Fat in the heart. A feature unique to the human species? Observational reflections on an unsolved problem.

Fat that is well demarcated from underlying muscle is found on the right ventricular free wall and around epicardial coronary vessels. Fat is not present in the left ventricle in normal subjects. In right ventricular dysplasia, fat and fibrosis may massively displace right ventricular myocardial tissue. It is frequently associated with some clusters of fat and fibrosis in the left ventricle. Adipocytes may be also found within fibrous tissue. In this situation it may be associated with inflammatory cellular infiltrates in both ventricles and this is called metaplastic fat. All these findings may be seen and sometimes are associated to a variable degree in the same myocardial specimen. However, fat may be interspersed with right ventricular myocardial fibres but without fibrosis or signs of inflammation. This situation is observed in more than half of the normal hearts and represents an over-looked pathologic condition only observed in the human species. The term "fat dissociation syndrome" is proposed to identify this condition. This new understanding of right ventricular myocardial structure which may be investigated by MRI may have important clinical consequences.