidocaine analogue, has been used in a number of chronic pain conditions although its use is not well characterized. OBJECTIVES: To report our experience using mexiletine in a chronic pain population, specifically looking at tolerability, side effects, and EKG changes. STUDY DESIGN: Retrospective, cohort study. SETTING: Three chronic pain clinics within a hospital system in Detroit, MI. METHODS: All patients who had a mexiletine prescription between August 2015 and August 2016 were queried via the electronic medical record. Each chart was examined for demographics, QTc changes on EKG, length of use, and reasons for stoppage. RESULTS: There were 74 total patients identified in the chronic pain management clinics as receiving at least 1 mexiletine prescription over the 1-year time period. Twice as many women as men received mexiletine prescriptions. Neuropathic pain was the most common primary diagnosis (64%) which included diabetic neuropathy, radiculopathy, and others. Fibromyalgia was the next most common primary diagnosis (28%). A QTc change on the EKG showed a mean decrease of 0.1 ms and median increase of 1.5 ms. At 6 months (180 days), approximately 30% of the patients remained on mexiletine therapy, and 28% remained on the therapy at 1 year (360 days). Median duration of use was 60 days and the mean was 288 days. Neurologic and gastrointestinal side effects were the most commons reason for stoppage. All side effects were mild and resolved with stoppage. After side effects, lack of response, or loss of efficacy, were the next most common reasons for stoppage. LIMITATIONS: Pain relief and outcomes were not specifically examined due to confounding factors including interventional treatments and multiple treatment modalities. This was a retrospective, cohort study limited to our specific clinic population with a relatively high loss to follow-up rate. CONCLUSION: Mexiletine is rarely a first line option for chronic pain management and is often used when multiple other modalities have failed. By reporting our experience, we hope other clinicians may have more familiarity with the drug's use in a chronic pain practice. It appears reasonably tolerable, may not require frequent EKG monitoring, and can be an appropriate adjunct in the chronic pain population. More research is needed regarding efficacy and dose titration for mexiletine in chronic pain. KEY WORDS: Chronic pain, mexiletine, IV lidocaine, pain, neuropathic pain, neuropathy, fibromyalgia, QTc, tolerability.

Study protocol for the MEXiletine hydrochloride administration trial: a placebo-controlled, randomised, double-blind, multicentre, crossover study of its efficacy and safety in spinal and bulbar muscular atrophy (MEXPRESS).

INTRODUCTION: Spinal and bulbar muscular atrophy (SBMA) is a slowly progressive neuromuscular disease. Cold exposure often leads to worsening of motor symptoms including paresis. Although mexiletine hydrochloride administration has been shown to be effective for the treatment of several muscular diseases, its effectiveness in SBMA has not been validated to date. The trial will test it as a symptomatic drug for cold paresis. This study is the first trial to evaluate the efficacy and safety of mexiletine hydrochloride administration in patients with SBMA. METHODS AND ANALYSIS: A placebo-controlled, randomised, double-blind, multicentre, crossover clinical trial will be conducted to assess the safety and efficacy of mexiletine hydrochloride in patients with SBMA. The eligible patients will be assigned randomly in a 1:1 ratio to two groups in a double-blind manner. Participants will take mexiletine hydrochloride (300 mg/day) or a placebo orally three times a day for 4 weeks (period 1). After a 1-week washout period, participants will take the other drug for 4 weeks (period 2). The primary endpoint is the difference in distal latencies between room temperature and cold exposure conditions. ETHICS AND DISSEMINATION: This study will be conducted in compliance with the Helsinki Declaration and the Ethical Guidelines for Medical and Health Research Involving Human Subjects by the Japanese government and has been approved by the ethics committee of Nagoya University Graduate School of Medicine, as a central institutional review board, and by each facility. The results will be disseminated in peer-reviewed journals and at scientific conferences. TRIAL REGISTRATION NUMBER: UMIN000026150; Pre-results.

Sensory ganglionopathy associated with drug-induced hypersensitivity syndrome caused by mexiletine.

