Mexiletine as a treatment for primary erythromelalgia: normalization of biophysical properties of mutant L858F NaV 1.7 sodium channels

Painful Raynaud's mimics

Raynaud's syndrome is a common finding in many autoimmune conditions. Accurately diagnosing Raynaud's, and differentiating it from mimicking conditions, is imperative in rheumatologic diseases. Raynaud's syndrome and Raynaud's mimickers, especially painful Raynaud's mimickers, can prove a diagnostic challenge for the practicing rheumatologist. Painful Raynaud's mimickers can lead to increased patient stress and unnecessary medical work up; Healthcare providers need to be aware of Raynaud's mimickers when evaluating patient concerns of skin color changes and pain. The present narrative review aims to highlight Raynaud's syndrome, important painful mimickers that may be seen, diagnosis, and updated management recommendations.

Genetics of diabetes-associated microvascular complications

Diabetes is associated with excess morbidity and mortality due to both micro- and macrovascular complications, as well as a range of non-classical comorbidities. Diabetes-associated microvascular complications are those considered most closely related to hyperglycaemia in a causal manner. However, some individuals with hyperglycaemia (even those with severe hyperglycaemia) do not develop microvascular diseases, which, together with evidence of co-occurrence of microvascular diseases in families, suggests a role for genetics. While genome-wide association studies (GWASs) produced firm evidence of multiple genetic variants underlying differential susceptibility to type 1 and type 2 diabetes, genetic determinants of microvascular complications are mostly suggestive. Identified susceptibility variants of diabetic kidney disease (DKD) in type 2 diabetes mirror variants underlying chronic kidney disease (CKD) in individuals without diabetes. As for retinopathy and neuropathy, reported risk variants currently lack large-scale replication. The reported associations between type 2 diabetes risk variants and microvascular complications may be explained by hyperglycaemia. More extensive phenotyping, along with adjustments for unmeasured confounding, including both early (fetal) and late-life (hyperglycaemia, hypertension, etc.) environmental factors, are urgently needed to understand the genetics of microvascular complications. Finally, genetic variants associated with reduced glycolysis, mitochondrial dysfunction and DNA damage and sustained cell regeneration may protect against microvascular complications, illustrating the utility of studies in individuals who have escaped these complications.

Clinical Characterization of Pediatric Erythromelalgia: A Single-Center Case Series

Erythromelalgia is a descriptive term for severe burning pain and erythema in the distal extremities relieved by cold and exacerbated by heat. Pediatric case series to date are relatively small. We extracted and analyzed medical record data for 42 pediatric patients to describe clinical characteristics, associated conditions, and responses to treatments. Informed consent was obtained according to an IRB-approved protocol that included gene discovery. Three patients had confirmed Nav1.7 sodium channelopathies, with six additional patients under investigation with novel gene candidates. There was a female predominance (2.5:1), and the median onset age was 12 years (IQR = 3-14). Patients saw a median of three specialists (IQR = 2-3) for a diagnosis. The majority (90%) reported bilateral symptoms. Cooling methods usually provided partial relief, while heat and exercise exacerbated pain. No medication appeared to be consistently effective; commonly prescribed medications included sodium channel blockers (n = 37), topical analgesics (n = 26), gabapentin (n = 22), and aspirin (n = 15). Based on the currently published literature, we believe this cohort is the largest pediatric study of erythromelalgia to date. Many findings are consistent with those of previously published case series. Work is in progress to establish a prospective cohort and multi-center registry.

Clinical Challenges in Primary Erythromelalgia: a Real-Life Experience from a Single Center and a Diagnostic-Therapeutic Flow-Chart Proposal

INTRODUCTION

Primary erythromelalgia (EM) is a rare clinical syndrome characterized by recurrent erythema, burning pain and warmth of the extremities. The symptoms greatly compromise the patients' quality of life leading to severe disability. SCN9A mutations can be the cause of the disease. Dermatologists are often the specialists these patients turn to for assistance.

