The left atrial septal pouch as a possible location for thrombus formation

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The left atrial septal pouch (LASP) was described in 2010 by Krishnan and Salazar1 as a potential source of embolism. It is formed by the caudal fusion of the area of overlap of the septum primum and the septum secundum, leaving a cranial opening towards the left atrium. There are a number of case reports demonstrating a thrombus arising from the cavity of the LASP, but the potential for thrombus formation inside this pouch remains debatable.2–4
An 82-year-old woman, without known cardiovascular history, was admitted to our institution for syncope. Several abnormal findings were noted: an atrial fibrillation and a systolic murmur. The patient was initiated on oral anticoagulation.
Transthoracic echocardiography showed a severe aortic stenosis and left atrial dilation. Significant left anterior descending artery stenosis and an important dilation of the ascending aorta were found on coronary angiography. The transoesophageal study showed a stenotic bicuspid aortic valve and a mobile isoechoic structure arising from a typical LASP. The three-dimensional images revealed a crescent-shaped mass, significantly larger than the two-dimensional images had anticipated (Fig. 1). We noted the presence of spontaneous contrast in the left atrium. The left atrial appendage was free of echoes. In the setting of atrial fibrillation, the mass was considered to be a probable thrombus. As the patient already had a clear indication for heart surgery, our diagnosis remained to be corroborated by direct pathological examination.
The patient underwent a Bentall procedure. The control transoesophageal echocardiogram before surgery noted the complete disappearance of the mass under anticoagulation. There were no signs of systemic embolism. A cranial computed tomographic (CT) scan ruled out a major stroke. She was implanted with a Mitroflow 23 aortic valve bioprosthesis and an aortic root Gelweave 30 prosthesis, associated with a left internal mammary bypass graft on the left anterior descending artery. The postoperative evolution was favourable.
The case presented here indirectly confirms the formation of thrombus inside the LASP in the setting of atrial fibrillation. The LASP is a potential site for thrombus formation, probably more frequently than previously thought, but it is present in nearly one-third of normal individuals. Several risk factors for thrombogenesis inside this pouch seem to stand out from the reports available so far: atrial fibrillation, especially valvular; elevated left ventricle filling pressure in heart failure; possible local endothelial injury; and hypercoagulable state.
We suggest that the presence of an LASP in the context of any of those risk factors should trigger a careful investigation of the interatrial septum, especially in case of ischaemic stroke. Recent reports also show the benefits of a multimodality imaging approach in the diagnosis of LASP masses.

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