Morphology of the cardiovascular system in conjoined twins: spatial and sequential segmental arrangements in 36 cases

Thirty-six pairs of conjoined twins are reviewed from the joint experience of the authors. The external forms of conjunctions were 18 thoracopagus (joined at chest), eight cephalothoracopagus (joined at head and chest), eight dicephalus (single trunk and two heads), one craniopagus (joined at head), and one omphalopagus (joined at abdomen). There was one case with a single conventional heart in association with isomerism of the left lungs and absence of the heart in the left-side twin. Six cases had two conventional hearts. All of them had associated lesions, which were more severe in three cases (50%) with abnormal laterality. All of the cephalothoracopagus (eight cases) had two shared hearts. Cardiac lesions were commonly seen in the posterior heart, but the arrangement of atrial appendages were normal in every heart. Twenty-one cases of either dicephalus or thoracopagus had a compound heart. Three cases (14%) had fusion only at the level of venous sinus; three others (14%) were fused at the atrial level only, but 15 cases (71%) had fusion at both atrial and ventricular levels. Abnormal laterality was an associated lesion in ten cases (48%) with a compound heart. The fusion at the atrial level always occurred between right atriums, but ventricular fusion was more commonly between two left ventricles. Four atrial or ventricular chambers, if fused, were arranged in a cruciate arrangement. Mode of conjunction was as important, in the formation of the cardiovascular system, as was the degree of fusion. Abnormal laterality, which is commonly associated in lateral or mixed lateral/facing conjunction, was the most important factor affecting the general morphology of the cardiovascular system.