Frequency of Hypoglycemia in Children With Beta-blocker Treated Long QT-syndrome

The goal of this observational study is to investigate the frequency and characteristics of hypoglycemic episodes in children with long QT syndrome (LQTS) and beta-blocker treatment, specifically those receiving propranolol, compared to healthy controls. The study focuses on children under seven years with genetically confirmed LQTS and beta-blocker therapy, as well as healthy siblings as controls. The main questions it aims to answer are: * Does hypoglycemic episodes occur in children with LQTS who are treated with propranolol? * Are there asymptomatic episodes of low blood glucose that could affect well-being and neurological development? * Are there individual differences in susceptibility to hypoglycemia, and do genetic factors explain these differences? * What is the incidence of hypoglycemia leading to emergency visits or hospitalizations among LQTS patients compared to healthy controls? Researchers will compare children with LQTS on propranolol to healthy siblings to see if the frequency and severity of hypoglycemia differs between groups. Participants will: Have a Dexcom G7 continuous glucose monitor installed to record glucose profiles for ten days, both during an healthy episode and during an infection (when risk for hypoglycemia is increased). If differences in susceptibility to hypoglycemia are observed, genetic data from the Helsinki biopank will be analyzed to explore potential genetic factors underlying these differences. Visits to emergency clinic and hospitalizations due to hypoglycemia will be retrospectively reviewed for all LQTS patients under 16 years old and compared to healthy controls. Additional Details: The study is conducted at Helsinki University Hospital (HUH) Children and Adolescents and the New Children's Hospital clinical research unit. The study aims to recruit 20-40 LQTS patients and 10 healthy controls. The study is ethically justified, with no extra laboratory tests or costs for participants, and written consent will be o