Congenital mitral valve abnormalities are rare and cause mitral stenosis, regurgitation, or a combination of the two. Three-dimensional echocardiography has provided new insight into the structure and function of both normal and abnormal mitral valves. Three-dimensional imaging permits accurate anatomic diagnosis and enhances two-dimensional echocardiographic data. Moreover, it enables echocardiographers to communicate effectively with cardiothoracic surgeons when displaying, analyzing, and describing pathology. The purpose of this report is to review congenital mitral valve disease, focusing on the benefits of three-dimensional echocardiography in its evaluation.
The true incidence of congenital mitral valve (MV) abnormalities is unknown, but they are likely rare (excluding MV prolapse [MVP]). Incidence rates have been reported in series of selected patients either at autopsy with known congenital heart disease (mitral stenosis [MS], 1.2%) or in patients presenting for echocardiographic assessment (MV disease, 0.5%).
The MV complex includes the annulus, leaflets, commissures, chordae, and papillary muscles, and any of these components can be affected in congenital MV disease. These abnormalities are seen with associated congenital heart defects in >50% of cases; associated defects range from simple septal defects to left heart obstructive lesions.
The MV has been extensively evaluated by three-dimensional (3D) echocardiography (3DE). In general, this has been done in the adult population, with attention to rheumatic MS, ischemic mitral regurgitation (MR) and MVP. Mitral annular function has also been a topic of interest, as has papillary muscle function in those with functional MR. Three-dimensional echocardiography has improved our understanding of normal and abnormal MV function and is increasingly used in clinical decision making. The diagnostic advantage of 3DE over two-dimensional (2D) echocardiography (2DE) in congenital MV disease is not specifically known, and further data are required. The purpose of this report is to review features of congenital MV disease focusing on lesions in which 3DE promises to be advantageous, from the limited published literature and from our experience.
Echocardiographers routinely use 2D echocardiographic images to educate their surgical colleagues about anatomic and functional characteristics of the MV, but the way in which tomographic images are displayed on 2DE is not intuitive to many surgeons and does not correspond in appearance to the anatomy as they view it. Using volumetric 3D echocardiographic acquisitions, “en face” views of the MV can be obtained that would enable both echocardiographers and surgeons to look at the images in an easily recognizable manner. With the images of the MV displayed in a surgical view from the left atrium, surgeons instantaneously appreciate the pathology, often with more confidence than echocardiographers. The other important aspect is that morphologic and functional assessment of the MV by 3DE is physiologic, whereas surgical evaluation by inspection and saline testing is not. This does not mean that echocardiographers should abandon 2D echocardiographic assessment of the MV, but they should strive to determine the incremental value of 3DE.
Embryologic and Developmental Perspective
The atrioventricular (AV) junction becomes prominent in the fourth week of gestation, after rightward looping of the heart tube. Superior and inferior endocardial cushions are formed and progressively fill the AV canal. Fusion of these structures in the sixth week of development divides the AV canal into right and left AV junctions, with a residual cleft present at the site of fusion. Recent embryologic literature refers to the leaflets of the normal MV as “aortic” and “mural,” and these terms are equivalent to “anterior” and “posterior” leaflets, respectively. The left ventricular component of the fused cushions will form the early aortic leaflet of the MV, while the right component forms the septal leaflet of the tricuspid valve. Lateral cushions are formed around the fifth week of development, with the left lateral cushion the progenitor of the mural leaflet of the MV. The lateral cushion is initially smaller than the fused inferior and superior components of the left AV junction. Expansion of the inferior component of the left AV junction occurs, including the lateral cushion, such that by 8 weeks of development, the mural leaflet constitutes two thirds of the MV. The aortic leaflet is formed from endocardial-derived mesenchyme from the superior and inferior cushions, whereas the mural leaflet is formed as a protrusion of myocardium with progressive mesenchyme formation on its surface. The myocardium is eventually removed by apoptosis.
The papillary muscles are formed from a horseshoe-shaped ridge of trabecular myocardium running from the posterior AV junction through the left ventricular apex to the anterior AV junction. This ridge is present from 5 weeks of development and is attached to both the inferior and the superior cushions. The anterior and posterior portions of the ridge progressively elongate and delaminate from the ventricular wall, becoming the papillary muscles, while the apical portion becomes part of the trabecular network of the left ventricular apex. The papillary muscles progressively compact, with fewer trabeculations. Chordae form from cushion tissue at 11 to 12 weeks, with the papillary muscles initially connected directly to the leaflets but with gaps starting to appear in the cushion tissue at their papillary muscle tips. Chordal formation is completed by 14 weeks of development.
