Skeletal Muscle Channelopathies

Prevalence of genetically confirmed skeletal muscle channelopathies in the era of next generation sequencing

We provide an up-to-date and accurate minimum point prevalence of genetically defined skeletal muscle channelopathies which is important for understanding the population impact, planning for treatment needs and future clinical trials. Skeletal muscle channelopathies include myotonia congenita (MC), sodium channel myotonia (SCM), paramyotonia congenita (PMC), hyperkalemic periodic paralysis (hyperPP), hypokalemic periodic paralysis (hypoPP) and Andersen- Tawil Syndrome (ATS). Patients referred to the UK national referral centre for skeletal muscle channelopathies and living in UK were included to calculate the minimum point prevalence using the latest data from the Office for National Statistics population estimate. We calculated a minimum point prevalence of all skeletal muscle channelopathies of 1.99/100 000 (95% CI 1.981-1.999). The minimum point prevalence of MC due to CLCN1 variants is 1.13/100 000 (95% CI 1.123-1.137), SCN4A variants which encode for PMC and SCM is 0.35/100 000 (95% CI 0.346 - 0.354) and for periodic paralysis (HyperPP and HypoPP) 0.41/100 000 (95% CI 0.406-0.414). The minimum point prevalence for ATS is 0.1/100 000 (95% CI 0.098-0.102). There has been an overall increase in point prevalence in skeletal muscle channelopathies compared to previous reports, with the biggest increase found to be in MC. This can be attributed to next generation sequencing and advances in clinical, electrophysiological and genetic characterisation of skeletal muscle channelopathies.

Blockers of Skeletal Muscle Nav1.4 Channels: From Therapy of Myotonic Syndrome to Molecular Determinants of Pharmacological Action and Back

The voltage-gated sodium channels represent an important target for drug discovery since a large number of physiological processes are regulated by these channels. In several excitability disorders, including epilepsy, cardiac arrhythmias, chronic pain, and non-dystrophic myotonia, blockers of voltage-gated sodium channels are clinically used. Myotonia is a skeletal muscle condition characterized by the over-excitability of the sarcolemma, resulting in delayed relaxation after contraction and muscle stiffness. The therapeutic management of this disorder relies on mexiletine and other sodium channel blockers, which are not selective for the Na_v1.4 skeletal muscle sodium channel isoform. Hence, the importance of deepening the knowledge of molecular requirements for developing more potent and use-dependent drugs acting on Na_v1.4. Here, we review the available treatment options for non-dystrophic myotonia and the structure-activity relationship studies performed in our laboratory with a focus on new compounds with potential antimyotonic activity.

Muscle channelopathies

Muscle channelopathies encompass a wide range of mainly episodic conditions that are characterized by muscle stiffness and weakness. The myotonic conditions, characterized predominantly by stiffness, include myotonia congenita, paramyotonia congenita, and sodium channel myotonia. The periodic paralysis conditions include hypokalemic periodic paralysis, hyperkalemic periodic paralysis, and Andersen-Tawil syndrome. Clinical history is key, and diagnosis is confirmed by next-generation genetic sequencing of a panel of known genes but can also be supplemented by neurophysiology studies and MRI. As genetic testing expands, so have the spectrum of phenotypes seen including pediatric presentations and congenital myopathies. Management of these conditions requires a multidisciplinary approach with extra support needed when patients require anesthetics or when pregnant. Patients with Andersen-Tawil syndrome will also need cardiac input. Diagnosis is important as symptomatic treatment is available for all of these conditions but need to be tailored to the gene and variant of the patient.

European Neuromuscular Centre consensus statement on anaesthesia in patients with neuromuscular disorders

BACKGROUND AND PURPOSE

Patients with neuromuscular conditions are at increased risk of suffering perioperative complications related to anaesthesia. There is currently little specific anaesthetic guidance concerning these patients. Here, we present the European Neuromuscular Centre (ENMC) consensus statement on anaesthesia in patients with neuromuscular disorders as formulated during the 259th ENMC Workshop on Anaesthesia in Neuromuscular Disorders.

METHODS

International experts in the field of (paediatric) anaesthesia, neurology, and genetics were invited to participate in the ENMC workshop. A literature search was conducted in PubMed and Embase, the main findings of which were disseminated to the participants and presented during the workshop. Depending on specific expertise, participants presented the existing evidence and their expert opinion concerning anaesthetic management in six specific groups of myopathies and neuromuscular junction disorders. The consensus statement was prepared according to the AGREE II (Appraisal of Guidelines for Research & Evaluation) reporting checklist. The level of evidence has been adapted according to the SIGN (Scottish Intercollegiate Guidelines Network) grading system. The final consensus statement was subjected to a modified Delphi process.

RESULTS

A set of general recommendations valid for the anaesthetic management of patients with neuromuscular disorders in general have been formulated. Specific recommendations were formulated for (i) neuromuscular junction disorders, (ii) muscle channelopathies (nondystrophic myotonia and periodic paralysis), (iii) myotonic dystrophy (types 1 and 2), (iv) muscular dystrophies, (v) congenital myopathies and congenital dystrophies, and (vi) mitochondrial and metabolic myopathies.

CONCLUSIONS

This ENMC consensus statement summarizes the most important considerations for planning and performing anaesthesia in patients with neuromuscular disorders.

A dangerous food binge: a case report of hypokalemic periodic paralysis and review of current literature

BACKGROUND

Hypokalemic periodic paralysis is a rare neuromuscular genetic disorder due to defect of ion channels and subsequent function impairment. It belongs to a periodic paralyses group including hyperkalemic periodic paralysis (HEKPP), hypokalemic periodic paralysis (HOKPP) and Andersen-Tawil syndrome (ATS). Clinical presentations are mostly characterized by episodes of flaccid generalized weakness with transient hypo- or hyperkalemia.

CASE PRESENTATION

A teenage boy presented to Emergency Department (ED) for acute weakness and no story of neurological disease, during the anamnestic interview he revealed that he had a carbohydrates-rich meal the previous evening. Through a focused diagnostic work-up the most frequent and dangerous causes of paralysis were excluded, but low serum potassium concentration and positive family history for periodic paralyses raised the diagnostic suspicion of HOKPP. After the acute management in ED, he was admitted to Pediatric Department where a potassium integration was started and the patient was counselled about avoiding daily life triggers. He was discharged in few days. Unfortunately, he presented again because of a new paralytic attack due to a sugar-rich food binge the previous evening. Again, he was admitted and treated by potassium integration. This time he was strongly made aware of the risks he may face in case of poor adherence to therapy or behavioral rules. Currently, after 15 months, the boy is fine and no new flare-ups are reported.

CONCLUSION

HOKPP is a rare disease but symptoms can have a remarkable impact on patients' quality of life and can interfere with employment and educational opportunities. The treatment aims to minimize the paralysis attacks by restoring normal potassium level in order to reduce muscle excitability but it seems clear that a strong education of the patient about identification and avoidance triggering factors is essential to guarantee a benign clinical course. In our work we discuss the typical clinical presentation of these patients focusing on the key points of the diagnosis and on the challenges of therapeutic management especially in adolescence. A brief discussion of the most recent knowledge regarding this clinical condition follows.