Objectives: To obtain minimum point prevalence rates for the skeletal muscle channelopathies and to evaluate the frequency distribution of mutations associated with these disorders.Methods: Analysis of demographic, clinical, electrophysiologic, and genetic data of all patients assessed at our national specialist channelopathy service. Only patients living in the United Kingdom with a genetically defined diagnosis of nondystrophic myotonia or periodic paralysis were eligible for the study. Prevalence rates were estimated for England, December 2011.Results: A total of 665 patients fulfilled the inclusion criteria, of which 593 were living in England, giving a minimum point prevalence of 1.12/100,000 (95% confidence interval [CI] 1.03-1.21). Disease-specific prevalence figures were as follows: myotonia congenita 0.52/100,000 (95% CI 0.46-0.59), paramyotonia congenita 0.17/100,000 (95% CI 0.13-0.20), sodium channel myotonias 0.06/100,000 (95% CI 0.04-0.08), hyperkalemic periodic paralysis 0.17/100,000 (95% CI 0.13-0.20), hypokalemic periodic paralysis 0.13/100,000 (95% CI 0.10-0.17), and Andersen-Tawil syndrome (ATS) 0.08/100,000 (95% CI 0.05-0.10). In the whole sample (665 patients), 15 out of 104 different CLCN1 mutations accounted for 60% of all patients with myotonia congenita, 11 out of 22 SCN4A mutations for 86% of paramyotonia congenita/sodium channel myotonia pedigrees, and 3 out of 17 KCNJ2 mutations for 42% of ATS pedigrees. Conclusion:We describe for the first time the overall prevalence of genetically defined skeletal muscle channelopathies in England. Despite the large variety of mutations observed in patients with nondystrophic myotonia and ATS, a limited number accounted for a large proportion of cases. Neurology â 2013;80:1472-1475 GLOSSARY ATS 5 Andersen-Tawil syndrome; CACNA1S 5 calcium channel, voltage-dependent, L type, a 1S subunit gene; CI 5 confidence interval; CLCN1 5 chloride channel, voltage-sensitive 1 gene; HyperPP 5 hyperkalemic periodic paralysis; HypoPP 5 hypokalemic periodic paralysis; KCNJ2 5 potassium inwardly rectifying channel, subfamily J, member 2 gene; MC 5 myotonia congenita; NDM 5 nondystrophic myotonias; PMC 5 paramyotonia congenita; PP 5 periodic paralyses; SCM 5 sodium channel myotonias; SCN4A 5 sodium channel, voltage-gated, type 4, a subunit gene.Nondystrophic myotonias (NDM) and periodic paralyses (PP) comprise a heterogeneous group of skeletal muscle disorders caused by mutations in genes encoding ion channels.e1 NDM include myotonia congenita (MC), paramyotonia congenita (PMC), and the sodium channel myotonias (SCM). PP encompass hypokalemic periodic paralysis (HypoPP), hyperkalemic periodic paralysis (HyperPP), and Andersen-Tawil syndrome (ATS). These are autosomal dominant disorders except for MC, which is inherited in either a dominant or recessive manner. e1 Although significant progress in the clinical, electrophysiologic, and genetic characterization of the skeletal muscle channelopathies has taken place in the past 2 decades, their actual prevalence rem...

Recent discoveries in the skeletal muscle channelopathies have increased our understanding of the genetics and pathophysiology of these diseases. Studies reporting imaging techniques raise the possibility of improved disease monitoring and better outcome measures for clinical trials. Randomized controlled trials to establish an evidence base upon which to recommend standard treatments are required.