The Left Atrial Appendage in Humans: Structure, Physiology, and Pathogenesis

Nabeela Karim; Siew Yen Ho; Edward Nicol; Wei Li; Filip Zemrak; Vias Markides; Vivek Reddy; Tom Wong

Disclosures

Europace. 2020;22(1):5-18. 

In This Article

Anatomy

The left atrial appendage (LAA) is a complex structure that develops in the 3rd week of gestation, arising from the left and superior side of the primary atrial tube.[1,2] Smaller than its counterpart on the right,[3] the LAA varies in size and shape, and in its relationship with surrounding structures.[4]

Surrounding Structures

Typically, the LAA lies inferior to the pulmonary artery, superior to the mitral valve, and anterior to the left pulmonary veins; most commonly the left upper pulmonary vein[4] (Figure 1). The ligament of Marshall serves as an epicardial landmark between the left lateral aspect of the LAA and left upper pulmonary vein.[5] Importantly, the appendage overlies the AV groove which contains the left circumflex artery and the great cardiac vein[3] (Figure 1). The left phrenic nerve courses on the surface of the fibrous pericardium that overlies the appendage.[6] Traversing across the interatrial groove and connecting the right and left atria is Bachman's bundle, which continues to encircle the epicardial aspect of the neck of the LAA.[3]

Figure 1.

LAA anatomy. (A) Left lateral view with LAA in situ. (B) Left lateral view with LAA retracted, to reveal coronary arteries and veins. (C) Left atrium cut through the os and LSPV showing the infolding of the atrial wall that looks like a ridge on the endocardial surface. (D) P-A view of the heart showing narrow os and LAA relationship to LCA, MV, and LIPV. (E) This dissection in similar orientation as panel C has the postero-lateral wall of left atrium deflected superiorly to allow a view into the pectinated LAA compared with the smooth endocardial surface of the left atrium. The broken line marks the os. (F) Transillumination reveals the thin membranous areas in between the pectinate muscles lining the LAA wall. GCV, Great cardiac vein; LAA, left atrial appendage; LAD, left anterior descending artery; LCA, left coronary artery; LCx, left circumflex artery; LIPV, left inferior pulmonary vein; LPA, left pulmonary artery; LSPV, left superior pulmonary vein; MV, mitral valve; os, ostim; RVOT, right ventricular outflow tract.

Ostium

On the endocardial aspect, the smooth anterior and lateral walls of the left atrium (LA) converge to meet with LAA pectinate muscles to form the LAA ostium.[7] The left lateral ridge; a fold in the atrium that separates the ostium from the orifice of the left upper pulmonary vein positioned postero-superiorly[8] (Figure 1). The ligament of Marshall runs along the epicardial side of this fold. The ostium reportedly occurs as four different shapes on computed tomography (CT) imaging: 'oval' in 68.9% of cases, 'foot-like' in 10%, triangular in 7.7%, 'water drop-like' in 7.7%, and 'round' in 5.7%.[9] An earlier anatomic study in 31 hearts, however, reported an oval-shaped ostium with major dimension of 17.4 ± 4 mm and minor dimension (perpendicular to the longest diameter) of 10.9 ± 4.2 mm.[8] Its size does not correlate with the size of the LA or LAA body.[8]

Body

The LAA body most commonly projects antero-superiorly, where it overlaps the left border of the right ventricular outflow tract/pulmonary trunk and the left main stem or left circumflex artery[3] (Figure 1). Alternatively, it can extend laterally and posteriorly, and in a minority, project behind the arterial pedicle where it sits in the transverse pericardial sinus.[3]

The length of the LAA is on average 45 mm long, with a range from 27 to 60 mm.[8] The LAA can have protrusions from the body referred to as lobes, the most distal protrusion can also be referred to as the tail, which itself represents a lobe.[10] In a sample of 500 autopsy cases, the most common LAA configuration was bilobed (54%) followed by trilobed (23%), then single-lobed (20%), with the least common being four or more lobes (3%).[10] Age and gender did not correlate with the number of lobes.[10]

Wall

Left atrial appendage muscle architecture consists of endocardial and epicardial fibres arranged in varying orientations creating a complex arrangement.[11] Wall thickness is non-homogenous, with parts being less than 0.5 mm thick.[3] Thicker bundles of pectinate muscle can sometimes be mistaken for thrombi or intra-atrial masses on imaging.[12]

Morphological Classification

Modern imaging techniques have enabled in vivo morphology to be analysed in detail, resulting in the LAA being classified first into those with a distinct bend and those without.[11] The former morphology is described as 'chicken wing' and the latter further classified into 'windsock', 'cauliflower', and 'cactus' (Figure 2).[9]

Figure 2.

LAA morphologies depicted on CT scan, illustrated with its corresponding anatomical figure. Makers have been placed to help identify the LAA. Panels are as follow: (A) Chicken wing, (B) Windsock, (C) Cauliflower, and (D) Cactus. CT, computed tomography; LAA, left atrial appendage.

The 'chicken wing' LAA is reportedly the most common morphology, occurring at a prevalence of 48%.[13] It has a central lobe with a bend in the proximal or middle part and may have secondary lobes.[13] The cactus shape is the second most common type (30%); it has a dominant central lobe with secondary lobes extending from the main body in various orientations.[13] The windsock LAA (19%) has a long dominant lobe and may have secondary lobes arising from it.[13] The cauliflower LAA is the least common with a prevalence of 3%; it tends to have an irregularly shaped ostium and complex internal characteristics with a varying number of lobes, none of which are dominant.[13]

It has been noted that the morphology of the LAA does not change with time in patients with atrial fibrillation (AF).[14] To make classification of the morphologies simpler, it has also been proposed to group LAA morphology into simply chicken wing and non-chicken wing.[14]

Left Atrial Appendage Imaging Techniques

Left atrial appendage structure, morphology, and thrombosis detection can be assessed by transoesophageal echo (TOE),[15–17] cardiac CT,[18,19] intracardiac echo,[20] and cardiac magnetic resonance.[21] Haemodynamic assessment of LAA velocities is best undertaken using Doppler on TOE.[1,22,23] Supplementary material online, Table S1 summarizes the disadvantages and advantages of each modality with regards to LAA imaging.

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