Abnormal MV development leads to MV pathology. Lack of fusion of the superior and inferior cushions is one of the developmental defects associated with producing AV septal defects. An isolated MV cleft is due to incomplete fusion of the leftward tip of the superior and inferior cushions. Abnormal excavation of the mural leaflet from the parietal ventricle wall produces Ebstein’s anomaly of the MV, with hinging of the mural leaflet displaced to within the ventricular cavity. Similarly, abnormalities in papillary muscle formation can lead to a parachute MV, likely due to underdeveloped space between the papillary muscles. Abnormal loosening of the cushion tissue at the papillary muscle insertion can potentially lead to abnormal chordal development and thickened or shortened chordae or the extreme form, MV arcade with direct attachment of papillary muscle to the leaflet.
Normal MV
The MV functions as a unit, with all components—the annulus, leaflets, commissures, papillary muscles, and supporting chordae—acting in unison to maintain normal valve function.
The valve annulus is saddle shaped to minimize leaflet stress, with two high and two low points ( Figure 1 ). This latter concept is important in MVP and will be discussed in detail later. The MV annulus changes shape throughout the cardiac cycle, and annular motion is heterogeneous because of the variable anatomic structures surrounding the MV. The commissure-to-commissure direction of the mitral annulus is bordered by muscle, while the anteroposterior direction has fibrous continuity with the aortic valve. During ventricular systole, annular shape contributes both to unimpeded blood flow through the left ventricular outflow tract and to efficient leaflet coaptation. Left atrial contraction contributes to reduction of the annulus area preceding ejection. The annular segments in the commissure-to-commissure direction start to contract in early systole. This is followed by diameter reduction in the anteroposterior direction and slight expansion in the commissure-to-commissure direction in mid and late systole, providing expansion of the aortic outflow tract as well as increasing leaflet coaptation. The shape of the MV annulus in children was described in a study of 17 patients. It demonstrated a similarly shaped MV annulus to that described in adults, with important interactions between the MV and the tricuspid valve. It also showed some differences in the annular dynamics. However, no studies have examined the longitudinal changes of MV shape and function with age or with disease processes in children or compared MV parameters in children between health and disease.
The normal MV leaflets have an element of tethering and prolapse, which are related to the insertion points of the chordae. The leaflets roll over each other to maintain maximum coaptation throughout systole. The anterior or aortic leaflet is smaller than its posterior or mural counterpart, with a curved zone of apposition between the two during systole ( Figure 2 ). The anterior and posterior leaflets are segmented into three individual scallops: A1, A2, and A3 for the anterior leaflet and P1, P2, and P3 for the posterior leaflet. The line of closure of the valve is along the atrial aspect, about one third of the distance from the free edge of the leaflets to their annular attachment. The area of MV coaptation can be measured with 3DE and can be considered functional reserve for regurgitation, with reduced coaptation area associated with increased regurgitation.
The leaflets are supported by two widely spaced papillary muscles and long, thin chordae. The chordae are multiple and complex. There are two main strut chordae that insert onto the anterior leaflet, with the attachments running from the free margins onto the belly of the leaflet ( Figure 3 ). These provide major support to the leaflet and are responsible for tethering in cases of ischemic MR with papillary muscle displacement. The free edges of the anterior leaflet are supported by rough zone chordae, which are smaller and multiple and prevent leaflet prolapse. Medially and laterally, there are commissural chordae that are shared by both the anterior and posterior leaflets. The posterior leaflet has a series of scallops, which are supported by cleft chordae.
The papillary muscles may have single or multiple heads. They have a broad base of attachment to the ventricular wall and maintain a constant angle with the annulus throughout the cardiac cycle. This helps minimize stress on the leaflets.
All of the above findings can be appreciated by 3DE, and indeed it is this technique that has provided these unique concepts of MV function. The valve can be visualized en face from the left atrial or left ventricular perspective, the latter suited for assessment of the anterior leaflet. The annulus of the MV is best appreciated from the left atrial perspective (“surgical view”), and dedicated software is available to quantify annular size and shape ( Figure 4 ). A recent American Society of Echocardiography guidelines document provides a comprehensive outline of the data acquisition and examination with 3DE, albeit with a focus on imaging in adults. Despite the adult focus, these views and acquisitions can be applied to the infant and pediatric population. In our opinion, the gated full-volume 3D acquisition is well suited to visualize the entire mitral apparatus. Cropping of this data set, addition of color Doppler to the acquisition, and offline software analysis are additional tools that permit further assessment of the MV apparatus, as well as quantification of stenosis or regurgitation (discussed below). Single-beat, real-time 3D images of the MV can be acquired in 3D zoom mode and may be preferred to full-volume mode in nonsedated infants. However, the full-volume mode has the advantage of higher frame rates compared with 3D zoom mode.
