Massive left ventricular hypertrophy (LVH) is considered a high-risk feature in pediatric hypertrophic cardiomyopathy (HCM), but its natural history has remained less clearly defined. In an analysis published in Circulation, data from the Sarcomeric Human Cardiomyopathy Registry (SHaRe) and International Paediatric Hypertrophic Cardiomyopathy Consortium (IPHCC) were used to compare pediatric-onset HCM patients with or without massive LVH. 

Massive LVH was defined as maximal left ventricular wall thickness (MLVWT) ≥30 mm or z score ≥+20 before age 18 years. Among 587 patients, 186 had massive LVH. Those with massive LVH were diagnosed younger than those without it (median 9.2 vs 13.6 years; P<0.001) and more often had sarcomeric genetic variants (72% vs 61%; P=0.034).

Massive LVH was associated with higher HCM-related mortality (hazard ratio [HR] 3.3; 95% confidence interval [CI] 1.2-9.7; P=0.026), major adverse cardiac events (HR 2.6; 95% CI 1.7-3.9; P<0.001), major ventricular arrhythmia (HR 3.1; 95% CI 1.8-5.2; P<0.001), and heart failure events (HR 1.9; 95% CI 1.1-3.1; P=0.013). These findings remained significant after age and sex adjustment.

Among 115 patients with serial imaging, median MLVWT increased from 26 mm to 31 mm (P<0.001), while 22% had a later measurement more than 5 mm below the peak value.

These findings suggest that massive LVH identifies a higher-risk pediatric HCM phenotype, although wall-thickness regression may occur in some patients.