A Review of the Anatomical and Histological Attributes of the Left Atrial Appendage with Descriptive Pathological Examination of Morphology and Histology.

The left atrial appendage (LAA) has a key role in the embolic complications of atrial fibrillation (AF). It has been studied extensively, from recent interest in the thrombotic implications of various LAA morphologies to LAA occlusion and ablation. We collected eleven post-mortem LAA samples for visual analysis, two were not included due to poor sample quality. On examination of the nine remaining samples, several common patterns of pectinate muscle orientation were noted. The LAA samples were noted to have a smooth circumferential neck of muscular tissue giving rise to a dominant singular smooth trunk of papillary muscle in 6 cases and two trunks in 3 cases. These trunks were either shallow (5 samples) or more muscular and raised (4 samples). Shallow trunks tended to be wider than the raised trunks and may even be circumferential (2 samples). The main trunk arborized to give off papillary muscle branches down to third or fourth order branches. The samples were visually assessed for the percentage of smooth papillary muscle versus non-papillary recesses and were found to have ≤50% smooth muscle in 3 samples, 50-75% in 3 samples and >75% in 3 samples. We performed histological analysis of further LAA samples collected during cardiac surgery in a parallel study. We identified a distinct pattern of myocyte orientation from the neck, mid-section and apical section of the LAA demonstrating arborization of myocyte fibers with minimal communication in distal segments of the LAA. We feel that this information may help understanding of the issues surrounding LAA ablation strategies.