Although various causes are reported for sensory ganglionopathy, drug-induced hypersensitivity syndrome (DIHS) has not been considered a possibility. We describe a 70-year-old woman, previously administered mexiletine hydrochloride for 4 weeks, who presented with systemic oedematous erythema and subacute progressive gait disturbance. Evaluation revealed lymphadenopathy with atypical lymphocytosis and eosinophilia, and human herpesvirus 6 (HHV-6) reactivation. Neurological examination indicated the almost complete loss of joint positional sense in her extremities; her tendon reflex was lost and there was marked pseudoathetosis and Romberg's sign. Skin biopsy revealed spongiosis with lymphocyte infiltration. Based on these findings, we diagnosed acute sensory ganglionopathy secondary to DIHS. Although her DIHS-induced symptoms subsided after methylprednisolone treatment, partial remission of sensory ganglionopathy occurred, even after subsequent intravenous immunoglobulin therapy. This case suggests the possibility that reactivation of HHV-6 may be involved in the pathomechanism of sensory ganglionopathy.

Mexiletine or catheter ablation after amiodarone failure in the VANISH trial.

INTRODUCTION: In patients with ischemic heart disease and ventricular tachycardia (VT) refractory to high dose amiodarone, the two most common therapeutic options are adjunctive mexiletine therapy or catheter ablation. There are little existing data on the efficacy of these strategies. We examined the relative efficacy of adjunctive mexiletine and catheter ablation among patients enrolled in the VANISH trial. METHODS: All subjects enrolled in the VANISH trial who had VT refractory to high dose (≥ 300 mg daily) amiodarone at baseline were included. Per protocol, subjects randomized to escalated drug therapy received adjunctive mexiletine. RESULTS: Nineteen of the 259 patients were receiving high-dose amiodarone at baseline and 11 were randomized to escalated therapy with mexiletine and 8 to ablation. The adjunctive mexiletine group had a higher rate of the primary composite outcome (death, VT storm, or appropriate shock) in comparison to catheter ablation (HR 6.87 [2.08-22.8]). Over 90% of the patients in the adjunctive mexiletine/group experienced a primary endpoint during a median 9.2 months' follow-up. There was no difference in the rate of adverse events between the two groups. CONCLUSIONS: Mexiletine has limited efficacy in the treatment of recurrent VT despite high-dose amiodarone therapy, in patients with ischemic heart disease. Catheter ablation is a superior strategy in this population.

Translational approach to address therapy in myotonia permanens due to a new SCN4A mutation.

OBJECTIVE: We performed a clinical, functional, and pharmacologic characterization of the novel p.P1158L Nav1.4 mutation identified in a young girl presenting a severe myotonic phenotype. METHODS: Wild-type hNav1.4 channel and P1158L mutant were expressed in tsA201 cells for functional and pharmacologic studies using patch-clamp. RESULTS: The patient shows pronounced myotonia, slowness of movements, and generalized muscle hypertrophy. Because of general discomfort with mexiletine, she was given flecainide with satisfactory response. In vitro, mutant channels show a slower current decay and a rightward shift of the voltage dependence of fast inactivation. The voltage dependence of activation and slow inactivation were not altered. Mutant channels were less sensitive to mexiletine, whereas sensitivity to flecainide was not altered. The reduced inhibition of mutant channels by mexiletine was also observed using clinically relevant drug concentrations in a myotonic-like condition. CONCLUSIONS: Clinical phenotype and functional alterations of P1158L support the diagnosis of myotonia permanens. Impairment of fast inactivation is consistent with the possible role of the channel domain III S4-S5 loop in the inactivation gate docking site. The reduced sensitivity of P1158L to mexiletine may have contributed to the unsatisfactory response of the patient. The success of flecainide therapy underscores the usefulness of in vitro functional studies to help in the choice of the best drug for each individual.

A randomized trial of mexiletine in ALS: Safety and effects on muscle cramps and progression.

OBJECTIVE: To determine the safety and tolerability of mexiletine in a phase II double-blind randomized controlled trial of sporadic amyotrophic lateral sclerosis (SALS). METHODS: Sixty participants with SALS from 10 centers were randomized 1:1:1 to placebo, mexiletine 300 mg/d, or mexiletine 900 mg/d and followed for 12 weeks. The primary endpoints were safety and tolerability. Secondary endpoints were pharmacokinetic study from plasma and CSF, ALS Functional Rating Scale-Revised (ALSFRS-R) score, slow vital capacity (SVC), and muscle cramp frequency and severity. RESULTS: The only serious adverse event among active arm participants was one episode of imbalance. Thirty-two percent of participants receiving 900 mg of mexiletine discontinued study drug vs 5% on placebo (p = 0.026). Pharmacokinetic study demonstrated a peak plasma concentration 2 hours postdose and strong correlation between plasma and CSF (p < 0.001). Rates of decline of ALSFRS-R and SVC did not differ from placebo. Analysis of all randomized patients demonstrated significant reductions of muscle cramp frequency (300 mg: rate = 31% of placebo, p = 0.047; 900 mg: 16% of placebo, p = 0.002) and cramp intensity (300 mg: mean = 45% of placebo, p = 0.08; 900 mg: 25% of placebo, p = 0.005). CONCLUSIONS: Mexiletine was safe at both doses and well-tolerated at 300 mg/d but adverse effects at 900 mg/d led to a high rate of discontinuation. Mexiletine treatment resulted in large dose-dependent reductions in muscle cramp frequency and severity. No effect on rate of progression was detected, but clinically important differences could not be excluded in this small and short-duration study. CLASSIFICATION OF EVIDENCE: This study provides Class I evidence that mexiletine is safe when given daily to patients with amyotrophic lateral sclerosis at 300 and 900 mg and well-tolerated at the lower dose.

