An epidemiological and genetic investigation of myotonia congenita was carried out in northern Finland. Altogether 58 patients were identified (of whom 54 lived in the study area) in 23 families, with a prevalence of 7.3 per 100000. The majority of the families originated from a sparsely populated area in western Lapland. The mean age at onset of the disease was 11 years with a range of 2 to 45 years. The mean time that had passed before verification of the clinical disease was 18 (SD 14) years. The sex ratio M/F was 2.2/1.0. Forty-seven cases were familial and 11 were sporadic. In six families/pedigrees the inheritance was compatible with autosomal recessive and in two families with autosomal dominant inheritance. In five additional families, in which autosomal recessive inheritance seemed most plausible, vertical transmission was also noticed. This could be explained either by consanguinity of the parents or by variant expression of the mutation(s) involved. Our results suggest that myotonia congenita is unusually frequent in northern Finland, most probably as a consequence of an enrichment of the gene mutation(s) in the population.
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