Congenital MV Abnormalities Causing MR
An understanding of the types of congenital abnormalities is important when embarking on an echocardiographic evaluation of the abnormal MV. In general, the lesions can be divided into those that produce predominantly stenosis or regurgitation, though there is some overlap ( Table 1 ). Abnormalities in any one of the components of the MV, including leaflets, chordal apparatus, and papillary muscles, have been identified as mechanisms of MR. These abnormalities include isolated anterior and/or posterior cleft of the MV, dysplasia of the MV, deficient leaflet tissue, double orifice, displacement of the MV of the Ebstein type, polyvalvular disease, and the mitral arcade. MR is frequently seen in association with aortic coarctation, because of either congenital MV abnormality, left ventricular dysfunction, or both. MV abnormalities associated with coarctation include congenital perforations, chordal rupture, isolated clefts, deficient leaflet tissue, and double-orifice mitral valve (DOMV). Papillary muscle ischemia or infarction resulting from limited coronary supply due to anomalous left coronary artery from pulmonary artery also causes MR.
| Pathology | Description | Frequency | MR | MS |
|---|---|---|---|---|
| MVP | Prolapse of one or more scallops, of either leaflet (predominantly posterior leaflet); leaflets often thickened and myxomatous Research in CHD using 3DE | 2.4% (population series, adult data) | + | − |
| Isolated cleft | Division of anterior leaflet by the cleft, complete (to the base of the leaflet) or partial Research in CHD using 3DE | 0.07% (echocardiographic series) | + | − |
| Straddling MV | A portion of the MV annulus “straddles” the ventricular septum, associated with an anterior ventricular septal defect, some subvalvar tissue (chords or entire papillary muscle) lies within the right ventricle | + | − | |
| MV dysplasia | Thickened leaflets, obliterated interchordal spaces are and deformed papillary muscles Research in CHD using 3DE | 0.18% (echocardiographic series) | + | + |
| DOMV | Two MV orifices supported by their own tension apparatus | 0.05% (echocardiographic series) | + | − |
| Supramitral ring | Fibrous ridge immediately above MV annulus, partial or complete | 0.03% (echocardiographic series) | − | + |
| Parachute MV | Single (or dominant) papillary muscle supports entire MV apparatus; often leaflet thickening, chordal shortening, and fusion | 0.18% (echocardiographic series) | − | + |
Isolated Cleft of the Anterior Leaflet
Isolated cleft of the anterior (aortic) mitral leaflet is one of the causes of MR. Division of the leaflet by the cleft, in association with a normally positioned MV annulus and intact AV muscular and membranous septum, characterizes this anomaly. The deformity may sometimes occur in association with ventricular septal defect or tetralogy of Fallot. In some cases, the cleft runs from the free edge of the leaflet to its base, while in others, the extension is incomplete. The severity of MR is in part determined by the extent of the cleft. The affected leaflet is often dysplastic, with rolled and thickened edges. The cleft differs from that seen in an AV septal defect, in that it points toward the left ventricular outflow tract rather than the interventricular septum and right ventricle. The exception to this is in hearts with abnormal ventriculoarterial connections, in which the clefts are often eccentric. The papillary muscles are in the normal location in those hearts with isolated clefts, unlike in an AV septal defect, in which the papillary muscles are more closely spaced with the posterior-medial papillary muscle rotated more laterally. In most cases, the free edges of the cleft are unsupported, but in some cases, there are accessory chordae supporting the leaflet, which usually results in valve competency. The accessory chordae may attach to the interventricular septum, while occasionally they cross the ventricular septum through a defect, resulting in MV straddling.
Although this lesion was first described by 2DE, the more recent 3D techniques permit a more complete evaluation of the extent of the cleft, as well as any associated chordal attachments. An isolated cleft can be reconstructed from either an apical four-chamber full-volume data set or a parasternal long-axis view. An apical rendered view from below ( Figure 5 , Video 1 ; available at www.onlinejase.com view video clip online) provides optimal images for reasons mentioned in the section on the assessment of the normal MV, but it can be displayed from above for surgeons ( Video 2 ; available at www.onlinejase.com view video clip online). From the parasternal long-axis view, an image cropped from above, such that the interventricular septum is removed, provides optimal images of the anterior mitral leaflet and a cleft. Neither of these two views is possible with 2DE.
