Sudden cardiac death (SCD) is the most common cause of death in childhood hypertrophic cardiomyopathy (HCM). Recently, two risk scores have been developed to estimate the 5-year risk of SCD.

We aimed to assess their respective performances in an independent cohort of primary prevention children with HCM.

All patients with HCM<18-year-old from a single-center were retrospectively included between 2003 and 2023. Secondary and syndromic causes of HCM were excluded as well as children with inaugural sustained ventricular arrythmias. HCM Risk-Kids and PRIMaCY risk scores were calculated at diagnosis and during follow-up. The primary composite outcome included sustained ventricular arrhythmia, appropriate ICD therapy, aborted cardiac arrest, or SCD.

Hundred primary prevention children were included (mean age 7.1±5.6years, 59.0% males), with a mean follow-up of 8.6±5.5years.13 (13.0%) patients experienced the primary composite outcome. When only considering events during the 5 first years, Harrel's C index was 0.52 (95% CI: 0.27–0.77) for HCM Risk-Kids (≥6%) and 0.70 (95% CI: 0.59–0.80) for PRIMaCY (>8.3%), with 1 patient potentially treated by ICD for every 25 ICDs implanted for HCM Risk Kids and 1 for every 14 ICDs implanted for PRIMaCY. When risk scores were repeated and all primary outcomes during follow-up considered, all events except one (93.2%) were correctly identified using both risk scores, with 1 patient potentially treated by ICD for every 5.6 ICDs implanted for HCM Risk Kids and 1 for every 5.3 ICDs implanted for PRIMaCY. Among 44 (44.0%) patients implanted with an ICD, all primary prevention patients who had≥one appropriate ICD therapy during follow-up had HCM Risk-Kids≥6% and PRIMaCY>8.3% at implantation.

Our findings suggest imperfect discrimination between low and high-risk HCM patients using these two risk scores. The performance or risk scores was substantially improved by periodic re-assessment during follow-up.