OBJECTIVES

To describe the demographic and clinical characteristics of patients with primary EM, to assess the presence and mutation types in the SCN9A gene, to evaluate the effectiveness of several therapeutic approaches, and to propose a diagnostic algorithm with therapeutic implications.

METHODS

A monocentric retrospective study using the database of patients with a discharge diagnosis of primary EM of our Center. Demographic, clinical, instrumental and laboratory data of patients were reviewed.

RESULTS

Eleven female patients (age range 16 to 57) were selected. All patients were affected in both the lower and upper extremities. Follow-up ranged from 2 to 9 years. Four patients had four different heterozygous variants of the SCN9A gene. Two patients, although genetically negative, had a suggestive family history. A variety of medications were tried in all our patients to alleviate symptoms, but their efficacy was variable, partial and/or transitory. The most effective therapies were antihistamines, venlafaxine, and mexiletine.

CONCLUSIONS

The diagnosis and treatment of EM remain challenging. Patients with this condition display a wide spectrum of clinical manifestations and severity, as well as a paucity of resources and structures to support them. Mutations in the SCN9A gene are not always detected.

Advances and challenges in neuropathic pain: a narrative review and future directions

Over the past few decades, substantial advances have been made in neuropathic pain clinical research. An updated definition and classification have been agreed. Validated questionnaires have improved the detection and assessment of acute and chronic neuropathic pain; and newer neuropathic pain syndromes associated with COVID-19 have been described. The management of neuropathic pain has moved from empirical to evidence-based medicine. However, appropriately targeting current medications and the successful clinical development of drugs acting on new targets remain challenging. Innovative approaches to improving therapeutic strategies are required. These mainly encompass rational combination therapy, drug repurposing, non-pharmacological approaches (such as neurostimulation techniques), and personalised therapeutic management. This narrative review reports historical and current perspectives regarding the definitions, classification, assessment, and management of neuropathic pain and explores potential avenues for future research.

Investigating genotype-phenotype relationship of extreme neuropathic pain disorders in a UK national cohort

The aims of our study were to use whole genome sequencing in a cross-sectional cohort of patients to identify new variants in genes implicated in neuropathic pain, to determine the prevalence of known pathogenic variants and to understand the relationship between pathogenic variants and clinical presentation. Patients with extreme neuropathic pain phenotypes (both sensory loss and gain) were recruited from secondary care clinics in the UK and underwent whole genome sequencing as part of the National Institute for Health and Care Research Bioresource Rare Diseases project. A multidisciplinary team assessed the pathogenicity of rare variants in genes previously known to cause neuropathic pain disorders and exploratory analysis of research candidate genes was completed. Association testing for genes carrying rare variants was completed using the gene-wise approach of the combined burden and variance-component test SKAT-O. Patch clamp analysis was performed on transfected HEK293T cells for research candidate variants of genes encoding ion channels. The results include the following: (i) Medically actionable variants were found in 12% of study participants (205 recruited), including known pathogenic variants: SCN9A(ENST00000409672.1): c.2544T>C, p.Ile848Thr that causes inherited erythromelalgia, and SPTLC1(ENST00000262554.2):c.340T>G, p.Cys133Tr variant that causes hereditary sensory neuropathy type-1. (ii) Clinically relevant variants were most common in voltage-gated sodium channels (Nav). (iii) SCN9A(ENST00000409672.1):c.554G>A, pArg185His variant was more common in non-freezing cold injury participants than controls and causes a gain of function of NaV1.7 after cooling (the environmental trigger for non-freezing cold injury). (iv) Rare variant association testing showed a significant difference in distribution for genes NGF, KIF1A, SCN8A, TRPM8, KIF1A, TRPA1 and the regulatory regions of genes SCN11A, FLVCR1, KIF1A and SCN9A between European participants with neuropathic pain and controls. (v) The TRPA1(ENST00000262209.4):c.515C>T, p.Ala172Val variant identified in participants with episodic somatic pain disorder demonstrated gain-of-channel function to agonist stimulation. Whole genome sequencing identified clinically relevant variants in over 10% of participants with extreme neuropathic pain phenotypes. The majority of these variants were found in ion channels. Combining genetic analysis with functional validation can lead to a better understanding as to how rare variants in ion channels lead to sensory neuron hyper-excitability, and how cold, as an environmental trigger, interacts with the gain-of-function NaV1.7 p.Arg185His variant. Our findings highlight the role of ion channel variants in the pathogenesis of extreme neuropathic pain disorders, likely mediated through changes in sensory neuron excitability and interaction with environmental triggers.

