Onset of myotonia congenital (Thomsen and Becker disease) is early in childhood (during the first or second decade of life), but usually earlier in Thomsen disease (Nagamitsu et al.
The majority of these mutations are found in the recessive form (Becker disease), whereas only a few have been described with the pure dominant form (Thomsen disease) (Pusch 2002, Colding-Jorgensen 2005).
Here we report clinical and molecular data from a family carrying a new mutation in the CLCN1 gene causing Becker disease and discuss the possible implications of the mutations and the function-structure-phenotype relationships in the CLCN1 channel.
According to the clinical results obtained in this study, we concluded that the clinical picture of this family is compatible with myotonia congenita, and its autosomal recessive inheritance pattern suggested the diagnosis of Becker disease. This is the first clinical report in Costa Rica of a family affected with Becker disease, but the second regarding a non-dystrophic myotonic condition (Morales et al.
(1996), reported that clinically unaffected heterozygous males, but no females, presented latent myotonia in six families affected with Becker disease, however, our clinically unaffected heterozygous males (I.2 and II.4, Fig.
The family we report here seems to represent a typical example of Becker disease: non-progressive, non-disabling and probably not as severe as other families affected with this disease.
It is inherited as either autosomal dominant or recessive known as Thomsen and Becker diseases, respectively.