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We describe a large Brazilian consanguineous kindred with 3 clinically affected patients with a Thomsen myotonia phenotype. They carry a novel homozygous nonsense mutation in the CLCN1 gene (K248X). None of the 6 heterozygote carriers show any sign of myotonia on clinical evaluation or electromyography. These findings confirm the autosomal recessive inheritance of the novel mutation in this family, as well as the occurrence of phenotypic variability in the autosomal recessive forms of myotonia., (Copyright © 2011 Wiley Periodicals, Inc.)

Myotonia permanens is associated with a G1306E mutation in the SCN4A gene. Two sporadic patients have been reported, but the clinical phenotype has not been fully characterized. The authors report a family in which the disease is autosomal dominantly inherited. The patients have severe myotonia, but the clinical picture is not qualitatively different from that seen in other nondystrophic myotonias.

Background: Over the past 3 years, the genetics of the myotonic diseases have been substantially elaborated. Three genetically different groups of myotonic disease can be discerned: (1) the chloride channel myotonias, (2) the adynamia-paramyotonia complex, and (3) myotonic dystrophy., Methods and Results: Electrophysiology has suggested and molecular biology has proven that the diseases belonging to the adynamia-paramyotonia complex, ie, paramyotonia congenita, hyperkalemic and normokalemic periodic paralysis, and some rare forms of myotonic disease, are caused by point mutations in the gene encoding the alpha subunit of the sodium channel in adult human skeletal muscle, located on chromosome 17q23. Thirteen different mutations have been described by various groups in the United States and Germany. The various mutations causing a particular form of the complex are not located in the gene in a predictable or easily understandable regular manner., Conclusions: Further study of the genotype-phenotype correlations should not only increase our understanding of the variability of signs in this group of diseases, it could also provide us with a deeper insight in the function of the various regions of the sodium channel protein.

Available experimental and clinical evidence strongly suggests that deficiency in type 2B muscle fibres in generalized myotonia and paramyotonia congenita will develop consequent to lasting myotonic activity as its adequate stimulus. Type 2B fibre deficiency need not be regarded as the product of a hypothetical genetic factor.