Incidence, Genetic Characteristics, and Outcomes of Paediatric Cardiomyopathy in a National Cohort (2003-2025).

Paediatric cardiomyopathy is a rare, serious inherited cardiac condition and a leading indication for paediatric heart transplantation. Population-based data describing paediatric cardiomyopathy within a single healthcare system complements multinational registry studies by providing important local epidemiological context. To describe the incidence, temporal trends, phenotypic characteristics, genetic findings, and clinical outcomes of paediatric cardiomyopathy in Northern Ireland over a 22-year period. A national retrospective cohort study of children diagnosed with cardiomyopathy in Northern Ireland between 2003 and 2025 was conducted. Population-based incidence rates were calculated for children aged 0-15 years affected by Dilated and Hypertrophic Cardiomyopathy using annual population denominators (2003-2023), with exact Poisson 95% confidence intervals. Temporal trends were assessed using quasi-Poisson and negative binomial regression models with population offsets. Clinical characteristics, genetics and outcomes (implantable cardioverter-defibrillator implantation, transplantation, and death) were analysed descriptively. Kaplan-Meier survival analysis and Cox regression were performed using a composite outcome of death or transplantation. Eighty-four children were diagnosed during the study period, corresponding to a mean annual incidence of 0.9 per 100,000 children. Annual incidence varied from 0.26 to 3.11 per 100,000. There was no statistically significant temporal increase in incidence (p = 0.058), although higher incidence values were observed in more recent years. HCM (n = 38, 45%) and DCM (n = 24, 29%) were the most common subtypes. Overall, 61% of patients had a pathogenic or likely pathogenic genetic variant or phenocopy condition identified. There were seven deaths, six cardiac transplants, and eighteen implantable cardioverter-defibrillator insertions. Children diagnosed before one year of age accounted for 65% of deaths or transplants. Compared with HCM, DCM was associated with a significantly increased risk of death or transplantation (hazard ratio 6.25, 95% CI 1.30-33.33, p = 0.0094). Paediatric cardiomyopathy remains rare in Northern Ireland with a similar incidence to previous international reports but is associated with substantial morbidity and mortality, particularly among infants and those with DCM. Although higher annual estimates were observed in more recent years, no statistically significant temporal increase was identified. Ongoing population-based surveillance and integration with international registries are essential to improve understanding and outcomes in these rare conditions.