Autosomal Recessive Becker's Form of Myotonia Congenita in Indian Families.

Myotonia congenita (MC), a rare inherited disease, is caused by variations in the skeletal muscle chloride voltage-gated channel one gene (CLCN1) and is characterized by symptoms of myotonia and muscle hypertrophy. We present a case report of two female patients aged nine and 10, from Andhra Pradesh, India, with a history of parental consanguinity, hypertrophy of arm and calf muscles, permanent weakness, and proximal muscle weakness. Patients were diagnosed with Becker's form of MC after genetic testing that reported the mutation c.1667T >A (p.lle556Asn) in exon 15 of the CLCN1, which is a pathogenic variant. Treatment with mexiletine showed improvement in the condition of patients. Because of its inherent nature, parents were given genetic counseling and the choice of antenatal diagnosis for upcoming pregnancies.