Surgical Treatment of Accessory Mitral Valve Tissue in a Toddler

Abstract

Accessory mitral valve tissue causing left ventricular outflow tract obstruction is exceedingly rare in children. We present the surgical treatment of a toddler with severe obstruction due to type IIa accessory mitral valve tissue. Intraoperatively, the accessory tissue resembled a duplicated mature mitral valve leaflet. Surgical treatment included a septal myectomy and imbrication of the accessory tissue without leaflet resection, which successfully relieved the obstruction and preserved valve function. This case emphasizes the importance of awareness of this diagnosis and classification system as standard mitral repair techniques may not be appropriate or effective, depending on the anatomic subtype.

Accessory mitral valve tissue (AMVT) is a rare congenital anomaly characterized by redundant membranous or fibrous tissue attached to any part of the mitral valve leaflets or annulus. This abnormality occurs in approximately 1 in 26,000 (0.004%) echocardiograms and is more common in male patients.¹ The exact mechanism of AMVT formation remains unclear, but it is hypothesized that the anomaly arises because of faulty separation of accessory mitral tissue from the endocardial cushions during embryogenesis.²

AMVT can lead to varying degrees of left ventricular outflow tract obstruction (LVOTO) and may be associated with a wide range of other cardiovascular anomalies, including atrial and ventricular septal defects, coronary anomalies, and aortic coarctation.³ The diagnosis of AMVT as a cause of LVOTO is extremely rare, with few adult and fewer pediatric cases reported in the literature.⁴ In children, LVOTO (all types) accounts for roughly 6% of all congenital heart abnormalities.⁵

We present the unusual case of a 23-month-old boy with LVOTO resulting from AMVT and provide details of the effective diagnosis, anatomic subtype, and surgical repair.

An asymptomatic toddler had been observed since birth for AMVT. The initial diagnosis was made when a transthoracic echocardiogram was obtained to evaluate an isolated 3/6 systolic ejection murmur. On return to the clinic at 23 months of age, the patient’s weight was 13.5 kg, his blood pressure was 98/53 mm Hg, and his heart rate was 122 beats/min. Oxygen saturation was 98%, with a respiratory rate of 20 breaths/min. His chest radiograph showed well-expanded lungs without edema or pleural effusion, and there was no cardiomegaly. An electrocardiogram revealed normal sinus rhythm and left ventricular hypertrophy.

His follow-up transthoracic echocardiogram demonstrated mild left atrial dilation, normal left ventricular dimensions with normal systolic function, and minimal left ventricular hypertrophy (Figure 1A). The mitral valve annulus measured 21 mm in diameter (z score, 2.2) and was severely dysplastic with abundant prolapsing AMVT, moderate regurgitation, and mild mitral stenosis (mean gradient, 6 mm Hg). In addition, the prolapsing AMVT was observed in the subvalvular region, with suspected attachments to the anterior leaflet and ventricular septum approximately 2 cm below the aortic valve annulus. The attachments prolapsed into the left ventricular outflow tract (LVOT), causing severe obstruction (Figures 1A-1C). After multidisciplinary discussion, the patient was referred for surgical intervention for severe LVOTO.

Figure 1.

Preoperative transthoracic echocardiography views of redundant mitral valve tissue (red arrows). (A) Apical 4-chamber view. (B) Parasternal long axis of left ventricular outflow tract. (C) Deep transgastric long axis 2-dimensional and color flow showing left ventricular outflow tract obstruction. (D) Intraoperative photograph showing accessory mitral valve tissue (large white arrow), viewed through the left atrium. Normal mitral leaflet tissue is not visible in this image.

With parental consent, the patient was brought to the operating room. Preoperative transesophageal echocardiography was performed, which confirmed the diagnosis of AMVT prolapsing into the LVOT and attached to the mitral leaflets. The resultant LVOTO was severe, with a transesophageal echocardiography–based peak gradient of 83 mm Hg through the LVOT (mean LVOT gradient, 51 mm Hg).

The operation was performed through a median sternotomy on cardiopulmonary bypass using bicaval cannulation and mild hypothermia. After aortic cross-clamping and cardioplegic arrest, the left atrium was opened, and the mitral valve was inspected and tested for competency by insufflating it with saline. The mitral valve was moderately regurgitant with abundant accessory mitral tissue resembling a mature duplicate of the mitral valve leaflet (Figure 1D). On closer inspection, the accessory tissue was entirely attached to the leading edge of the anterior leaflet of the mitral valve and the chordae below (type IIa). It appeared that resection of the accessory tissue would damage the subvalvular apparatus of the anterior mitral leaflet, risking prolapse and worsened regurgitation. To adequately inspect the LVOT and its relationship to the accessory mitral tissue, an aortotomy was performed. Through the aortic approach, a limited septal myectomy was performed, which significantly opened the outflow tract and provided better exposure of the anterior mitral leaflet. From the left atrial approach, the accessory tissue was then imbricated (rolled under to flatten it) and tacked with interrupted mattress sutures to the anterior mitral leaflet to prevent it from billowing into the outflow tract. This approach also eliminated the secondary restriction of the anterior mitral leaflet that contributed to the mitral regurgitation observed preoperatively. No mitral valve tissue was resected.

The postoperative transesophageal echocardiogram showed resolution of the LVOTO and the mitral regurgitation (Figure 2A). The patient tolerated the procedure well and had an uneventful recovery. His discharge echocardiogram demonstrated no LVOTO, trivial mitral regurgitation, and no significant residual mitral stenosis (mean, 3 mm Hg; Figures 2B, 2C).

Figure 2.

Postoperative transesophageal echocardiography and transthoracic echocardiography (TTE) images. (A) Transesophageal echocardiography deep transgastric long axis with color flow showing no residual accessory mitral valve tissue (denoted by star) in left ventricular outflow tract. (CPB, cardiopulmonary bypass). (B) TTE parasternal long axis of left ventricular outflow tract with color flow showing no residual accessory mitral valve tissue (denoted by star) in left ventricular outflow tract. (C) TTE apical 4-chamber view obtained 16 months after surgery showing postoperative appearance of mitral valve and improvement of left atrial dilation.

Comment

It is important to consider AMVT in the differential diagnosis for a child with LVOTO and an isolated murmur. Although AMVT is a rare congenital anomaly, it can lead to progressive, severe LVOTO requiring surgical intervention.⁶ This case study highlights the importance of appropriate surgical exposure for thorough intraoperative assessment of the valve anatomy, including the subvalvular apparatus, and a customized mitral valve repair strategy for AMVT based on the anatomic subtype.

Here, the accessory mitral tissue was entirely attached to the leading edge of the anterior leaflet of the mitral valve and the chordae below, classified as type IIa.² Given recent increases in the detection of AMVT through echocardiography,⁷ a classification system for AMVT has been proposed on the basis of the anatomic site of attachment. Type I AMVT has attachments on the left atrial side of the leaflets (supravalvular); type II has attachments on the mitral leaflets (IIa, accessory tissue is involving only the mitral leaflets or ventricular aspect of the anterior mitral valve leaflet; IIb, AMVT may have insertion on the subaortic level of the intraventricular septum or aortic valve); and type III has attachments on the left ventricular side of the mitral leaflets (subvalvular).²

The successful surgical outcome in this 23-month-old patient underscores the efficacy of imbrication in treating LVOTO without the need for tissue resection. Accurate diagnosis and appropriate surgical strategies are crucial for managing such cases and improving patient outcomes.⁸ Future treatment strategies and patient management will benefit from considering AMVT in the differential diagnosis for LVOTO.