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Congenital Mitral Valve Stenosis: Anatomic Variants and Surgical Reconstruction

      Congenital mitral valve stenosis is a heterogeneous group of lesions that can occur as an isolated defect or, more commonly, in association with other left heart obstructive defects. Age at presentation, presence and severity of pulmonary hypertension, and location of the primary obstructing lesion have been shown to be important risk factors for survival and long-term outcomes. Anatomic features vary, and obstructing tissue or tethering structures can be present at all levels of the valve, including supra-annular, intra-leaflet, and sub-valvar. Surgical techniques aim to remove abnormal tissue that is causing the obstruction or impediment to adequate leaflet mobility, and improve the mobility of the sub-valve structures. We describe a series of surgical options for mitral valve reconstruction that address the specific pathologies frequently found in congenital mitral stenosis.

      Introduction

      Congenital mitral stenosis (MS) is a spectrum of defects that result in functional and anatomic obstruction of inflow into the left ventricle. In the more common forms, the anatomic defects encompass the valve leaflets and the sub-valve apparatus. In less common forms, there is accessory tissue that forms at or above the valve annulus causing obstruction or tethering of the valve annulus and preventing growth. The clinical presentation depends on the degree of obstruction, the presence of regurgitation, the presence and severity of associated pulmonary hypertension, and the presence of associated lesions (eg, hypoplastic left heart, aortic stenosis, and coarctation).

      Classification and Anatomic Features

      A number of classifications have been proposed for congenital MS. Ruckman and Van Praagh
      • Ruckman R.N.
      • Van Praagh R.
      Anatomic types of congenital mitral stenosis: report of 49 autopsy cases with consideration of diagnosis and surgical implications.
      proposed a simple classification based on pathologic findings of autopsy specimens.
      • Ruckman R.N.
      • Van Praagh R.
      Anatomic types of congenital mitral stenosis: report of 49 autopsy cases with consideration of diagnosis and surgical implications.
      Four categories were described, which included typical MS with short chordae tendineae, obliteration of interchordal spaces and reduction of interpapillary distance; hypoplastic congenital MS almost always associated with a hypoplastic left heart syndrome; supramitral ring; and parachute mitral valve. This morphologic classification, while useful in comparing pathologic groups, did not clearly reflect the mechanisms responsible for obstruction and the fact that multiple pathologies are often seen in the same valve. More recently, Carpentier and Chauvaud
      • Carpentier A.
      • Chauvaud S.
      • Mihaileanu S.
      Classification of congenital malformations of the mitral valve and their surgical management.
      described a functional classification based on the location of the major lesion (Table 1).
      Table 1Functional Classification of Congenital Mitral Stenosis
      Type A (normal papillary muscles)
      Supra-valvular ring
      Leflet fusion (intra-leaflet ring)
      Type B (abnormal papillary muscles
      Parachute deformity
      Papillary muscle abnormality
      The etiology of congenital MS is not known, but many of the morphologic features seen in young children resemble the stages of early valve formation (Fig. 1) . During the earliest phase of formation of the endocardial cushion, myocardial cells and collagen-forming mesenchymal cells are intermixed as the compact layer begins to develop. As the valve leaflets form, distinct muscle pillars begin to form, which eventually will become the papillary muscles of the mitral valve. The regulatory mechanisms for this process are now being worked out, although the abnormality responsible for the arrested or malformation of the mitral valve apparatus is not yet known.
      • Armstrong E.J.
      • Bischoff J.
      Heart valve development: endothelial cell signaling and differentiation.
      Figure thumbnail gr1
      Figure 1Mitral valve development from early endocardial cushions to mature valve formation.
      (from Layman and Edwards
      • Layman T.E.
      • Edwards J.E.
      Anomalous mitral arcade A type of congenital mitral insufficiency.
      )
      The most frequently observed anatomic features of congenital MS are short or even absent chordae, posterior tethering of the papillary muscles to the left-ventricular free wall, fusion or poor development of the commissures, and often the two papillary muscles are connected by a thick fibrous or muscle band that keeps them close together (Fig. 2) . While leaflets are often thicker than normal, they are usually pliable in younger children and do not pose a significant restriction. There can exist, however, a thick band of connective tissue along the entire edge of the leaflets and extending to each papillary muscle, which severely restricts leaflet motion. The functional result can be both stenosis and severe regurgitation, particularly in young infants. In older children, this fibrous band can be very prominent and the term mitral “arcade” has been used to describe the appearance of the anterior leaflet.
      • Layman T.E.
      • Edwards J.E.
      Anomalous mitral arcade A type of congenital mitral insufficiency.
      In some cases, the annulus of the mitral valve can be dilated resulting in severe mitral regurgitation. Severe left atrial and left ventricular enlargement is often seen in association. This combination of anatomic and function defects can be very challenging to repair.
      Figure thumbnail gr2
      Figure 2Typical congenital mitral stenosis with marked reduction of the space between the anterolateral (ALP) and the posteromedial (PMP) papillary muscle groups, which are identifiable but fused. Both papillary muscle groups are present and both receive insertions from the shortened and thickened chordae tendineae, but the anomaly may be regarded as a forme fruste of parachute mitral valve. The mitral valve orifice is markedly reduced; there is only one small opening above the interpapillary muscle space (IPMS) and another small aperture at the anterolateral commissure (ALC) (from Ruckman and Van Praagh
      • Ruckman R.N.
      • Van Praagh R.
      Anatomic types of congenital mitral stenosis: report of 49 autopsy cases with consideration of diagnosis and surgical implications.
      ). FR, fibrous ring.
      Other forms of congenital MS include a fibrous ring of tissue adherent to the mid portion of the leaflets also creating fusion of the two leaflets at the commissures that restricts mobility and inflow across the mitral leaflets. The term “supra-mitral ring” is commonly used to describe the presence of this structure, which can vary in thickness from very thin inelastic membrane to a thick fibroelastic layer of tissue. This tissue can encircle the valve partially or completely, and can include the commissures resulting in fusion. There are two forms generally recognized. One is a true supra-mitral ring, which attaches above the native valve annulus, restricting its growth and causing obstruction by forming a doughnut-like ring structure, analogous to the membrane seen in discreet sub-aortic stenosis. The second is an intra-mitral ring of tissue, also varying in thickness and extent around the circumference of the valve.
      • Toscano A.
      • Pasquini L.
      • Iacobelli R.
      • et al.
      Congenital supravalvar mitral ring: an underestimated anomaly.
      The latter type is more common and directly restricts leaflet motion.

