A 54-year-old woman with a history of rheumatic heart disease with severe mitral stenosis was scheduled for a mitral valve replacement procedure. The preoperative echocardiogram revealed severely thickened and fibrosed mitral valve leaflets with extensive commissural and subvalvular fusion. Apart from the mitral stenosis with an enlarged left atrium (LA), moderately high pulmonary arterial pressures, and mild tricuspid regurgitation, the rest of the echocardiogram was within normal limits. A preoperative electrocardiogram revealed atrial fibrillation.
After induction of anesthesia, a S7-2 Omni transesophageal echocardiogram (TEE) probe (Philips, Andover, MA) was inserted into the esophagus and a comprehensive TEE examination was performed using a Philips HD11XE (Andover, MA) ultrasound machine. On the midesophageal (ME) 4-chamber view, an enlarged LA with the presence of spontaneous echo contrast was seen. An oscillating, pedunculated, rounded LA mass close to the interatrial septum (IAS) (Supplemental Digital Content, Video 1, http://links.lww.com/AA/B81) and its appearance closely resembling a myxoma, which was not evident on the preoperative TEE. Close examination of her LA appendage revealed no thrombus and the rest of the TEE findings were similar to that of the preoperative TEE. Focusing on the IAS, while increasing the multiplane angle to 45°–50°, helped obtain the ME aortic valve short-axis view, in which the LA mass was seen to oscillate (with cardiac contractions) in and out of view (Supplemental Digital Content, Video 1, http://links.lww.com/AA/B81). Increasing the multiplane angle to 90° helped visualize the IAS using the bicaval view, where a small “pouch-like” space embedded within the IAS, close to the floating mass, was clearly evident (Fig. 1A). Reducing depth, narrowing sector width, and using the zoom function helped focus on the septal pouch (Fig. 1B). A small color flow Doppler (CFD) window with a reduced velocity scale (<20 cm/s), placed over this space, helped elicit the flow of blood in and out of the small cavity within this pouch into the LA (Supplemental Digital Content, Video 1, http://links.lww.com/AA/B81). The mass was seen freely oscillating close to this potential pouch-like space opening into the LA cavity. An agitated saline contrast injection was performed, which failed to show any communication (patent foramen ovale [PFO]) between the 2 atria.
During surgery, after opening the LA, the IAS was carefully examined. A left atrial septal pouch (LASP) was evident on the intraoperative TEE. The LA mass was retrieved with a stalk-like attachment found within the septal pouch. The cut section appeared firm and the mass was sent for biopsy, which confirmed it to be an organized clot. The LASP was closed surgically.
The IAS is formed by varying degrees of overlapping and fusion between the septum primum (SP) and septum secundum (SS). The SP overlaps the opening of the SS, where the SP functions as a door and the SS functions as a doorframe.1 A PFO is a channel of communication across the IAS formed by the failure of fusion between the SP and SS (Fig. 2A). Over time, development of adhesions between the SP and SS serve to close the foramen in up to 75% of adults.1 In the remaining 25%, where adhesions fail to occur, a PFO results.2 However, when fusion does occur between the SP and SS, in the majority of cases, it does not occur along the entire zone of overlap, giving rise to blind pockets or pouches in approximately 44% of the general population.1 A septal pouch that opens into the LA cavity is a LASP (Fig. 2B), and a septal pouch opening into the right atrial cavity is called the right atrial septal pouch (RASP) (Fig. 2C). Clearly the condition has existed and been recognized, and the term ASP seems to be more of a recent change in semantics and nomenclature.1 Whether or not an ASP can serve as a nidus for thrombus formation and be a potential source of stroke remains controversial.3
The best views to examine the IAS using TEE are the ME 4 chamber, ME aortic valve short-axis, and ME bicaval views. Two-dimensional echo, especially while reducing depth, narrowing the sector width (thus increasing frame rate, producing crisper images), and using the zoom function, helps to identify anatomical details, like the relatively small-sized ASPs. However, the narrow patent channel between the SS and SP forming a PFO may not be evident on 2-dimensional echo. CFD with a reduced velocity scale (<20 cm/s) may help identify the leak through a PFO. Interestingly, the presence or absence of a PFO also helps differentiate between a septal pouch and a double IAS.4 Although a PFO may be identifiable in a double atrial septum, its absence is the hallmark in a case of a septal pouch (Fig. 2, B and C). The double IAS4 is anatomically a much larger entity. The presence of an accessory atrial septum (Fig. 2D) and easily recognizable flow of blood into and out of the large interatrial space under CFD is the hallmark in a case of double atrial septum. In contrast, flow in and out of septal pouches under CFD is relatively difficult to ascertain because of the much smaller cavity spaces in them.
In our case, a thrombus was associated with the LASP. Additional factors that promote stasis of blood such as an enlarged atrial cavity with atrial fibrillation, which were present in this case, further favored the formation of a thrombus from within the pouch. Whether the presence of a pouch alone, in the absence of other factors supporting thrombus formation, e.g., atrial fibrillation, can promote thrombus formation is debatable.
For similar reasons, a right ASP, like a LASP, has been reported to be a potential source of embolus.5 The majority of ASPs (90%) are left-sided.1
In conclusion, an ASP is a new descriptive terminology to a previously known entity introduced in 2010.1 Prior knowledge of such an entity with its potential risks may prompt the intraoperative echocardiographer to closely examine the IAS not only in high-risk patients with or without a history of thromboembolism, but also routinely as part of the comprehensive TEE examination. Although surgical closure of the septal pouch in established cases of thrombus arising from it is desirable, the decision is not clear whether to close it or leave it as is when there is no history, other risks, or evidence of thrombus in a patient.
Clinician’s Key Teaching Points
By Kent H. Rehfeldt, MD, Massimiliano Meineri, MD, and Martin J. London, MD
- The atrial septum is formed by variable degrees of fusion between the SP and SS. A lack of fusion between the SP and SS, occuring in approximately 25% of individuals, gives rise to a PFO.
- An incomplete fusion of the SP and SS results in a blind pocket or pouch between the 2 overlapping septal components without a shunt. This small space is called an ASP and may be found in up to 44% of individuals. The ASP usually communicates with the left atrium (left ASP), although in 10% of cases, it communicates with the right atrium (right ASP).
- In this case, intraoperative TEE imaging of the atrial septum (enhanced by the zoom function to maximize resolution and frame rate) in the midesophageal aortic valve short-axis and midesophageal bicaval views detected a mass in the left atrium arising from a small space between the SP and SS. Color Doppler imaging using a reduced velocity scale (<20 cm/s) demonstrated communication between the left atrium and the atrial septal space, whereas agitated saline injection excluded the presence of a PFO, confirming the diagnosis of a left ASP.
- Although this common anatomic variant has been recognized for decades, the term ASP is relatively new. Although the mass arising from the ASP in this patient with risk factors for left atrial stasis (mitral stenosis and atrial fibrillation) proved to be thrombus, the role of an ASP in facilitating thromboembolic events is debatable and an individualized approach to management is warranted.
Name: Saikat Bandyopadhyay, DA, MD, FCCM, FTEE.
Contribution: This author drew the diagrams, designed and labelled the images, compiled and edited the videos, and wrote the manuscript.
Attestation: Saikat Bandyopadhyay approved the fi nal manuscript.
Name: Kayapanda Mandana, MBBS, MS, MCh, FETCS.
Contribution: This author helped write the manuscript.
Attestation: Kayapanda Mandana approved the final manuscript.
This manuscript was handled by: Martin J. London, MD.
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