Open Label Study in Adolescents and Children With Myotonic Disorders

"Patients will be enrolled sequentially into 2 cohorts. Cohort 1 - (patients aged 12 to \< 18 years): approximately 8 weeks - 4 weeks of dose titration period + 4 weeks of maintenance period.~Cohort 2 - (patients aged 6 to \< 12 years,): approximately 8 weeks - 4 weeks of dose titration period + 4 weeks of maintenance period.~Enrolment for Cohort 2 will begin after initial safety assessment of patients in Cohort 1 by the DSMB and no safety concerns are observed. The dose level for cohort 2 will be confirmed by PK modelling study."