Erythromelalgia: A Review of Medical Management Options and Our Approach to Management

Erythromelalgia (EM) is a rare disorder characterized by episodic, burning pain associated with erythema and warmth of the extremities. The feet and hands are most commonly affected. The pain can be so severe that patients may engage in behaviors, sometimes extreme, to cool the affected areas and change their lifestyle to avoid precipitating factors, such as exercise and increased ambient heat. A literature search was performed with PubMed and MEDLINE with the search term erythromelalgia. Inclusion criteria were studies on EM published after 1985 until January 1, 2022, in the English language and studies that provided information on medical treatment of EM. Studies were excluded if they were duplicates or did not include treatment data. No guidelines exist for the treatment of this complex disorder. Lifestyle modifications and pharmacologic treatments (topical and systemic) are discussed in this article, which provides a comprehensive review of published medical management options for erythromelalgia and a proposed approach to management.

Extensive Lumbar Sympathetic Ganglion Block Combined With Epidural Block for Primary Erythromelalgia: A Case Report

A 19-year-old-woman experienced severe burning pain in the lower extremities with erythema and swelling. She was diagnosed with primary erythromelalgia (PE). The pain was unresponsive to medications but relieved by immersing her feet in cold water. We performed a multilevel lumbar sympathetic ganglion block (LSGB) with 5% phenol at second lumbar vertebra (L2) and third lumbar vertebra (L3), and additional fourth lumbar vertebra (L4) levels. An epidural block was intermittently combined. The pain and skin lesions dramatically improved after the procedures, and she no longer needed medications or to soak her feet in cold water. This case demonstrated that extensive LSGB may be a therapeutic option for intractable PE.

[Erythromelalgia: skin redness and pain]

Erythromelalgia is a rare disease that is associated with hemato-oncological diseases or after taking certain drugs and toxins, but it can also occur as an independent clinical picture, for example, due to mutations in the sodium channel Na_V1.7. Clinically, there is a characteristic triad of attack-like burning pain and skin redness in the area of the distal extremities, which can be alleviated by excessive cooling. The attacks are triggered by heat, exertion, and stress. The diagnosis is primarily made clinically and can be confirmed by genetic testing if a sodium channel Na_V1.7 mutation is present. Important differential diagnoses are complex regional pain syndrome, the non-freezing cold injury, and small fiber neuropathies. Therapy is multidisciplinary and has to be planned individually and include physical therapy and psychotherapy as well as drug therapy as integral components.

The Role of Voltage-Gated Sodium Channels in Pain Signaling

Acute pain signaling has a key protective role and is highly evolutionarily conserved. Chronic pain, however, is maladaptive, occurring as a consequence of injury and disease, and is associated with sensitization of the somatosensory nervous system. Primary sensory neurons are involved in both of these processes, and the recent advances in understanding sensory transduction and human genetics are the focus of this review. Voltage-gated sodium channels (VGSCs) are important determinants of sensory neuron excitability: they are essential for the initial transduction of sensory stimuli, the electrogenesis of the action potential, and neurotransmitter release from sensory neuron terminals. Nav1.1, Nav1.6, Nav1.7, Nav1.8, and Nav1.9 are all expressed by adult sensory neurons. The biophysical characteristics of these channels, as well as their unique expression patterns within subtypes of sensory neurons, define their functional role in pain signaling. Changes in the expression of VGSCs, as well as posttranslational modifications, contribute to the sensitization of sensory neurons in chronic pain states. Furthermore, gene variants in Nav1.7, Nav1.8, and Nav1.9 have now been linked to human Mendelian pain disorders and more recently to common pain disorders such as small-fiber neuropathy. Chronic pain affects one in five of the general population. Given the poor efficacy of current analgesics, the selective expression of particular VGSCs in sensory neurons makes these attractive targets for drug discovery. The increasing availability of gene sequencing, combined with structural modeling and electrophysiological analysis of gene variants, also provides the opportunity to better target existing therapies in a personalized manner.

