Pathogenic SCN5A Mutation and Thyrotoxicosis-Related Neurological Syndrome: Casual or Causal Relationship?

Background: Various neurologic complications of hyperthyroidism are reported, and most of these complications are reversible with the amelioration of thyrotoxicosis. We report a previously undescribed concurrence of hyperthyroid-associated exercise-induced myalgia and stiffness, pyramidal tract dysfunction, and myoclonic movements that make an initial clinical diagnosis difficult. Case presentation: A 17-year-old male was hospitalized in the department of neurology, presenting with a 4-year history of severe exercise-induced myalgia and stiffness, weakness of lower limbs, and myoclonic movements. Laboratory investigations unexpectedly revealed hyperthyroidism. MRI of the brain and spine, electrophysiology, and whole exome sequencing were also performed. Antithyroid therapy led to marked improvement of neurologic symptoms, accompanied by a significant improvement of the time-dependent decline in compound muscle action potentials (CMAP) amplitudes after exercise and normalization of the prolonged QTc interval. Genetic analysis identified a rare variant in SCN5A. Conclusion: This case report provides important insights into the relationship between hyperthyroidism and neurologic/cardiac complications, particularly in those with a genetic predisposition. SCN5A mutation possibly plays a role in the complex neurological syndrome associated with hyperthyroidism. Further studies are warranted to better understand the underlying mechanisms and potential therapeutic options for these complex conditions.