Nonobstructive Vertical Folding Supramitral Membrane

Dear Editor,

A 21-year-old woman with complaints of palpitation, atypical chest pain, and dyspnea on exertion (New York Heart Association Functional Class II) was referred to our echocardiography laboratory for further evaluation. The patient’s physical examinations and electrocardiography were unremarkable. Transthoracic echocardiography showed normal left and right ventricular size and systolic function, normal left atrial (LA) size, a bicuspid aortic valve (type zero) without significant stenosis, a nonstenotic subaortic membrane, and a membrane above the mitral valve (MV) accompanied by an increased MV gradient (the mean pressure gradient = 5 mmHg) and mild eccentric to lateral mitral regurgitation [Figure 1a and Video 1]. Two-dimensional transesophageal echocardiography demonstrated a floppy, redundant folding membrane attached to the interatrial septum and the mid-anterior MV leaflet restricting left ventricular inflow [Figure 1b, c and Video 2]. Three-dimensional (3D) transesophageal echocardiography clearly demonstrated the vertical to MV position of this membrane and additional attachment to the lateral wall of the LA, resulting in a left ventricular inflow orifice area of 1.6 cm², but there was no valvular mitral stenosis (the MV area by 3D planimetry = 3.1 cm²), suggestive of the functional obstruction of the MV [Figure 2 and Video 3].

Figure 1.

(a) A membrane in the left atrium (LA) is attached to the anterior mitral valve (MV) leaflet in the parasternal long-axis view of transthoracic echocardiography. A membrane in the LA is attached to the mid-anterior MV leaflet (arrow) in the modified upper-esophageal five-chamber view of transesophageal echocardiography (b) and to the interatrial septum (arrow) in the mid-esophageal four-chamber view (c). MV = Mitral valve, LA = Left atrium, LV = Left ventricle, AO = Aorta

Figure 2.

A membrane (arrow) in a vertical position to the mitral valve (MV) is attached to the interatrial septum and the left atrium lateral wall in the MV surgical view of three-dimensional transesophageal echocardiography in diastole (a) and systole, showing the folding of the membrane (arrow) (b). AV = Aortic valve, IAS = Interatrial septum, LAA = Left atrial appendage, MV = Mitral valve

To the best of our knowledge, this report is the first case of a vertical folding membrane above the MA that was floppy and redundant with attachment at three points to the interatrial septum, the lateral atrial wall, and the mid-anterior MV leaflet.

The presence of the LA band in various cases was demonstrated previously.[1] However, the width of the bands is small while the width of the membrane is large. Hence, our case was more compatible with a membrane than a band.

A prior study reported a case of an isolated aneurysmal supravalvular mitral ring mimicking cor triatriatum in a young adult.[2] Another investigation reported a supramitral membrane attached to the posterior MV leaflet and the LA wall mimicking posterior MV prolapse.[3] Cor triatriatum sinister and supramitral rings are rare congenital heart diseases dividing the LA into two chambers; however, the position of the membrane differs between these two anomalies.[4] In cor triatriatum sinister or cor triatriaum dexter, a sagittal membrane or a fibromuscular band divides the atrium into two chambers.[5] In cor triatriatum sinister one of them containing the MV and the LA appendage and the other one containing the pulmonary veins. In supramitral rings, there is a sagittal membrane or ring just above the MV, the LA appendage and the pulmonary veins are in the superior chamber, and the MV apparatus is beneath the membrane or the ring. Atrial chordae tendineae can be considered a differential diagnosis; nonetheless, atrial chordae tendineae is thin and seen in only some planes of echocardiography not easily obtainable.[6]

Compared with the literature, our case was a mimicker of cor triatriatum or supramitral rings. Nevertheless, in our case, the membrane was vertical, and its appearance was a floppy, redundant folding membrane above the MV that attached at three points to the interatrial septum, the lateral LA wall, and the mid-anterior MV leaflet. Accordingly, this type of LA membrane, as a kind of supramitral membrane, should be considered in the differential diagnosis of cor triatriatum sinister and supramitral rings.

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