DISCUSSION
In some cases, AVMT is a rare anomaly in childhood and may be detected as an incidental finding or cause LVOT obstruction. This malformation is associated with other congenital heart malformations, such as atrial septal defect, ventricular septal defect, transposition of the large arteries, patent ductus arteriosus, aortic coarctation, bicuspid aortic valve, mitral cleft, and dextrocardia.7-9 The embryological mechanism of AMVT formation is not clear and may be related to abnormal or incomplete separation of the mitral valve from the endocardial cushions.9-11 In the anatomical classification, there are three types of AVMT: type I: AMVT with attachments at the supra leaflet level, type II: AMVT attachments on the mitral leaflets and type III: AMVT attachments below the mitral leaflets.
Most cases of AMVT are associated with other cardiac malformations. The clinical signs and symptoms of this anomaly vary depending on its location and concomitant cardiac malformations. As in this case, congenital AMVT may remain asymptomatic for many years. In adult patients, AMVT may be diagnosed incidentally intraoperatively. Although AMVT can remain asymptomatic in patients, it can lead to LVOT obstruction in some patients. As in our case, most adolescent and adult cases are patients with vague symptoms in whom an echocardiography was performed for another indication. The most common symptoms are chest pain, dyspnea and palpitations. In addition, a few cases have been found to be associated with transient ischemic attack (TIA) and AMVT.7,10-13 The most common complaints in our patient were shortness of breath and atypical chest pain. Her neurological examination was normal and no TIA was detected in her medical history. The diagnosis of AMVT should be considered especially in patients with an unexplained cerebrovascular accident with a heart murmur and tissue causing signs of LVOT obstruction in the subaortic area.
Accessory mitral valve tissue resection is recommended in symptomatic isolated AMVT patients with a significant LVOT gradient (mean gradient ≥25 mmHg) or in patients who were scheduled for cardiac surgery for additional cardiac pathology.7,10 Diagnosis of AMVT may be difficult before or during surgery. The echogenicity of AMV tissue is similar to that of endocardial structures. Therefore, surgeons should know the clinical and pathological features of AMVT to avoid overlooking it during surgery.1,7-10
Echocardiography is considered the gold standard for imaging AMVT. Transthoracic echocardiography (TTE) is sufficient for diagnosis, but perioperative TEE is important both for confirming the diagnosis and completing the surgical procedure, and for assessing postoperative mitral valve functions and possible complications that may occur after excision of AMVT.1-6,8 In particular, three-dimensional (3D) TEE allows assessment of the location of the AMVT attachment and the presence of an accompanying mitral lesion. We performed a TTE and a 3D-TEE which showed that the AMVT attached to the anterior mitral leaflet, with a subaortic attachment, resulting in an increase in the LVOT gradient.
Yasui et al. reported a patient who could not be separated from cardiopulmonary bypass due to LVOT caused by an overlooked AMVT, and pointed out that detection of this anomaly during surgery is difficult without an accurate presurgical diagnosis.14 Surgical excision of the AMVT should be carefully performed without affecting the functions of the mitral valve and surrounding structures.8,9,14