Extreme Left Ventricular Hypertrophy in Pediatric Hypertrophic Obstructive Cardiomyopathy.

Hypertrophic cardiomyopathy (HCM) in childhood is characterized by marked phenotypic heterogeneity and poses significant challenges in risk stratification for sudden cardiac death (SCD). We report the case of a 9.5-year-old girl in whom transthoracic echocardiography revealed progressive extreme interventricular septal hypertrophy (up to 33 mm; Z-score > +11), marked left atrial enlargement, reduced left ventricular cavity size, and significant left ventricular outflow tract obstruction. Although repeated ambulatory Holter monitoring did not reveal ventricular arrhythmias, the patient exhibited one episode of unexplained syncope at rest. These findings placed her in a high-risk category according to contemporary pediatric risk stratification approaches. Following insufficient response to medical therapy, surgical septal myectomy was performed. Given the persistent high-risk profile, an implantable cardioverter-defibrillator was implanted for primary prevention of sudden cardiac death. This case emphasizes the importance of individualized, multimodal risk stratification in guiding management decisions in pediatric HCM, even in the absence of documented malignant arrhythmias.