Straddling MV
Straddling MVs are seen more frequently in hearts with abnormal ventriculoarterial connections, in particular transposition of the great arteries with ventricular septal defect or double-outlet right ventricle. There is almost always an associated cleft in the aortic leaflet, resulting in MR. The degree of straddling is important, as it determines whether a biventricular or functionally single ventricle repair is undertaken in the presence of balanced ventricles. The straddling MV may have chordal insertions onto the crest of the interventricular septum or onto the septal surface or lateral wall of the contralateral ventricle. In the former situation, a biventricular repair can be considered, but in the latter two lesions, single-ventricle palliation is usually the treatment of choice.
Because of its higher frame rates, 2DE provides superior detail when imaging fine chordal structures, but it is inferior when attempting to determine the spatial relationship of these structures in 3D space. Whether a ventricular septal defect can be closed is dependent on these relationships, and this is where 3DE is valuable. Most important, it provides surgeons with a mental image of what they will encounter, and it may prevent unnecessary surgical exploration and improved preoperative planning. Because MV chords always straddle through an anterior defect in the interventricular septum, they are best appreciated by 3DE from the parasternal location, as this images the chordae in the axial plane and provides optimal resolution. The view we have found most useful is that obtained when the anterior crop plane is adjusted, as the septal defect, chords, and any accessory papillary muscles are readily appreciated ( Figure 6 ).
MV Dysplasia
In MV dysplasia, the leaflets are thickened, the interchordal spaces are obliterated, and papillary muscles are deformed. This lesion usually results in MR and/or stenosis (see the section on MS) because of leaflet deficiency, tethering, and poor zones of coaptation. A dysplastic valve is often globally hypoplastic. In general, the posterior mitral leaflet is more affected than its anterior counterpart. Two-dimensional echocardiography demonstrates leaflets that appear thickened and relatively immobile, often tethered with areas of poor coaptation. Three-dimensional echocardiography can demonstrate similar findings, but the amount of tethering and leaflet area can be determined. Commissural details and the supporting apparatus of the dysplastic valve can be appreciated on 3DE from the “left ventricular” perspective.
DOMV
In this anomaly, two complete orifices of the MV are supported by their own tension apparatuses. The anomaly may be responsible for MR in some cases, while in others, it occurs as an incidental finding during evaluation for an associated lesion. The orifices may vary in size from being equal to one orifice being much smaller. Three types of DOMV have been described on the basis of site of accessory orifice: at the anterolateral commissure, at the posteromedial commissure, or equal-size orifices with a central fibrous bridge. Although this anomaly is seen more commonly in patients with AV septal defects, it can be seen rarely in patients with intact AV septa. One or both orifices may be regurgitant, which is problematic when attempting repair, because if the regurgitant orifice is closed, the other may be become stenotic. DOMV frequently occurs with associated cardiovascular malformations, including septal defects and tricuspid valve abnormalities.
There are many case reports and series of 2D echocardiographic diagnosis of DOMV, but there are limited data on its accuracy. A large series of patients with AV septal defects showed correct diagnosis in only one third of cases of DOMV by 2DE. By 3DE, the entire valve, from annulus to papillary muscle, can be seen in one view. In addition, 3DE permits evaluation of the extent of leaflet immobility, and the area of each orifice can be imaged accurately. Whether 3DE is superior to 2DE in the evaluation of this lesion will be determined only from longitudinal studies. From an imaging standpoint, a full-volume data set acquired from the apical four-chamber or parasternal long axis view should provide optimal imaging to help aid in the diagnosis of this lesion ( Figure 7 ).
MVP
Isolated MVP is rarely encountered in the pediatric population. This lesion occurs more commonly in association with abnormalities of elastic tissue (Marfan syndrome or Ehlers-Danlos syndrome), but it may also occur in association with other forms of congenital heart disease. The mural (posterior) leaflet of the MV is less well supported at its free edges compared with the aortic (anterior) leaflet, predisposing the former to prolapse. The affected leaflet is thickened, often with myxomatous changes and with annular dilation on the atrial aspect. The true incidence of MVP in the general population is about 2.4%, much lower than was previously reported. The reason for overdiagnosis was limited understanding of the normal structure of the MV annulus. This was corrected by important early work in 3DE, providing an understanding of the saddle-shaped mitral annulus, with two high and two low points. Evaluation by 2DE in the parasternal long-axis view provides a more accurate assessment, imaging the two high points of the annulus, while the apical four-chamber view demonstrates the two low points.
