Introduction The non-dystrophic myotonias (NDM) and periodic paralyses (PP) are a group of skeletal muscle disorders caused by mutations in genes encoding ion channels. To date, very few studies have systematically evaluated the prevalence of these disorders, never in England, and most of them predate genetic diagnosis.

Objective To obtain prevalence data on skeletal muscle channelopathies and to evaluate the relative frequency of common mutations.

Methods The study covered all patients with NDM or PP living in the UK that were referred to the UK national reference centre for assessment. Inclusion criteria were clinical and electrophysiological features of NDM or PP, and confirmed mutations in genes encoding ion channels (96% of cases). England was selected as the geographical area for prevalence analysis.

Results From a total of 582 patients identified, 286 had myotonia congenita, 70 paramyotonia congenita, 23 sodium-channel myotonias, 97 hypokalemic PP, 66 hyperkalemic PP, four normokalemic PP, and 36 Andersen-Tawil syndrome (ATS). 530 patients were from England, giving a point prevalence of 1/100 000. Significant allelic heterogeneity was associated with NDM and ATS. However, a limited number of mutations were responsible for most cases.

Conclusion We have analysed the largest series of patients with skeletal muscle channelopathies reported so far, and documented for the first time their overall prevalence. The spectrum of mutations was similar to that previously reported.