Hypertrophic cardiomyopathy: still connecting the dots between genotype and phenotype

The current era of cardiac magnetic resonance (CMR) imaging has resulted in a resurgence of interest in hypertrophic cardiomyopathy (HCM). We are now much more aware of the various phenotypic expressions of this disease, which can manifest with typical septal thickening or other variants of hypertrophy, such as apical, midwall or concentric. Additionally, CMR has enabled us to appreciate the wide variety of abnormalities in papillary muscle morphology, which in some cases can contribute to left ventricular outflow tract or mid-cavitary level obstruction (1). In some subjects, mitral valve elongation, myocardial crypts, prominent apical trabeculation, papillary muscle thickening (with or without apical displacement) and abnormal chordal attachments may be subtle phenotypic HCM variants, despite normal left ventricular wall thickness. These abnormalities may be symptomatic to varying degrees or remain subclinical and be noted incidentally or on HCM familial screening programs. Whether these subjects will go on to develop progressive left ventricular hypertrophy (LVH) or whether they are manifesting incomplete phenotypic penetrance of the disease process remains unclear.