Anatomical characteristics of the human atrial appendages at 17–28 weeks of intrauterine development

BACKGROUND: Anatomical studies of the human right and left atrial appendages (RAA and LAA respectively) during the antenatal period are essential for identifying age-related characteristics and patterns of heart structure and development, establishing atrial identity in congenital heart defects, and informing fetal cardiac surgery. AIM: To provide an anatomical description of the structure and position of the RAA and LAA in morphologically normal human hearts at 17–28 weeks of gestation, and to propose criteria for their identification. METHODS: Fifty-four specimens of morphologically normal human hearts fixed in 10% formalin during atrial diastole were studied. The sizes of the heart, atria, RAA and LAA were measured using a digital caliper under an Olympus SZX2-ZB10 stereomicroscope. The shape of the appendages was assessed according to the classification of S.S. Mikhailov (1987), the LAA shape according to the classification of Y. Wang et al. (2010) as modified by K. Słodowska et al. (2021), and the number of LAA lobes according to R. Kamiński et al. (2015). A comparative analysis of the anatomical characteristics of the atrial appendages was performed across two distinct gestational periods: 17–22 weeks (n=20) and 23–28 weeks (n=34). RESULTS: At 17–28 weeks of gestation, the median dimensions were: RAA: length 9.80 mm, width 9.00 mm, thickness 3.40 mm; LAA: length 9.95 mm, width 8.25 mm, thickness 2.30 mm. The length of the RAA and LAA exceeded their width 1.1- and 1.2-fold, and the thickness 2.9- and 4.4-fold, respectively. Each of these dimensions significantly increased from the 17–22-week group to the 23–28-week group (p 0.010). The RAA was triangular (44.4%), round (20.4%), trapezoidal (16.7%), or possessed an additional small lobe, exhibiting a “mitten-like” shape (14.8%). The LAA resembled a “chicken wing” (35.2%), “cauliflower” (42.6%), “arrowhead” (22.2%). According to another classification, the LAA presented a “cockscomb” (53.7%), lobular (38.9%), or vermiform (7.4%) shape, and consisted of one (13%), two (56%) or three lobes (31%). Seventy seven point eight percent of RAA and 61.1% of LAA had an apex. The RAA apex was located to the right and/or anterior to the aortic root. The LAA apex was located in the anterior interventricular groove or faced the pulmonary trunk. Variations in the shape of the atrial appendages and the position of their apexes in the 17–22- and 23–28-week groups occurred with equal frequency. CONCLUSION: The RAA and LAA were similar in length, but the LAA was narrower, longer, and flatter, characterized by a lobular structure and marked variability in dimensions, shape, and apex position. The most significant predictors of the length and width of both appendages and the thickness of the RAA were the heart length and the width of the atrial complex; the best predictor of LAA thickness was the length of the ventricular complex. The growth of the atrial appendages from 17 to 28 weeks of gestation occurs proportionally and is interconnected with the growth of the atrial complex and the heart as a whole.