Identifies a Val1589Met mutation in the skeletal muscle sodium channel gene associated with a mild phenotype of potassium-aggravated myotonia.
Extends SCN4A spectrum in myotonia; hypothesis-generating and should not yet change practice.
Clinical, electrophysiological, and molecular genetic features were investigated in two patients from a family a with dominantly inherited myotonic disease, characterised by painful cramps, stiffness without weakness, fluctuation of symptoms, and cold sensitivity. A reduction in amplitude of the compound muscle action potential was demonstrated on cooling and administration of potassium, although no clinical exacerbation was seen. A heterozygote mutation Val1589Met was identified in the alpha-subunit of the skeletal muscle sodium channel gene in both patients, consistent with the diagnosis of potassium-aggravated myotonia. The phenotype in this family is much milder than that previously described in another family with a mutation at this site.
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Orrell et al. (1998) studied this question.
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