left atrial appendage (LAA)
The left atrial appendage (LAA) (also known as left auricle) is a pouch-like projection from the main body of the left atrium, which lies in the atrioventricular sulcus in close proximity to the left circumflex artery, the left phrenic nerve, and the left pulmonary veins.
Gross anatomy
Morphological types
Four main morphological types have been described:
- chicken wing: 48%
- cactus: 30%
- windsock: 19%
- cauliflower: 3%.
Development
It is derived from the left wall of the primary atrium, which forms during the fourth week of embryonic development. It has developmental, ultrastructural, and physiological characteristics distinct from the left atrium proper.
Related pathology
- left atrial appendage thrombus: can be the source of more than 90% of cardiac-based emboli.
Radiographic features
Transesophageal echocardiography
The assessment of suspected thrombus formation in the left atrial appendage has emerged as one of the most common indications to utilize transesophageal echocardiography. Echocardiographically derived measures of LAA contractility and flow characteristics are highly predictive of future thromboembolic stroke risk. While this is a well-known complication of atrial fibrillation, LAA dysfunction is a strong independent risk factor for ischemic stroke and TIA.
The multilobulated architecture, frequent variant anatomy, and pectinate muscles make securing several orthogonal views essential for detection or exclusion of pathology. Evaluation of the left atrial appendage should be accompanied by a structural and functional assessment of related structures, including the :
- left atrium
- elevated left atrial pressures may affect LAA morphology, contractile function, and outflow velocities
- pulmonary veins
- abnormal pulmonary venous Doppler filling profiles associated with LAA dysfunction
- left ventricle
- dimensions and function (systolic and diastolic)
- elevated end-diastolic diameter associated with LAA dilation and thrombogenesis
- mitral valve
- including pulsed wave Doppler derived inflow pattern
- diastolic dysfunction and elevation in LVEDP are associated with LAA contractile dysfunction
Specific assessment of the left atrial appendage may then proceed with the following views :
- mid-oesophagal (ME) 2 chamber view
- initial identification of the LAA, which will be a roughly crescentic shaped extrusion from the anterolateral left atrium
- anterior extension typically parallels LUPV
- variation may occur, including a posterolateral course
- assess for gross LAA structural abnormalities, including dilation or aneurysm
- ostium typically measures between 1.0 and 2.5 cm
- body most commonly consists of two lobes
- found just below the entrance of the left upper pulmonary vein and the intervening fold of Marshall
- initial identification of the LAA, which will be a roughly crescentic shaped extrusion from the anterolateral left atrium
- mid-esophageal left atrial appendage (LAA) view
- spontaneous echo contrast (SEC) or uniformly echogenic luminal densities representing thrombus should be specifically sought
- rough estimation of contractile status should also be noted
- color flow Doppler can define an area of maximal flow within the appendage
- aliased flow within the LAA decreases the likelihood of thrombus formation
- a pulsed wave Doppler sample volume should be advanced toward the identified blood flow (no more than 1-2 cm within the LAA)
- four deflections may be observed in sinus rhythm
- positive deflection representing early diastolic filling, with a peak between 20 and 40 cm/second
- followed by the markedly positive late diastolic LAA contraction, with a velocity of 50–60 cm/s
- subsequent negative deflection of similar amplitude represents LAA filling
- variable, undulating systolic reflection waves follow
- four deflections may be observed in sinus rhythm
- bidirectional velocities measured > 40 cm/s amplitude reassuring for normal LAA contractile function, and low risk for thrombus
- whereas velocities < 20 cm/s are concerning
- spontaneous echo contrast (SEC) or uniformly echogenic luminal densities representing thrombus should be specifically sought
- mid-oesophagal aortic short-axis view
- maybe used to measure orifice width and the depth of the LAA
Practical points
Some authors report that the presence of trabeculations and a smaller left atrial appendage orifice diameter may be associated with greater stroke prevalence in atrial fibrillation; i.e. the risk is lowest with chicken wing, with other types carrying higher risk: cactus (x 4.08), windsock (x 4.5), and cauliflower (x 8) .
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