Isolated Posterior Mitral Leaflet Elongation as a Rare Mechanism of Systolic Anterior Motion in Hypertrophic Cardiomyopathy: A case report of successful Treatment with Mavacamten

Background Hypertrophic cardiomyopathy (HCM) is the most common inherited cardiac disorder. Left ventricular outflow tract (LVOT) obstruction typically results from systolic anterior motion (SAM) of the anterior mitral leaflet. Case summary We describe a 26-year-old man with HCM in whom SAM was generated by isolated elongation of the posterior mitral leaflet, a rare mechanism with important clinical implications. He was diagnosed during childhood by a pediatric cardiologist and followed until transition to adult care. On presentation, he was in NYHA class II with asymmetric septal hypertrophy (25 mm) and posterior leaflet elongation causing LVOT obstruction. Cardiac magnetic resonance (CMR) confirmed hypertrophy and late gadolinium enhancement (LGE) at the site of maximal wall thickness. Genetic testing revealed a pathogenic MYH7 variant (p.Ile263Thr) and a MYH6 classified as of uncertain significance. Treatment with mavacamten 5 mg led to a reduction of the LVOT gradient from 63 to 32 mmHg and symptomatic improvement. Discussion This case illustrates an unusual anatomical substrate for LVOT obstruction in HCM and emphasizes the role of multimodality imaging in diagnosis. It also highlights the therapeutic potential of myosin inhibition with mavacamten in avoiding invasive interventions in complex phenotypes.