PO08 Unexplained early onset isolated hypertrophic cardiomyopathy in a symptomatic 18 year old male

Background Hypertrophic cardiomyopathy (HCM) is a genetically heterogeneous disorder characterised by left ventricular hypertrophy (LVH) and can present with obstructive features, arrhythmias, and sudden cardiac death, particularly in young individuals. Unexplained cases with isolated cardiomyopathy in adolescents may suggest novel genetic variants, warranting comprehensive evaluation. Purpose Case report of unexplained hypertrophic obstructive cardiomyopathy (HOCM) in an 18-year-old patient, highlighting diagnostic challenges and the role of multimodal imaging, biopsy, and genetic testing. Methods An 18-year-old male who recently emigrated from Cuba presented with unexplained HCM was evaluated via echocardiography, endomyocardial biopsy, cardiac magnetic resonance imaging (MRI) with and without contrast, and genetic testing via a targeted panel and sent to be confirmed by Whole Exome Sequencing (WES). Results Echocardiogram revealed ejection fraction (EF) 45-50%, mild global left ventricular (LV) hypokinesis, massive LVH, resting left ventricular outflow tract (LVOT) gradient 30-40 mmHg, provoked LVOT gradient 60 mmHg, Grade I diastolic dysfunction, mildly reduced right ventricular (RV) systolic function, trace mitral and tricuspid regurgitation, and normal RV pressure. Biopsy showed cardiomyocyte hypertrophy with architectural disarray and cytoplasmic vacuolisation; electron microscopy demonstrated myocardial fiber disarray, vacuolar, and mitochondrial changes, suggesting HOCM versus sarcomeric or vacuolar cardiomyopathies. Cardiac MRI confirmed significant myocardial thickening (left anteroseptal wall up to 5.8 cm), systolic anterior motion (SAM) with mitral regurgitation, diffuse heterogeneous delayed enhancement involving 30-40% of LV myocardium, global hypokinesis of both ventricles (LVEF 40%), and small pericardial effusion. Genetic panel identified a heterozygous benign pseudodeficiency allele in the GAA gene (c.2065G>A), deemed non-pathogenic and unrelated to the phenotype. Conclusions This case represents a likely novel HOCM presentation as a diagnosis of exclusion in a young patient with negative standard genetic panel and unremarkable muscle and neurological symptoms. Whole exome sequencing is pending for confirmation, underscoring the need for advanced genomic analysis in unexplained HCM.