Myotonia congenita is non-dystrophic myotonia caused by mutations of CLCN1 gene, which encodes human skeletal muscle chloride channel 1. Col12A1 gene encodes protein expressed in extracellular matrix of skeletal muscles and mutation gives rise to distal muscular dystrophy (Ullrich or Bethlem). These mutations can be inherited either in autosomal recessive or dominant forms. Myotonia congenita is characterized by muscle stiffness since childhood; all striated muscle groups including extraocular and bulbar muscles may be involved. I would like to present a case of clinical myotonia with negative family history but dual mutations.

A 13-years-old boy with reduced intra-natal fetal movements and progressive difficulty in standing from sitting position since the age of almost 1 year but achieved milestones at normal age.  There is no family history of muscle diseases but his parents are consanguineous. His examination showed normal cognition, generalized muscle hypertrophy with clinical myotonia and minimal distal appendicular but no axial muscular weakness. His needle electromyogram showed myotonic discharges but no myopathic units, normal creatinine kinase (159) and aldolase (13.7) levels. DNA sequence analysis showed homozygous mutation for CLCN1 and Col12A1, his parental testing revealed heterozygous status for both parents for similar mutations. None of other kids in family are involved clinically (3 boys and one girl).

My patient has homozygous CLCN1 and Col12A1 mutations with negative family history which point towards autosomal recessive inheritance. Dual mutations have been noted in increasing number of patients as genetic testing is performed more frequently in present era. This particular case stresses the importance of comprehensive history and examination as genetic testing only can be deceptive. Genetic testing should be done to confirm clinical diagnosis. As our knowledge of different genetic mutations with clinical manifestations (phenotype-genotype matching) is increasing so is the complexity of interpretation with multiple mutations in individual patient.