A 3-year-old girl presented with a history of progressively increasing breathlessness on exertion. An echocardiogram done at an outside hospital identified an ostium secundum atrial septal defect (ASD). The ASD physiology alone could not explain the degree of exertional dyspnea. A preoperative transthoracic echocardiogram revealed severely elevated pulmonary artery systolic pressures in addition to a membranous ridge attached to the left atrial (LA) wall circumferentially, just above the mitral annulus. The rest of the preoperative evaluation was within normal limits.
After induction of anesthesia, a S7-3t transesophageal echocardiography (TEE) probe was introduced and a comprehensive TEE examination was performed using a Philips HD11 XE (Bothell, WA) ultrasound platform. The midesophageal 4-chamber (ME 4C) view revealed an ostium secundum ASD with right atrial and right ventricular dilation consistent with the preoperative echocardiogram findings. An additional membrane (Fig. 1; Supplemental Digital Content 1, Video 1, http://links.lww.com/AA/B174) was seen in the LA cavity attached to the lateral atrial wall and discontinuous toward the interatrial septum (as seen on the ME 4C view), lying parallel to and just above the mitral annulus. This supramitral membrane was circumferentially attached to the LA wall; however, in the ME 4C view it appeared deficient in the anteromedial aspect (facing the interatrial septum). It appeared this might have been because of a small opening in the membrane, which was not central but located anteromedially close to the ostium secundum ASD. With color flow Doppler (CFD) (Nyquist limit of 50 cm/sec), intense aliasing was observed as blood flow was restricted through the narrow opening in the supramitral membrane before it entered the left ventricular cavity via the mitral valve. Increasing the transducer rotation to 45°, a ME aortic valve short-axis view was obtained demonstrating the ASD. However, the supramitral membrane could not be clearly visualized in this view. Progressively increasing the multiplane angle to 60° (ME commissural view), 90° (ME 2C view), and 120° (ME long-axis view), the supramitral membrane could be seen clearly from different angles, its circumferential attachment to the LA wall well appreciated with its free margin directed anteromedially toward the ascending aorta (Supplemental Digital Content 1, Video 1, http://links.lww.com/AA/B174). Obtaining an ME 2C view and turning the probe to the left helped locate the LA appendage, and the attachment of the supramitral membrane to the floor of the LA appendage was evident (Fig. 2). Reducing the multiplane angle back to 0° an ME 4C view was obtained, from where the probe was further advanced into the stomach, anteflexed and withdrawn until the fish-mouth appearance of the mitral valve appeared, obtaining the basal transgastric (TG) short-axis view. Increasing the multiplane angle to 90° from there demonstrated the basal TG 2C view (Fig. 3; Supplemental Digital Content 2, Video 2, http://links.lww.com/AA/B175) in which the mitral leaflets and the supramitral membrane could be clearly seen. With CFD, turbulent blood flow through the constricting ring in the membrane could be seen as previously demonstrated using the ME views.
During surgery, the right atrium was opened and the sheet of membrane (supramitral) could be seen through the ASD. A narrow opening was seen in the sheet of membrane situated close to the ASD, which acted as a constricting ring allowing blood to flow through it under pressure. After resection of the membrane and closure of the ASD, the patient was weaned from cardiopulmonary bypass uneventfully. TEE revealed adequate resection of the supramitral membrane with normal flow across the mitral valve apparatus (Supplemental Digital Content 3, Video 3, http://links.lww.com/AA/B176). The postoperative course was unremarkable.
Supramitral ring is also known as supravalvar mitral ring. A mitral ring is a rare type of congenital mitral stenosis.1 Two types of mitral rings have been described in the literature: the supramitral ring and the intramitral ring.2 In patients with a supramitral ring, the mitral apparatus is usually normal. Intramitral rings are often associated with complex valve pathology, and therefore have a worse prognosis compared with supramitral rings.2 Echocardiography plays a crucial role in the evaluation of the mitral valve apparatus in these cases. It helps identify the types of malformation, the associated hemodynamic implications, and also helps prognosticate outcome.
The supramitral ring may be described as a horizontally oriented membrane or ridge of connective tissue3 located just above the mitral valve annulus with a narrow circular opening in the sheet of membrane/fibrous ridge which allows blood to flow from the larger upper chamber of the LA to the smaller lower chamber enclosed between the supramitral membrane above and the mitral valve apparatus below. Because the opening in the membrane is small, it acts as a constricting ring that allows blood to flow under pressure from the upper LA chamber to the lower chamber and into the left ventricle mimicking a mitral stenosis physiology. TEE, owing to the close proximity of the probe transducer to the LA, can examine the supramitral ring with precision. The best ME views to examine it are ME 4C, commissural, 2C, and long-axis views. These views are best supplemented with an additional TG view, namely the basal TG 2C view in which the supramitral membrane can be seen on the right of the screen and the mitral valve and left ventricle can be seen to the left (Fig. 3).