      Surgical Approaches

      Exposure

      For visualization of the mitral valve, a number of approaches have been described, including a vertical trans-atrial septal incision, transverse atrial septal incision, and a left atrial incision at the inter-atrial groove, with or without division of the superior vena cava. Each approach has its limitations, including limited incision size in the vertical trans-septal, potential for injury to atrioventricular (AV) node with the transverse trans-septal, and sino-atrial (SA) node dysfunction and/or superior vena cava (SVC) stenosis in the latter approach. Regardless of the approach, in experienced hands, all of these incisions can work well. Combining the incision with strategically placed traction sutures, the valve can be pulled into an optimal angle and usually ensures adequate visualization of the entire valve and sub-valve structures. Opening the pericardium and left pleura widely also facilitates rotation of the apex of the heart posteriorly and leftward, improving the view of the sub-valve structures (Fig. 3A).
      Figure thumbnail gr3
      Figure 3(A) Exposure to the mitral valve is shown via a trans-septal approach. The atrial surface of both mitral leaflets and the thickened ring of tissue restricting the leaflet motion and fusing the commissures can be seen. (B) Supravalve mitral ring. Resection by either sharp or occasionally blunt dissection developing a plane between the leaflet and the ring of connective tissue can be achieved by releasing the leaflets and separating the adherent leaflets at their commissures (top); leaflet commissures are more clearly defined once the adherent layer of fibroelastic tissue is removed (bottom). Ao, aorta; SVC, superior vena cava; RA, right atrium; IVC, inferior vena cava; MV, mitral valve.

      Congenital supravalvar mitral ring

      The valve repair technique must be tailored to the mechanism of dysfunction. For supra-mitral ring attaching above the valve annulus, direct resection can be performed and usually results in release of the restricted valve annulus with an adequate diameter in most cases. For the intra-mitral ring where thickened tissue covers the atrial surface of the leaflet and often crosses the commissures, resection can be difficult, especially in young infants because the ring tissue is usually very adherent to the native leaflet. With care however, the majority of this tissue can be removed, which releases the leaflet substantially. Similarly, this tissue must be peeled or resected from the leaflets at the commissures to enlarge the effective orifice (Fig. 3B). This lesion is different than the mitral arcade described by Layman and Edwards,
      • Layman T.E.
      • Edwards J.E.
      Anomalous mitral arcade A type of congenital mitral insufficiency.
      where the thickening is at the leading edge of the leaflets and often there are abnormal chordal attachments, particularly to the posterior leaflet.

      Leaflet thickening and short chords

      In cases where the leaflet mobility is restricted, often it is caused by growth of fibroelastic tissue as a membrane on the leaflets themselves or as a thick chord, such as that seen in mitral arcade. The anterior leaflet is usually affected more by this process than the posterior leaflet, but in older children, both leaflets often are involved. The membrane type of tissue most often can be peeled off the native leaflet using a combination of sharp and blunt dissection, similar to the technique of endarterectomy in a diseased vessel (Fig. 3B). Often the fibroelastic membrane has dense attachments at the leaflet free edge, particularly at the edge chord insertions. Sharp dissection must be used for complete removal. In the arcade type defect, this fibrous tissue is densely adherent to the leaflet edge and must be excised sharply. The leaflet edge is often rolled and extra care must be taken to prevent significant leaflet injury. Here direct traction on the leaflet to create tension of the tethering abnormal tissue facilitates its removal and also gives excellent exposure to the fused chords seen in these cases (Fig. 4) . Removal of the connective tissue layer causing chordal fusion can sometimes be performed in young infants. In older children, this tissue is densely adherent to the chords and therefore chordal splitting with sharp dissection is usually required. Because the leaflet edge can be foreshortened, repair may require leaflet extension, usually at the annulus attachment, with pericardium.
      Figure thumbnail gr4
      Figure 4Posterior leaflet retracted by suture to place tension on the tethering connective tissue that needs to be removed. Similar traction can be used on the anterior leaflet to facilitate removal of adherent connective tissue on the edge of the leaflet.

