Novel CLCN1 mutation in a young male with Becker Myotonia and unique clinical phenotype

Aims: In the late 50s, Becker (1957) discovered an autosomal recessive mode of inheritance in his myotonia congenita patients with a more severe phenotype than in autosomal dominant Thomson Myotonia and henceforth called it Myotonia congenita Becker. To date, approximately 130 different CLCN1 gene mutations have been identified and genotype-phenotype relationships have been vividly discussed.

We report on a young male of German-Indian descent with Becker Myotonia and an untypical clinical phenotype who carries a novel CLCN1– mutation.

Methods: We performed detailled clinical examinations, mutational analysis and electrophysiological examinations of the patient and the patients parents. Electromyogram (EMG) was done before therapeutic intervention.

Results: The 16 year old patient proved to be heterozygous carrier of a novel c.53_65delGTGACCCCCAGTA; p.(Ser18Thrs*55) mutation within the CLCN1 gene.

The patient presented himself with a unique phenotype with adipose body stature and a BMI of 31 without typical generalized muscle hypertrophy.

Close examination though revealed disabling myotonic symptoms, muscle stiffness and warm-up phenomenon.

Initial medical treatment with a GABA-analogue (Gabapentin) was not successful. Carbamazepine (sodium channel blocker) has been started with promising initial results.

Conclusion: Myotonia Becker patients typically present with a triad of an athletic body stature, muscle stiffness and Warm-up phenomenon. EMG examination usually reveals pathognomonic myotonic discharges.

The diagnosis of Becker Myotonia in pediatric patients may be suspected by the presence of the symptomatic triad even before a patient is sent to painful EMG-testing.

However, in some patients the clinical phenotype may be divergent thereby misleading the diagnosing physician. In these patients, EMG may easily prove diagnosis.

However, these phenotypical differences possibly imply that the prevalence of Becker Myotonia might be higher than actually thought.

Becker Myotonia - CLCN1 gene - novel mutation - genotype-phenotype diversity - EMG