Unexpected echocardiographic findings ： Accessory Mitral Valve Tissue in Adults

Background: Accessory mitral valve tissue (AMVT) is a rare congenital cardiac anomaly, which is associated with other congenital heart diseases. It is diagnosed in neonates or childhood and rarely in adulthood. Nevertheless, AMVT is an incidental finding or described as isolated. Echocardiography, especially three-dimensional(3D) echocardiography is considered as an optimal imaging technique for AMVT diagnosis. Case presentation ： We herein presented the two asymptomatic adult cases with AMVT, who presented with varying degrees of symptomatic left ventricular outflow tract (LVOT) obstruction. One presented with mild LVOT obstruction and no surgery was required, and another one with significant LVOT obstruction was recommended for surgical excision. Conclusions: We emphasized the usefulness of echocardiography in the morphology detection of AMVT, and the importance of operation guidance and follow-up.


Background
Assisted mitral valve tissue (AMVT) was first reported as early as 1842 by Chevers et al 1 , as a rare congenital cardiac anomaly. Symptomatology of the patient commonly manifest asymptomatic heart murmur or symptoms of LVOT obstruction, such as chest pain, syncope, or palpitations 4,5 .
Although AMVT is often associated with other congenital heart diseases 2 , it may be seen as isolated.
It is often detected in neonates or childhood and rarely in adults 3 . Three-dimensional(3D) echocardiography is considered as an optimal imaging technique for AMVT diagnosis. We herein presented the two asymptomatic adult cases with AMVT, who presented with varying degrees of symptomatic left ventricular outflow tract (LVOT) obstruction. One presented with mild LVOT obstruction and no surgery was required, and another one with significant LVOT obstruction was recommended for surgical excision.

Case 1
A 33-year-old man with a medical history of ventricular septal defect (VSD) repair surgery in seven years ago, and no other comorbidity, was referred to cardiology services for assessment on July, 2018. He didn't have any history of chest pain, dyspnea or syncope. On admission, his vital signs were normal. At the left sternum, physical examination revealed a systolic murmur of the third and fourth intercostal spaces were 2/6 grade medium, and there was no radiation to the neck. The electrocardiogram was unremarkable. Other laboratory data were also normal. Two-dimensional Even if the AMVT was not identified in the patient's previous echocardiographic examinations in another cardiac institute, the current diagnosis of AMVT was clear. Given that there were no significant LVOT obstructions or symptoms, we considered that surgery is unnecessary. The patient was advised echocardiographic follow up and aspirin therapy to reduce the risk of thromboembolism events. At one year of follow-up, no significant changes in morphology and LVOT obstruction were caught by echocardiography. This patient did not have any new cardiac adverse events.

Case 2
A 46-year-old woman was transferred from cardiovascular Surgery Department to our clinic for the reassessment of patient before surgery. There was no any sign on physical examination other than a 3/6 systolic ejection murmur with faint radiation to the neck. TTE showed normal-sized cardiac chambers and the left ventricular ejection fraction was 0.79. Concurrently, ascending aorta dilatation was also found with diameter up to 37 mm with mild to moderate aortic regurgitation.

Discussion
AMVT is a rare congenital malformation, which may be caused by abnormal or incomplete separation of the mitral valve from the endocardial cushions 1 . It may be isolated or combined with other congenital heart anomalies. The usual age range for diagnosis of AMVT was from newborn to 77 years (average 8.6 years), while the incidence of AMVT in adults was 1/26,000 based on echocardiography 4 . Prifti et al 6 . provided a classification of this anomaly based on intraoperative description and anatomic presentation. As to AMVT morphology, sac-like, balloon-like, parachutelike, sail, leaflet-like, sheet, membrane, or pedunculated mass were demonstrated 8 . The patients in our case had a mobile leaflet-like structure and no well-developed chordae tendineae. Therefore, we classified our cases into Type IIB1. Echocardiography can clarify the morphology and attachment points of the AMVT, particularly in patients scheduled for surgery 11,12 . Typically, we need to differentiate AMV from redundant mitral valve chordae and other structures in LVOT. For example, redundant mitral valve chordae may be involved in chordal systolic anterior motion with dynamic LVOT obstruction and similar to AMVT in case of chordal rupture 10 . We could also choose cardiac computed tomography and CMR, especially CMR to provide not only validation of AMVT types but also volume quantification for obstruction. Identifying AMVT during operation is not always possible, bypass causing collapse of the thin structure in empty and arrested left ventricle. In the patient of our case 1, the AMVT was not identified when VSD was repaired seven years ago.
Symptoms of patients with AMVT depend on the degree of LVOT obstruction, knock-on effects on the aortic and mitral valves and on concomitant cardiovascular malformations 7 . Just like the patient in our case 2, whom has an obvious LVOT obstruction (> 50mmHg) with associated symptoms and eventually undergo surgical treatment. In addition, AMVT is susceptible to embolism of the neurological events, so surgery is also recommended 13 . For patients without severe LVOT obstruction, a serial echocardiographic follow-up is recommended to assess the progression of the gradient without the need for prophylactic operation.

Conclusion
AMVT is a rare congenital cardiac abnormality with various clinical manifestations, which also results in one of the rare causes of LVOT obstruction. It should be always considered in the differential diagnosis of LVOT obstruction, especially in pre-or post-operative of congenital heart disease patients. Hence, echocardiography plays a crucial role in diagnosis, treatment and follow-up.