Pediatric Erythromelalgia and SCN9A Mutations: Systematic Review and Single-Center Case Series

OBJECTIVES

To evaluate the clinical features of erythromelalgia in childhood associated with gain-of-function SCN9A mutations that increase activity of the Na_v1.7 voltage-gated sodium channel, we conducted a systematic review of pediatric presentations of erythromelalgia related to SCN9A mutations, and compared pediatric clinical presentations of symptomatic erythromelalgia, with or without SCN9A mutations.

STUDY DESIGN

PubMed, Embase, and PsycINFO Databases were searched for reports of inherited erythromelalgia in childhood. Clinical features, management, and genotype were extracted. Case notes of pediatric patients with erythromelalgia from the Great Ormond Street Hospital Pain Service were reviewed for clinical features, patient-reported outcomes, and treatments. Children aged over 10 years were recruited for quantitative sensory testing.

RESULTS

Twenty-eight publications described erythromelalgia associated with 15 different SCN9A gene variants in 25 children. Pain was severe and often refractory to multiple treatments, including nonspecific sodium channel blockers. Skin damage or other complications of cold immersion for symptomatic relief were common (60%). SCN9A mutations resulting in greater hyperpolarizing shifts in Na_v1.7 sodium channels correlated with symptom onset at younger ages (P = .016). Variability in reporting, and potential publication bias toward severe cases, limit any estimations of overall prevalence. In our case series, symptoms were similar but comorbidities were more common in children with SCN9A mutations. Quantitative sensory testing revealed marked dynamic warm allodynia.

CONCLUSIONS

Inherited erythromelalgia in children is associated with difficult-to-manage pain and significant morbidity. Standardized reporting of outcome and management in larger series will strengthen identification of genotype-phenotype relationships. More effective long-term therapies are a significant unmet clinical need.

Review of primary and secondary erythromelalgia

Erythromelalgia is a condition characterized by episodic pain, erythema and temperature of the extremities, which is relieved by cooling and aggravated by warming. It is useful to review this topic in light of recent discoveries of the genetic mutations that now define primary erythromelalgia, as opposed to secondary erythromelalgia, which is often associated with underlying medical disorders.

The efficacy and safety of first-line therapies for preventing chronic post-surgical pain: a network meta-analysis

Background

Due to conflicting evidence regarding first-line therapies for chronic post-surgical pain (CPSP), here we comparatively evaluated the efficacy and safety of first-line therapies for the prevention of CPSP.

Materials and Methods

MEDLINE, EMBASE, and Cochrane CENTRAL databases were searched for randomized, controlled trials (RCTs) of systemic drugs measuring pain three months or more post-surgery. Pairwise meta-analyses (a frequentist technique directly comparing each intervention against placebo) and network meta-analyses (a Bayesian technique simultaneously comparing several interventions via an evidence network) compared the mean differences for primary efficacy (reduction in all pain), secondary efficacy (reduction in moderate or severe pain), and primary safety (drop-out rate from treatment-related adverse effects). Ranking probabilities from the network meta-analysis were transformed using surface under the cumulative ranking analysis (SUCRA). Sensitivity analyses evaluated the impact of age, gender, surgery type, and outlier studies.

