Comparative micro-computed tomography assessment of left-sided septal pouch and left atrial appendage morphometry

The left-sided septal pouch (LSSP) is a blind-ending recess of the interatrial septum that opens into the left atrium and has been proposed as a potential site of blood stasis and thrombus formation. Although its clinical relevance has been discussed in relation to cryptogenic stroke, detailed three-dimensional morphometric data are limited. This study used high-resolution micro-computed tomography to compare the morphology and volume of the LSSP with those of the left atrial appendage (LAA), a well-established source of cardiac embolism, thereby contextualizing the size and spatial orientation of the LSSP. Forty structurally intact human hearts with an LSSP were examined. After controlled fixation and micro-computed tomography scanning, three-dimensional reconstructions of the LSSP and LAA luminal spaces were generated and used for morphometric analysis. The mean LSSP volume was 0.43 ± 0.16 ml, whereas the mean LAA volume was 3.81 ± 2.21 ml ( p < 0.001). The mean specimen-level LSSP/LAA volume ratio was 14.5 ± 7.6% (range: 4.2–59.3%). The mean left atrial cavity volume was 37.4 ± 10.2 ml, while the mean specimen-level LSSP/LA cavity and LAA/LA cavity volume ratios were 1.4 ± 0.6% and 11.3 ± 7.2%, respectively. The LSSP ostium was oriented superiorly, whereas the LAA orifice was arranged in a horizontal plane. The LSSP was smaller than the LAA reference structure across the assessed volumetric, ostial, and linear dimensions, including ostial or orifice area (39.7 ± 19.0 mm² vs. 121.7 ± 74.1 mm 2 ), depth or length (9.1 ± 4.2 mm vs. 23.6 ± 7.5 mm), and width (7.1 ± 3.6 mm vs. 17.7 ± 6.9 mm). In conclusion, our findings provide the first micro-computed tomography-based three-dimensional morphometric assessment of the LSSP and show that, despite its small size, it represents a measurable left atrial recess with distinct spatial anatomy and quantifiable volume.