A 6-Year-Old Saudi Boy with Myotonia Congenita

Abstract

Background: myotonia congenita is an inherited form of myotonia that is due to mutations in the skeletal muscle chloride channel CLCN1. These mutations lead to reduced sarcolemmal chloride conductance, causing delayed muscle relaxation that is evident by clinical exam and myotonic discharges on electromyogram. Two forms of myotonia congenita are recognized: autosomal recessive (Becker variant) or autosomal dominant (Thomsen variant). The recessive form tends to be more severe and has an earlier onset than the dominant one. The dominant form varies in severity from asymptomatic to moderately severe. These two forms may be distinguished by clinical presentation, inheritance pattern and age of onset. Methods: we described the clinical presentation and genetic testing results of one individual with the autosomal recessive variant of myotonia congenita. Results: the initial diagnosis was made based on the clinical presentation then it was confirmed based on electromyographic findings of myotonic discharges and CLCN1 gene sequencing which revealed homozygous disease-associated C ->A transverse mutation. The patient achieved a modest response to treatment with phenytoin, carbamazepine or acetazolamide. His condition remained stable with minimal weakness and muscle hypertrophy. Conclusions: myotonia congenita is a rare genetic disorder of muscle relaxation. The diagnosis is made based on clinical features and is confirmed by sequencing CLCN1 gene. Response to treatment is variable. Recommended medications included mexiletine, phenytoin, carbamazepine and acetazolamide among others.