Results

Twenty-four RCTs were included. Mexiletine and ketamine ranked highest in primary efficacy, while ketamine and nefopam ranked highest in secondary efficacy. Simultaneous SUCRA-based rankings of the interventions according to both efficacy and safety revealed that nefopam and mexiletine ranked highest in preventing CPSP. Through the sensitivity analyses, gabapentin and ketamine remained the most-highly-ranked in terms of efficacy, while nefopam and ketamine remained the most-highly-ranked in terms of safety.

Conclusions

Nefopam and mexiletine may be considered as first-line therapies for the prevention of CPSP. On account of the paucity of evidence available on nefopam and mexiletine, gabapentin and ketamine may also be considered. Venlafaxine is not recommended for the prevention of CPSP.

How a Simple Ankle Sprain Turned Into Neuropathic Pain: Complex Reflex Sympathetic Dystrophy Versus Erythromelalgia

A 36-year-old woman sustained a Grade 2 ankle sprain at work. Two days after the injury, the ankle and foot became red and she complained of “intense burning pain.” First diagnosed with complex reflex sympathetic dystrophy, the employee was prescribed medications that provided some pain relief; a subsequent temporary nerve block provided additional relief. However, the symptoms returned and she was treated unsuccessfully with surgical sympathectomy. The employee was referred to a neurologist and diagnosed with primary erythromelalgia, a rare pain disorder that can be mistaken as complex reflex sympathetic dystrophy.

Erythromelalgia

Erythromelalgia is a rare syndrome characterized by the intermittent or, less commonly, by the permanent occurrence of extremely painful hyperperfused skin areas mainly located in the distal extremities. Primary erythromelalgia is nowadays considered to be a genetically determined neuropathic disorder affecting SCN9A, SCN10A, and SCN11A coding for NaV1.7, NaV1.8, and NaV1.9 neuronal sodium channels. Secondary forms might be associated with myeloproliferative disorders, connective tissue disease, cancer, infections, and poisoning. Between the pain episodes, the affected skin areas are usually asymptomatic, but there are patients with typical features of acrocyanosis and/or Raynaud's phenomenon preceding or occurring in between the episodes of erythromelalgia. Diagnosis is made by ascertaining the typical clinical features. Thereafter, the differentiation between primary and secondary forms should be made. Genetic testing is recommended, especially in premature cases and in cases of family clustering in specialized genetic institutions after genetic counselling. Multimodal therapeutic intervention aims toward attenuation of pain and improvement of the patient's quality of life. For this purpose, a wide variety of nonpharmacological approaches and pharmacological substances for topical and systemic use have been proposed, which are usually applied individually in a step-by-step approach. Prognosis mainly depends on the underlying condition and the ability of the patients and their relatives to cope with the disease.

Small fiber neuropathy: Diagnosis, causes, and treatment

Small fiber neuropathy, which affects the sensory Aδ and C fibers, is now a major diagnostic and therapeutic challenge. Nearly 7% of the general population have chronic neuropathic pain responsible for severe quality-of-life impairments. Awareness must therefore be raised among clinicians of the somatosensory and autonomic symptoms that can reveal small fiber neuropathy, appropriate diagnostic investigations, most common causes, and best treatment options for each patient profile. To help achieve this goal, the present review article discusses the clinical presentation of neuropathic pain and paresthesia and/or autonomic dysfunction due to involvement of nerves supplying exocrine glands and smooth muscle; normal findings from standard electrophysiological investigations; most informative diagnostic tests (epidermal nerve fiber density in a skin biopsy, laser-evoked potentials, heat- and cold-detection thresholds, electrochemical skin conductance); main causes, which consist chiefly of metabolic diseases (diabetes mellitus, glucose intolerance), dysimmunity syndromes (Sjögren's syndrome, sarcoidosis, monoclonal gammopathy), and genetic abnormalities (familial amyloidosis due to a transthyretin mutation, Fabry disease, sodium channel diseases); and the available symptomatic and etiological treatments.