Although 2-dimensional TEE views can produce excellent images to confirm the diagnosis, adding CFD on to the 2-dimensional window can help assess the intensity of aliasing (using a Nyquist limit of 50–60 cm/sec) and hence assess the degree of obstruction to blood flow created by the supramitral ring. Continuous wave Doppler guided by CFD, with the cursor placed over the turbulent region of blood flow, may help assess the gradient across the ring. However, the Doppler alignment should be parallel to the flow across the constriction for correct estimation of the gradient across it. In our case, Doppler alignment was a problem with the angle of insonation of the Doppler beam across the constriction >30°, and hence correct estimation of a gradient across the ring was not possible. A supramitral ring must be differentiated from cor triatriatum sinister (Fig. 4) because in both these entities the LA is divided by a membranous septum into upper and lower chambers. Table 1 summarizes the key features by which these 2 congenital abnormalities can be clearly demarcated. The hallmark feature that helps differentiate the 2 is that in a supramitral ring, the membrane is attached below the opening (or on the floor) of the LA appendage (Fig. 2), whereas in cor triatriatum sinister, the membrane is always located above the opening of the LA appendage.4
Supravalvular mitral ring rarely occurs as an isolated defect.5 Other congenital heart defects frequently coexist (90%).5 In Shone’s syndrome, it coexists with parachute mitral valve, subvalvular aortic stenosis, and coarctation of aorta.6 In our patient, an ostium secundum ASD was associated with the supramitral membrane.
In conclusion, supramitral ring is a rare congenital anomaly that presents as supravalvular mitral stenosis. A comprehensive TEE examination is warranted in all cases not only to identify/describe associated structural and/or functional derangements of mitral valve apparatus but also to exclude other associated congenital anomalies and to confirm adequate surgical resection.
Clinician’s Key Teaching Points
By J. Skubas, MD, and Roman M. Sniecinski, MD
- Several types of horizontally, parallel to the mitral annulus, oriented membranes can occur within the left atrium, functionally dividing it into 2 chambers. If the orifice between the chambers is small enough, physiologic mitral stenosis may result.
- Two such membranes that cause similar echo findings in the left atrium are a supravalvular mitral ring, a rare type of congenital membrane located just proximal to the mitral annulus, and a cor triatriatum sinister, a duplication or dissection of the interatrial septum that presents as a divided left atrium. In a supravalvular mitral ring, the proximal chamber is always the largest, with its floor distal to the opening of the left atrial appendage. In cor triatriatum sinister, the membrane is always located above the orifice of the left atrial appendage and the distal chamber is usually largest.
- In this case of a child with an ostium secundum atrial septal defect, a supravalvular mitral ring was imaged in the midesophageal and transgastric basal long-axis views. Color flow Doppler across the opening between the 2 chambers demonstrated turbulent flow with significant flow restriction into the lower chamber. Transesophageal echocardiography (TEE) was also used to exclude a parachute mitral valve, subvalvular aortic stenosis, and aortic coarctation, which along with a supravalvular mitral ring make up a constellation of findings termed Shone syndrome.
- Because the TEE probe images from directly behind the left atrium, it is better suited than transthoracic echocardiography to determine which type of atrial membrane may be present, as well as assess the severity of physiologic mitral stenosis if present. This is important because the risk of surgical resection may vary depending on what structures (the mitral valve or left circumflex coronary artery, e.g.) are in close proximity to the membrane and may be damaged by surgical intervention.
Name: Saikat Bandyopadhyay, MBBS, DA, MD, FTEE.
Contribution: This author is the chief consultant cardiac anesthesiologist who was clinically responsible for the case. This author drew the diagrams, designed and labelled the images, compiled and edited the videos, and helped write the manuscript.
Attestation: Saikat Bandyopadhyay approved the final manuscript.
Name: Kayapanda Mandana, MBBS, MS, MCh, FETCS.
Contribution: This author is the chief operating cardiac surgeon, and helped write the manuscript.
Attestation: Kayapanda Mandana approved the final manuscript.
This manuscript was handled by: Martin London, MD.
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