Three-dimensional echocardiography has also been important in demonstrating that the normal MV has inbuilt prolapse and tethering, which are intrinsic components of normal valve function ( Figure 4 E). By permitting imaging of the annulus and leaflets from multiple imaging planes, 3DE provides a very sensitive evaluation of localized prolapse. Viewing en face images of the MV from the surgical view (left atrial view) provides accurate diagnosis of the region and severity of leaflet prolapse and is readily understood by surgeons ( Figure 8 , Video 3 ; available at www.onlinejase.com view video clip online). In comparison, 2DE requires multiple standard 2D planes and “mental” reconstruction of the MV anatomy to determine the precise location of MVP. Commercially available quantitative packages allow the volume of prolapse to be measured. Indeed, this has been one of the major steps forward in the evaluation of MVP. Therefore, the current evidence is that 3DE provides greater anatomic and functional data than its 2D counterpart.
MV in Congenitally Corrected Transposition and Ebstein’s Malformation
The MV is frequently abnormal in congenitally corrected transposition of great arteries. This has a significant impact on decision making in hearts for which anatomic repair is considered. The valve may have a cleft in the anterior leaflet, abnormal chordae or papillary muscles, or variable numbers of cusps. The abnormal chordal attachments may obstruct the pulmonary outflow tract or straddle an anterior ventricular septal defect. Two-dimensional echocardiography can be helpful in the diagnosis of a cleft and straddling of chordal apparatus. On the other hand, 3DE can provide greater detail of the cleft, as well as defining the importance of subvalvar tissue involved in outflow tract obstruction. In patients with MR, the vena contracta site and size are more readily defined by 3DE. Imaging from the four-chamber view usually provides an optimal data set, but the valve frequently lies at more of an angle to the transducer beam, resulting in less optimal imaging than with its tricuspid counterpart.
Ebstein’s malformation of a morphologic MV is rare. In this lesion, the mural (posterior) leaflet develops abnormally and is dysplastic and displaced such that it hinges below the AV junction. In contrast to Ebstein’s malformation of a morphologically tricuspid valve, atrialization of the ventricular inlet is usually not seen with the MV anomaly.
Congenital MV Abnormalities Causing MR
An understanding of the types of congenital abnormalities is important when embarking on an echocardiographic evaluation of the abnormal MV. In general, the lesions can be divided into those that produce predominantly stenosis or regurgitation, though there is some overlap ( Table 1 ). Abnormalities in any one of the components of the MV, including leaflets, chordal apparatus, and papillary muscles, have been identified as mechanisms of MR. These abnormalities include isolated anterior and/or posterior cleft of the MV, dysplasia of the MV, deficient leaflet tissue, double orifice, displacement of the MV of the Ebstein type, polyvalvular disease, and the mitral arcade. MR is frequently seen in association with aortic coarctation, because of either congenital MV abnormality, left ventricular dysfunction, or both. MV abnormalities associated with coarctation include congenital perforations, chordal rupture, isolated clefts, deficient leaflet tissue, and double-orifice mitral valve (DOMV). Papillary muscle ischemia or infarction resulting from limited coronary supply due to anomalous left coronary artery from pulmonary artery also causes MR.
| Pathology | Description | Frequency | MR | MS |
|---|---|---|---|---|
| MVP | Prolapse of one or more scallops, of either leaflet (predominantly posterior leaflet); leaflets often thickened and myxomatous Research in CHD using 3DE | 2.4% (population series, adult data) | + | − |
| Isolated cleft | Division of anterior leaflet by the cleft, complete (to the base of the leaflet) or partial Research in CHD using 3DE | 0.07% (echocardiographic series) | + | − |
| Straddling MV | A portion of the MV annulus “straddles” the ventricular septum, associated with an anterior ventricular septal defect, some subvalvar tissue (chords or entire papillary muscle) lies within the right ventricle | + | − | |
| MV dysplasia | Thickened leaflets, obliterated interchordal spaces are and deformed papillary muscles Research in CHD using 3DE | 0.18% (echocardiographic series) | + | + |
| DOMV | Two MV orifices supported by their own tension apparatus | 0.05% (echocardiographic series) | + | − |
| Supramitral ring | Fibrous ridge immediately above MV annulus, partial or complete | 0.03% (echocardiographic series) | − | + |
| Parachute MV | Single (or dominant) papillary muscle supports entire MV apparatus; often leaflet thickening, chordal shortening, and fusion | 0.18% (echocardiographic series) | − | + |
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