A 19-year-old woman was sent for a two-dimensional transthoracic echocardiography because of dyspnea on mild exertion of recent onset. An echocardiography revealed a membrane dividing the left atrium (LA) into a posterosuperior chamber (SC) and an anteroinferior chamber (IC) (figure 1A, video 1). However, no Doppler colour flow acceleration (figure 1B, video 2) or pressure gradient was noted. LA appendage was included in IC (figure 1C). The right heart and pulmonary artery pressure were both normal. A three-dimensional echocardiography showed the membrane in greater detail and revealed the orifice through which the blood flowed …