Avoiding overdiagnosis of hypertrophic cardiomyopathy: a CMR–based diagnostic strategy for ventricular septal bulge

Introduction Cardiac magnetic resonance (CMR) imaging combined with electrocardiography (ECG) is essential for differentiating hypertrophic cardiomyopathy (HCM) from it phenocopies. One commonly observed feature in clinical practice is basal septal thickening, known as the basal septal bulge (BSB). However, standardized diagnostic criteria for BSB when it presents as a phenocopy of HCM (BSB/HCM) remain lacking. Purpose This study aims to systematically characterize the clinical, electrocardiographic, and imaging features of BSB and to clarify its distinction from true HCM. Methods BSB was defined as focal hypertrophy of the basal interventricular septum with a wall thickness ≥12 mm and a mid-to-distal septal thickness ratio ≥1.3; as measured by CMR. From over 1,425 consecutive CMR studies, only those requested for suspected HCM were selected. Among these; 24 patients with a confirmed HCM diagnosis served as controls and were compared to 76 patients diagnosed with BSB/HCM. Results Patients with HCM/BSB phenocopy exhibited normal wall thickness outside the prominent basal septal bulge, with a significant mean difference of 2 mm compared to HCM, as well as severe aorto-septal angulation (OR: 12.66; p<0.0001). They showed anterior systolic motion (OR: 4.4; p=0.007) and small left ventricle (OR: 5.6; p=0.004), with a high incidence of ventricular hyperkinesia (84%). Late gadolinium enhancement was almost exclusive to HCM (OR: 0.031; p=0.0001), coinciding with areas of greater hypertrophy, and absent in phenocopies. The typical electrocardiographic pattern of HCM was notably absent in the HCM/BSB phenocopy. Conclusions Clinical, electrocardiographic, and imaging features distinctly differentiate basal septal bulge hypertrophy (BSC/HCM) as a phenocopy of hypertrophic cardiomyopathy from true HCM. These findings lay a robust foundation for the development of specific diagnostic criteria and contribute to a more accurate and nuanced diagnostic strategy in routine clinical practice.Results. Forest diagramROC curve