CLCN1 mutations could lead to atypical myotonic symptoms and abnormities on electromyography

Inactivation of the skeletal muscle chloride channel ClC-1 results in myotonia congenita (MC), which occur with mutations of CLCN1 gene. However, there is no clear correlation between genotype and phenotype. Clinical data of a patient and his parents with MC were collected retrospectively, including the symptoms and signs, results of blood tests, electromyography, MRI images, and examination results of biceps brachii pathology by histopathology. The patient was diagnosed according to next-generation sequencing. Sanger sequencing was then carried out on his parents' blood samples to verify their mutations. The patient had typical clinical characteristics of Becker myotonia with compound heterozygous mutations of the CLCN1 gene, inherited from his mother (M560T), who showed only mild symptoms and cold induced myotonic motor unit potentials, and from an unaffected father (c.697-2delA on the intron 5 resulting in exon 6 skipping). In view of the compound heterozygous mutations, he could be classified into Becker myotonia congenita. In conclusion, these results suggested that CLCN1 mutations could lead to atypical myotonic symptoms and abnormities on electromyography (EMG). EMG after muscle cooling test and exercise tests should be completed in the relatives of patients with MC and some patients with atypical syndrome.