Sodium channel biophysics, late sodium current and genetic arrhythmic syndromes

Arrhythmias arise from breakdown of orderly action potential (AP) activation, propagation and recovery driven by interactive opening and closing of successive voltage-gated ion channels, in which one or more Na⁺ current components play critical parts. Early peak, Na⁺ currents (I_Na) reflecting channel activation drive the AP upstroke central to cellular activation and its propagation. Sustained late Na⁺ currents (I_Na-L) include contributions from a component with a delayed inactivation timecourse influencing AP duration (APD) and refractoriness, potentially causing pro-arrhythmic phenotypes. The magnitude of I_Na-L can be analysed through overlaps or otherwise in the overall voltage dependences of the steady-state properties and kinetics of activation and inactivation of the Na⁺ conductance. This was useful in analysing repetitive firing associated with paramyotonia congenita in skeletal muscle. Similarly, genetic cardiac Na⁺ channel abnormalities increasing I_Na-L are implicated in triggering phenomena of automaticity, early and delayed afterdepolarisations and arrhythmic substrate. This review illustrates a wide range of situations that may accentuate I_Na-L. These include (1) overlaps between steady-state activation and inactivation increasing window current, (2) kinetic deficiencies in Na⁺ channel inactivation leading to bursting phenomena associated with repetitive channel openings and (3) non-equilibrium gating processes causing channel re-opening due to more rapid recoveries from inactivation. All these biophysical possibilities were identified in a selection of abnormal human SCN5A genotypes. The latter presented as a broad range of clinical arrhythmic phenotypes, for which effective therapeutic intervention would require specific identification and targeting of the diverse electrophysiological abnormalities underlying their increased I_Na-L.

Neuropathic pain

Neuropathic pain is caused by a lesion or disease of the somatosensory system, including peripheral fibres (Aβ, Aδ and C fibres) and central neurons, and affects 7-10% of the general population. Multiple causes of neuropathic pain have been described and its incidence is likely to increase owing to the ageing global population, increased incidence of diabetes mellitus and improved survival from cancer after chemotherapy. Indeed, imbalances between excitatory and inhibitory somatosensory signalling, alterations in ion channels and variability in the way that pain messages are modulated in the central nervous system all have been implicated in neuropathic pain. The burden of chronic neuropathic pain seems to be related to the complexity of neuropathic symptoms, poor outcomes and difficult treatment decisions. Importantly, quality of life is impaired in patients with neuropathic pain owing to increased drug prescriptions and visits to health care providers, as well as the morbidity from the pain itself and the inciting disease. Despite challenges, progress in the understanding of the pathophysiology of neuropathic pain is spurring the development of new diagnostic procedures and personalized interventions, which emphasize the need for a multidisciplinary approach to the management of neuropathic pain.

Neuropathic pain

Neuropathic pain is caused by a lesion or disease of the somatosensory system, including peripheral fibres (Aβ, Aδ and C fibres) and central neurons, and affects 7-10% of the general population. Multiple causes of neuropathic pain have been described and its incidence is likely to increase owing to the ageing global population, increased incidence of diabetes mellitus and improved survival from cancer after chemotherapy. Indeed, imbalances between excitatory and inhibitory somatosensory signalling, alterations in ion channels and variability in the way that pain messages are modulated in the central nervous system all have been implicated in neuropathic pain. The burden of chronic neuropathic pain seems to be related to the complexity of neuropathic symptoms, poor outcomes and difficult treatment decisions. Importantly, quality of life is impaired in patients with neuropathic pain owing to increased drug prescriptions and visits to health care providers, as well as the morbidity from the pain itself and the inciting disease. Despite challenges, progress in the understanding of the pathophysiology of neuropathic pain is spurring the development of new diagnostic procedures and personalized interventions, which emphasize the need for a multidisciplinary approach to the management of neuropathic pain.

