To define a new type of dominant myotonic muscle disorder and to identify the gene lesion.
Case series, clinical examination and electromyography, measurements of grip force and relaxation time, and DNA analysis to probe for mutation in the gene for the skeletal muscle sodium channel.
Outpatient clinic and home.
Three families studied; all together, 17 affected and nine unaffected individuals.
The findings in these three families confirm the existence of myotonia fluctuans as we described it previously in another family. Myotonia (prolongation of relaxation time) developed 20 to 40 minutes after exercise. Potassium caused generalized myotonia. Cooling had no major effect on muscle function. Three families had a common mutation in exon 22 and one family had a mutation in exon 14 of the gene for the sodium channel α subunit.
Myotonia fluctuans is a disorder of the muscle sodium channel. There are at present two other distinct clinical muscle disorders associated with mutations in the sodium channel: hyperkalemic periodic paralysis and paramyotonia congenita. The findings in the present report indicate that myotonia fluctuans belongs to a third type of sodium channel disorder. Further work is needed to understand the complex genotype-phenotype correlations in sodium channel disorders.