Introduction and objectives: Early-onset nonsyndromic cardiomyopathies (eoNSCM) are rare in pediatrics and are often associated with cardiac events (CE). The prognostic impact of multiple genetic variants in eoNSCM remains incompletely characterized. This study aimed to describe clinical outcomes in a pediatric cohort with eoNSCM and to analyze their association with genetic burden.

Methods: Single-center retrospective study analyzing clinical and genetic data from patients with eoNSCM. The primary outcome was the occurrence of CE: ventricular tachycardia or fibrillation, appropriate implantable cardioverter-defibrillator shocks, cardiac arrest or sudden cardiac death, heart failure stages C and D, or heart transplantation.

Results: From 2014 to 2024, 202 pediatric patients were evaluated for cardiomyopathy, of whom 84 were diagnosed with eoNSCM (36.9% female; median age at onset, 13 [interquartile range, 2.5-15] years. Diagnoses included 53 hypertrophic cardiomyopathies (63%), 21 dilated cardiomyopathies (25%), and 10 arrhythmogenic cardiomyopathies (12%). A conclusive genetic diagnosis was obtained in 52 of 84 (62%). Twenty-four patients (29%) experienced a significant CE, of whom 19 of 24 (79%) carried multiple genetic variants. Children with multiple variants exhibited a significantly higher risk of CE compared with carriers of a single causal variant (HR, 7.27; 95%CI, 2.47-21.44; P < .001). The highest risk was observed in patients carrying 2 pathogenic or likely pathogenic variants (P < .001). In dilated cardiomyopathy, adding baseline phenotype (left ventricular dilation and systolic dysfunction) to a genetics-only model improved discrimination for CE (integrated discrimination improvement, 0.528; 95%CI, 0.312-0.683; P < .001).

Conclusions: Pediatric eoNSCM shows a high prevalence of rare variants. In this cohort, a higher genetic burden was associated with an increased risk of CE.