Coexistence of sodium and chloride channelopathies: A complex phenotype with features of myotonia congenita and paramyotonia congenita

Non-dystrophic myotonias (NDM) encompass chloride and sodium channelopathy disorders. This case study presents a 34-year-old female patient with a complex phenotype involving features of both myotonia congenita (MC) and paramyotonia congenita (PMC). Initially presenting with asthenia, weakness, and limb stiffness in her second decade of life, her symptoms gradually worsened over time. She experienced frequent retrosternal compression, tachycardia, stiffness, and symmetrical pain in her lower limbs. Clinical examination and electromyography confirmed severe lid lag myotonia, hypertrophy in all four limbs, and myotonic discharges. Exposure to cold triggered her symptoms, significantly impacting her daily life. Initial treatment with mexiletine was switched to acetazolamide due to gastrointestinal side effects. Genetic analysis revealed a new variant, p.Glu1607del, in SCN4A and homozygosity for p.Met485Val in CLCN1. This case underscores the importance of accurate diagnosis and appropriate therapy guided by an expert neurologist. Keywords: myotonia, paramyotonia, channelopathies, CLCN1, SCN4A