Nonlinear effects of hyperpolarizing shifts in activation of mutant Nav1.7 channels on resting membrane potential

The Na_v1.7 sodium channel is preferentially expressed within dorsal root ganglion (DRG) and sympathetic ganglion neurons. Gain-of-function mutations that cause the painful disorder inherited erythromelalgia (IEM) shift channel activation in a hyperpolarizing direction. When expressed within DRG neurons, these mutations produce a depolarization of resting membrane potential (RMP). The biophysical basis for the depolarized RMP has to date not been established. To explore the effect on RMP of the shift in activation associated with a prototypical IEM mutation (L858H), we used dynamic-clamp models that represent graded shifts that fractionate the effect of the mutation on activation voltage dependence. Dynamic-clamp recording from DRG neurons using a before-and-after protocol for each cell made it possible, even in the presence of cell-to-cell variation in starting RMP, to assess the effects of these graded mutant models. Our results demonstrate a nonlinear, progressively larger effect on RMP as the shift in activation voltage dependence becomes more hyperpolarized. The observed differences in RMP were predicted by the "late" current of each mutant model. Since the depolarization of RMP imposed by IEM mutant channels is known, in itself, to produce hyperexcitability of DRG neurons, the development of pharmacological agents that normalize or partially normalize activation voltage dependence of IEM mutant channels merits further study.NEW & NOTEWORTHY Inherited erythromelalgia (IEM), the first human pain disorder linked to a sodium channel, is widely regarded as a genetic model of neuropathic pain. IEM is produced by Na_v1.7 mutations that hyperpolarize activation. These mutations produce a depolarization of resting membrane potential (RMP) in dorsal root ganglion neurons. Using dynamic clamp to explore the effect on RMP of the shift in activation, we demonstrate a nonlinear effect on RMP as the shift in activation voltage dependence becomes more hyperpolarized.

[Erythromelalgia: Diagnosis and therapeutic approach]

Erythromelalgia is a rare intermittent vascular acrosyndrome characterized by the combination of recurrent burning pain, warmth and redness of the extremities. It is considered in its primary form as an autosomal dominant neuropathy related to mutations of SCN9A, the encoding gene of a voltage-gated sodium channel subtype Nav1.7. Secondary erythromelalgia is associated with myeloproliferative disorders, drugs (bromocriptine, calcium channel blockers), or clinical conditions such as rheumatic diseases or viral infection. Primary familial erythromelalgia include genetics and sporadic forms associated with small fibers neuropathy. Aspirin is a useful treatment of erythromelagia associated with myeloproliferative disorders. Treatment of primary erythromelalgia is difficult, individualized, with sodium channel blockers such as lidocaine, carbamazepine and mexiletine.

shRNA mediated knockdown of Nav1.7 in rat dorsal root ganglion attenuates pain following burn injury

Background

Abnormal acute pain after burn injury still torments patients severely. In this study, we investigated that one voltage gated sodium channel Nav1.7 plays a vital role in lowering heat pain threshold after burn injury, and the hypothesis that knockdown of Nav1.7 attenuates pain following burn injury.

Methods

Sixty eight adult male Sprague–Dawley rats were divided into 4 treatment groups: (1) sham, which hind paw was put on the room temperature metal plate for 15 s (2) burn model, which hind paw was put on the 85 °C metal plate for 15 s. (3) Burn injury + lentiviral vector -SCN9AsiRNA-GFP (LV- SCN9AsiRNA-GFP group, n = 18), which receive the DRG microinjection of LV- SCN9AsiRNA-GFP on the zero day. (4) Burn injury + lentiviral vector negative control (LV-NC-GFP group, n = 18), which receive the DRG microinjection of empty lentiviral vector on the zero day.

Results

Both mechanical and heat threshold were measured from day 1 to 21. Meanwhile, expression of sodium channels Nav1.7 in injured dorsal root ganglia were measured on post-operative days 7(POD 7). Rats exhibited decreased thresholds on both mechanical allodynia and thermal withdrawl latency, accompanied by increased Nav1.7 and c-fos expression in dorsal root ganglion (DRG). And knockdown of Nav1.7 in L5DRG led to the attenuation of burn injury-induced mechanical allodynia and thermal hyperalgesia in the rats.

Conclusion

We provide evidence that shRNA mediated knockdown of Nav1.7 attenuates burn induced pain in rats as well as decreased the activiation of c-fos protein.

