Anomalous ridge on the left atrial side of the atrial septum

A 75-year-old male with a history of persistent atrial fibrillation, type II diabetes mellitus, hypertension and hypercholesterolemia underwent transesophageal echocardiography (TEE) to evaluate for a left atrial appendage thrombus prior to direct current cardioversion. The TEE was performed using a real-time 3-dimensional (3D) TEE probe connected to the iE 33 Philips ultrasound system (Philips Healthcare, Andovor, MA, USA). The 3D TEE revealed a ridge-like structure on the left atrial side of the atrial septum protruding into the left atrial lumen [Figure 1a]. This ridge was formed along the superior edge of the oval fossa in the left atrium, and extended to the left atrial free wall medially and superiorly [Figure 1b]. This ridge-like structure overhung posteriorly, creating a hollow-like structure [Figure 1c and d]. Although we did not perform an agitated saline contrast study, which is more sensitive than color Doppler, to evaluate a right to left atrial shunt, the color Doppler interrogation of this ridge did not reveal interatrial shunting. No thrombus or spontaneous echo contrast was noted beneath the overhung area. The atrial septum appeared normal from the right atrial side [Figure 2a and b].Figure 1: In panel a, an overhanging membrane-like ridge is demonstrated in the mid esophageal view of 2-dimensional transesophageal echocardiography (TEE). The white arrow indicates a ridge-like structure. AS - atrial septum; RA - right atrium; LA - left atrium; AA - ascending thoracic aorta. In panel b, superiorly coursing membrane-like ridge in the left atrium is demonstrated in the en face surgeon's view (looking at the supine patient from head to toe from the left side of the patient) with full-volume 3-dimensional TEE. The black arrow indicates a ridge-like structure. The arrow heads denote the spatial orientation. LA - left atrium; RA - right atrium; AV - aortic valve; OF - oval fossa; SVC - superior vena cava; Ant - anterior side of the patient; Rt - right side of the patient; *stitch artifact due to respiratory motion. In panels c and d, a folding membrane-like character creating a slit of the ridge structure (black arrow) is demonstrated in 3-dimensional TEE. The arrow heads denote the spatial orientation. AS - atrial septum; RA - right atrium; LA - left atrium; OF - oval fossa; ant, anterior side of the patient; rt - right side of the patient; sup - head side of the patient; inf - back side of the patientFigure 2: The views of atrial septum (AS) from the right atrium are shown with 3-dimensional transesophageal echocardiography. The AS is viewed from the right atrial free wall in panel (a) The oval fossa (OF) is demonstrated as slight depression of the atrial septum. The atrial septum is viewed in standard bicaval view in panel. (b) SVC denotes superior vena cava. The ridge formation is depicted by a white arrow headIn contrast to the ridge-like structures of the right atrium, such as an Eustachian valve or crista terminalis, ridge-like structures on the left atrial side have not been well described. [12] In the left atrial side of the atrial septum, redundancy of atrial septal membrane forming the oval fossa is often noted, and atrial septal aneurysm is common. [12] Sweeny et al. describe a network of muscular trabeculations that can be seen in the left atrial septum. Some of these may be seen along the anterior border of the oval fossa representing the remnants of the ostium secudum, and have been named the cresentic muscular arch. [34] However, a prominent membrane-like ridge formation along the oval fossa that overhangs and extends to the atrial free wall superiorly has not been clearly documented in the previous literature. In addition, this variation could interfere with navigating guiding wires and catheters in the left atrium through transeptal approach. Recently, two discrete types of mesenchymal cells have been identified that form the mesenchymal cap that closes the primary atrial foramen by fusing with the arterioventricular cushions. [5] One population is from the endocardium and the other is from the dorsal mesocardium outside the heart, indicating that both intracardiac and extracardiac mesenchymal cells participate in the formation of the atrial septum. [5] This study underscores the complexity of atrial septum formation and a need for further understanding of this embryonic development. Our case is a very rare anomaly. The number of cardiac patients who undergo inverventional transeptal approach has risen substantially over the past few years, further underlying the importance of understanding left atrial anatomy and anomalies. Delineation of complex atrial septal structures before such procedures by TEE with the aid of 3-dimensional TEE informs the cardiologist of potential procedural challenges and improves patient safety. [6] Source of Support: Nil, Conflict of Interest: None declared.