Aims: Earlier studies in monozygotic (MZ) and dizygotic (DZ) twins showed genetic variance on echocardiographically determined heart size. However, cardiovascular magnetic resonance (CMR) is more precise and reproducible. We performed a twin study relying on CMR, focusing on left ventricular (LV) mass and papillary muscle, since there are no genetic reports on this structure.
Methods and results: We measured left heart dimensions of 25 healthy twin pairs with a 1.5T MR scanner, analysed with the mass, Medis Software. We performed heritability analysis and tests for genetic influences shared between cardiac structures. We found that CMR-based heritability estimates (h(2) = 84%) substantially exceeded estimates based on echocardiography. We also found significant genetic influence on papillary muscle mass (h(2) = 82%). Bivariate analysis of papillary and LV muscle mass revealed significant genetic influences shared by both phenotypes (genetic correlation 0.59) and suggested an additional genetic component specific to papillary muscle. We observed correlations between body mass index, surface area, and systolic blood pressure with cardiac dimensions, even in this small study. Environmental influences were relevant as well, indicating reciprocal influences on papillary vs. LV muscle mass.
Conclusion: Cardiovascular magnetic resonance, even with few subjects, allows a genetic assessment of cardiac structures that cannot be attained with echocardiography. Hitherto fore unappreciated relationships can be uncovered by this method.