CONGRESS ABSTRACT

Background: Isolated left posterior fascicular block (LPFB) is rare in youth and may signal underlying LV fibrosis and cardiomyopathy. We report a case of sudden cardiac arrest (SCA) followed by death (SCD) in a previously healthy adolescent with known LPFB and autopsy-proven hypertrophic cardiomyopathy (HCM). Case: A previously healthy 16-year-old male, only child, with known left posterior fascicular block (LPFB) since age 7 described at a pre sport participation resting ECG, presented with out-of-hospital cardiac arrest (OHCA) while on a chair lift during a ski trip with family. After 8–10 minutes of no-flow, cardiopulmonary resuscitation was started and an automatic external defibrillator delivered two shocks for very low-voltage ventricular fibrillation. On arrival of advanced life support, the patient was asystolic. Intravenous adrenaline and endotracheal intubation were performed, with return of spontaneous circulation (ROSC) after 10 minutes. First 12-lead ECG (at 60 minutes from ROSC) showed sinus rhythm at 120 bpm, LPFB, diffuse ST-segment depression with transient ST elevation in aVR (subsequently normalized), and a QTc of 490 ms. Transthoracic echocardiography showed mild left ventricular (LV) hypertrophy (13 mm) and reduced systolic function (LVEF 43%) due to diffuse hypokinesia. Posterior mitral valve leaflet hypoplasia with mild, asymmetrical mitral regurgitation and elevated LV filling pressures were noted. LVEF nearly normalized within 48 hours. Admission CT angiography showed no lesions; cardiac MRI could not be performed. The patient died two days later. Differential diagnosis included acute myocarditis, supported by a recent febrile episode, channelopathies, which were deemed unlikely given the absence of diagnostic or suspicious ECG features on a 24-hour, 12 leads, Holter ECG performed one day post-OHCA, cardiomyopathies. Genetic testing revealed a heterozygous variant of uncertain significance (p.Arg56Lys) in MYOZ2 gene, codifying for myozenin 2, a calcineurin-binding sarcomeric protein potentially associated with hypertrophic cardiomyopathy (HCM). Autopsy confirmed concentric LV hypertrophy (17–20 mm) with myocyte hypertrophy and interstitial fibrosis consistent with HCM. Conclusions: This adolescent case reinforces LPFB as a potential marker of LV fibrosis and SCD risk in youth. The MYOZ2 gene and VUS warrants further study and family evaluation. Post-mortem findings were pivotal in establishing HCM as the etiologic substrate.