Variant Anatomy of the Right Ventricular Moderator Band in Human Heart in Early Antenatal Period of Development

INTRODUCTION: The information about the variants of the structure and topography of the moderator band (MB) that connects the interventricular septum with the anterior papillary muscle and the anterior wall of the right ventricle in the heart of a fetus and a newborn, is of great importance for cardiac surgery. 
AIM: To establish the prevalence of the MB and describe the variants of its shape, structure and position in the right ventricle of the normal human heart in the early antenatal period of development. 
MATERIALS AND METHODS: Using Olympus SZX2-ZB10 stereomicroscope with 4.725X to 15X magnification, formalin-fixed hearts of fetuses and stillborns of 1728 weeks were studied. The results are presented in the form of median, 25th and 75th percentiles, extreme values. Correlation analysis was performed. Significance of the difference of proportions was evaluated by one-sided t-test. The results are presented in the form of median (Me), 25th and 75th percentiles (Q25%Q75%). 
RESULTS: MB was found in 73 of 90 preparations (81.1%), in 48 cases (66%) it was bridge-like, and in 24 of 72 (33.3%) ― parietal. MB had flattened (crest-like) or cylindrical shape (62.5% vs 33%; p = 0.0002). The most common was flattened bridge-like variant. The length of MB was 2.2 (1.753.0) mm, width 1.35 (0.91.75) mm, thickness 1.0 (0.651.5) mm. The band mainly originated from the interventricular septum between the middle and apical thirds of the longitudinal axis, and the anterior and middle thirds of the transverse axis of the interventricular septum. It typically terminated with the attachment to the anterior papillary muscle (47.7%), or to the anterior wall of the right ventricle immediately in front of this muscle (38.5%). In 22.2% of cases, the MB had papillary muscles on it, and in 37.5%, the secondary trabeculae extended from it to the apex of the ventricle. 
CONCLUSION: MB is a normal, but not obligatory structure of the heart in the antenatal period, its normal anatomy is variable and is manifested by typical and rare variants of the form, position, beginning and end, which in many cases can impede diagnostics and treatment of the pathology of the right ventricle.