The Complexity of Pain Management in Patients with Erythromelalgia

A 15-year-old girl diagnosed with erythromelalgia was admitted to the hospital with severe pain in her feet associated with burning, pruritus, erythema, and swelling. She had not responded to conventional management and received some relief only from cold bath immersions, which resulted in chronic blistering and multiple episodes of superinfection. After a successful trial of spinal cord stimulation, she had a permanent implantation procedure. The spinal cord stimulator relieved her pain and improved function but not the sensation of burning pain. However, this pain resolved after she started daily mexiletine. This case demonstrates that erythromelalgia sometimes can be managed successfully with a combination of pharmacologic and interventional procedures.

Genetic neurological channelopathies: molecular genetics and clinical phenotypes

Evidence accumulated over recent years has shown that genetic neurological channelopathies can cause many different neurological diseases. Presentations relating to the brain, spinal cord, peripheral nerve or muscle mean that channelopathies can impact on almost any area of neurological practice. Typically, neurological channelopathies are inherited in an autosomal dominant fashion and cause paroxysmal disturbances of neurological function, although the impairment of function can become fixed with time. These disorders are individually rare, but an accurate diagnosis is important as it has genetic counselling and often treatment implications. Furthermore, the study of less common ion channel mutation-related diseases has increased our understanding of pathomechanisms that is relevant to common neurological diseases such as migraine and epilepsy. Here, we review the molecular genetic and clinical features of inherited neurological channelopathies.

Inhibition of hERG potassium channel by the antiarrhythmic agent mexiletine and its metabolite m-hydroxymexiletine

Mexiletine is a sodium channel blocker, primarily used in the treatment of ventricular arrhythmias. Moreover, recent studies have demonstrated its therapeutic value to treat myotonic syndromes and to relieve neuropathic pain. The present study aims at investigating the direct blockade of hERG potassium channel by mexiletine and its metabolite m-hydroxymexiletine (MHM). Our data show that mexiletine inhibits hERG in a time- and voltage-dependent manner, with an IC₅₀ of 3.7 ± 0.7 μmol/L. Analysis of the initial onset of current inhibition during a depolarizing test pulse indicates mexiletine binds preferentially to the open state of the hERG channel. Looking for a possible mexiletine alternative, we show that m-hydroxymexiletine (MHM), a minor mexiletine metabolite recently reported to be as active as the parent compound in an arrhythmia animal model, is a weaker hERG channel blocker, compared to mexiletine (IC₅₀ = 22.4 ± 1.2 μmol/L). The hERG aromatic residues located in the S6 helix (Tyr652 and Phe656) are crucial in the binding of mexiletine and the different affinities of mexiletine and MHM with hERG channel are interpreted by modeling their corresponding binding interactions through ab initio calculations. The simulations demonstrate that the introduction of a hydroxyl group on the meta-position of the aromatic portion of mexiletine weakens the interaction of the drug xylyloxy moiety with Tyr652. These results provide further insights into the molecular basis of drug/hERG interactions and, in agreement with previously reported results on clofilium and ibutilide analogs, support the possibility of reducing hERG potency and related toxicity by modifying the aromatic pattern of substitution of clinically relevant compounds.

Mutations in Sodium Channel Gene SCN9A and the Pain Perception Disorders

Voltage-gated sodium channels (NaV) play a crucial role in development and propagation of action potentials in neurons and muscle cells. NaV1.7 channels take a special place in modern science since it is believed that they contribute to nerve hyperexcitability. Mutations of the gene SCN9A, which codes the α subunit of NaV1.7 channels, are associated with pain perception disorders (primary erythermalgia, congenital analgesia, and paroxysmal pain disorder). It is considered that the SCN9A gene mutations may cause variations in sensitivity to pain, from complete insensitivity to extreme sensitivity. Further research of the SCN9A gene polymorphism influence on pain sensitivity is essential for the understanding of the pathophysiology of pain and the development of the appropriate targeted pain treatment.