      Papillary muscle tethering and multiple papillaries (“Hammock” valve)

      In the most common form of congenital MS, the major abnormality is maldevelopment or arrested development of the papillary and chordal structures. There are often more than two papillary muscles, with the additional papillaries attaching primarily to the posterior leaflet. Secondary chords arising from the posterior free wall of the ventricle frequently attach to the mid portion of the posterior leaflet and are often short and thick, resulting in complete immobility of that leaflet (Fig. 5) . Division of these chords is facilitated by detachment of the posterior leaflet. Elongated papillary muscles that attach directly to the anterior leaflet must be split and fused chords must also be incised axially to gain some mobility. In young infants, this can be very challenging because the chords cannot be easily identified, risking transection and creation of a flail segment of the leaflet. In older children, chordal replacement offers a solution to an otherwise difficult problem. Further leaflet mobility can be gained by leaflet extension with pericardium. This type of extension is most useful following detachment of the posterior leaflet but can also be used in cases where the anterior leaflet is deficient. For posterior leaflet augmentation, it is best to use an oval-shaped patch that extends to the central portion of the leaflets (P1 and P3 by Carpentier's criteria; Fig. 6) . Extending the leaflet to the commissures can result in folding of the patch, creating a gutter effect limiting the mobility of the posterior leaflet.
      Figure thumbnail gr5
      Figure 5Congenital MS with a combination of a connective tissue layer on the valve leaflets, short fused chords, multiple papillary muscles, and accessory chords tethering the posterior leaflet. To gain access to the subvalvar structures, detachment of the posterior leaflet close to the annulus facilitates access to the accessory chords and papillary muscles. The incision needs to extend from commissure to commissure. Aggressive delamination of the papillary muscles leaving only the apical-most attachments helps mobilize these structures for improved leaflet mobility. Inset, lower right: cross-sectional view of the leaflet incision and papillary muscle dissection. LA, left atrium; LV, left ventricle.
      Figure thumbnail gr6
      Figure 6Extension of the posterior leaflet can be achieved with autologous treated pericardium or similar xenograft material such as porcine intestinal submucosa or “matrix” patch. Care must be taken not to extend the patch completely to the commissures because this can result in folding of the patch limiting its usefulness.

      Results

      The success of mitral valve reconstruction for congenital MS is affected by factors such as age at presentation, the presence of associated mitral regurgitation along with significant stenosis, and the extent of involvement of the valve and sub-valve apparatus. In our experience, mitral valve replacement has been rare as a first intervention in children. In most cases, improvement in valve function can be achieved with the reconstructive procedures described above. The operative mortality has also been relatively low. In the 6 years from 2005 to 2010, 49 children were operated at our center for primary congenital MS; 23 presented with pure MS and 26 had combined congenital MS and significant mitral regurgitation. Operative survival was 94% (46/49 patients), and there were no late deaths in a follow-up period of up to 60 months. The current results compare favorably with our previously reported experience.
      • McElhinney D.B.
      • Sherwood M.C.
      • Keane J.F.
      • et al.
      The presence of mitral regurgitation was not associated with increased mortality. However, age at presentation was a risk factor, with two of the three deaths in the group being neonates (Fig. 7; Table 2).
      Figure thumbnail gr7
      Figure 7Freedom from reoperation in children following mitral valve reconstructive surgery shown by primary indication for surgery. Mixed lesion with significant mitral valve stenosis and regurgitation (left). Freedom from reoperation shown by age group (right). MR, mitral valve regurgitation; MS, mitral valve stenosis; MS/MR, mixed lesion with significant mitral valve stenosis and regurgitation.
      Table 2Operative Survival by Age Group
      Death
      Age GroupsAliveExpiredTotal
      Neonates426
      Infants13114
      Children29029
      Total46349
      The incidence of reoperation was also relatively low, considering the age at presentation and the wide range in severity of the lesions (Fig. 7A). Seven patients required reoperation during the follow-up period, for a freedom from reoperation of 86% at 5 years. In some cases, re-repair was feasible; however, in most, replacement was required.

      Summary

      Using a tailored surgical approach, most cases of MS can be repaired with significant relief of obstruction. Even in cases where significant mitral regurgitation is also present, the risk of the procedure and the late outcomes were not different than with MS alone. However, the most challenging cases for the surgeon are lesions with abnormal leaflets and severe regurgitation. While there is an ongoing need for re-operation, in most current series, the need for valve replacement is uncommon.

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        • Chauvaud S.
        • Mihaileanu S.
        Classification of congenital malformations of the mitral valve and their surgical management.
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