Mexiletine and its Interactions with Classical Antiepileptic Drugs: An Isobolographic Analysis.

Using the mouse maximal electroshock test, the reference model of tonic-clonic seizures, the aim of the present study was to determine the type of interaction between mexiletine (a class IB antiarrhythmic drug) and classical antiepileptics: valproate, carbamazepine, phenytoin, and phenobarbital. Isobolographic analysis of obtained data indicated antagonistic interactions between mexiletine and valproate (for fixed ratio combinations of 1:1 and 3:1). Additivity was observed between mexiletine and valproate applied in proportion of 1:3 as well as between mexiletine and remaining antiepileptics for the fixed ratios of 1:3, 1:1, and 3:1. Neither motor performance nor long-term memory were impaired by mexiletine or antiepileptic drugs regardless of whether they were administered singly or in combination. Mexiletine did not significantly affected brain concentrations of carbamazepine, phenobarbital or phenytoin. In contrast, the antiarrhythmic drug decreased by 23 % the brain level of valproate. This could be, at least partially, the reason of antagonistic interaction between the two drugs. In conclusion, the observed additivity suggests that mexiletine can be safely applied in epileptic patients treated with carbamazepine, phenytoin or phenobarbital. Because of undesirable pharmacodynamics and pharmacokinetic interactions with valproate, mexiletine should not be used in such combinations.

Mexiletine as an adjunctive therapy to amiodarone reduces the frequency of ventricular tachyarrhythmia events in patients with an implantable defibrillator.

The most effective pharmacological management of frequent ventricular tachyarrhythmia events in patients with an implantable defibrillator who failed or did not tolerate amiodarone is unknown. The aim of this retrospective cohort study was to assess the efficacy and tolerability of mexiletine in such patients. The patients served as self-controls. The number of treated ventricular tachyarrhythmia episodes (primary outcome); mortality, shocks from the defibrillator, and electrical storm events (secondary outcomes) during mexiletine therapy was compared with a matched duration of observation just before initiating mexiletine in 29 patients who were treated with a median dose of 300 mg/d of mexiletine and were followed for a median of 12 months. None of the patients had to stop mexiletine due to side effect. There was a significant reduction in the incidence of ventricular tachycardia/fibrillation episodes (median 2 vs. 12 events, P = 0.001) and shocks (median 0 vs. 2 events, P = 0.003) in the first 3 months of treatment, but long-term efficacy was only observed among patients who continued amiodarone therapy. In conclusion, mexiletine, when added to amiodarone in case of amiodarone inefficacy, reduces ventricular tachycardia/fibrillation events and appropriate therapies in patients with an implantable cardioverter defibrillator. A randomized trial should validate the efficacy and safety of mexiletine as an adjunctive therapy to amiodarone.

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.

Primary erythromelalgia in a 12-year-old boy: positive response to sodium channel blockers despite negative SCN9A mutations.

Erythromelalgia is a rare disorder characterized by recurrent pain attacks, swelling and redness in the distal extremities. The primary forms of the disorder are caused by mutations in voltage-gated sodium channels. Treatment is difficult and controlled therapeutic studies offer little to no guidance. We report on a 12-year-old boy and his first occurrence of primary erythromelalgia. Genetic findings for mutations in the SCN9A gene, which encodes for the α-subunit of sodium channel NaV1.7, were negative. Although initial treatment with sodium nitroprusside was ineffective, subsequent medication with lidocaine and mexiletine, in combination with gabapentin, was successful. Despite negative findings for mutations in the sodium channels, the use of sodium channel blockers should be considered in these patients.