The mitral valve is defined as the inlet valve to the anatomic left ventricle, regardless of ventriculoarterial concordance (i.e., whether the left ventricle functions as the systemic or pulmonary ventricle) [1]. The incidence of isolated congenital abnormalities of the mitral valve is relatively rare [2, 3]. Congenital mitral valve disease frequently occurs in combination with coarctation or hypoplasia of other left‐sided structures, including the left ventricle, aortic valve, and aortic arch [4]. These congenital lesions can be subdivided into mitral stenosis, mitral regurgitation, or a mixture of both. Functionally similar to a mitral valve, the trileaflet inlet valve to the systemic ventricle in a complete atrioventricular (AV) canal defect may result in stenosis or regurgitation as well. Acquired injury to the mitral valve (e.g., endocarditis, rheumatic heart disease, ischemic injury, traumatic injury) is another important cause of stenosis or regurgitation. However, management of acquired lesions will not be discussed extensively here. This chapter will focus instead on the diagnosis, evaluation, treatment, and outcomes of various forms of congenital mitral valve disease in patients with a systemic left ventricle.

Historical Note

Surgical management of mitral valve disease in children began in the 1920s with treatment of rheumatic mitral stenosis. In 1923, Cutler operated on an 11‐year‐old girl with rheumatic mitral stenosis who survived for an additional four years following this intervention [5]. Most early repairs included closed techniques for mitral valvotomy, with techniques for open repair of congenital mitral stenosis being developed in the 1950s. Starkey reported his experiences with open repair of congenital mitral stenosis and insufficiency in 1959 [6]. In 1962, Creech described a successful operation for congenital mitral insufficiency in a 2‐year‐old child [7]. In 1964, Young and Robinson reported successful replacement of the mitral valve in a 10‐month‐old infant [8]. In 1976, Carpentier devised a surgically oriented classification system for congenital lesions of the mitral valve and proposed a systematic approach to surgical management of congenital mitral valve disease [9]. With the diagnostic improvement of echocardiography in the 1980s and advancement of surgical techniques for mitral valve repair and replacement, treatment of congenital mitral valve disease now yields better and more reproducible clinical results, with most patients being amenable to valve repair techniques [10–14].

Normal Mitral Valve Anatomy

The normal mitral valve can be subdivided into the annulus, leaflets, chordae tendineae, and papillary muscles [1]. The annulus is a circumferential fibrous structure that separates the atrium from the ventricle and attaches to the two leaflets of the mitral valve. The anterior one‐third of the annulus subtends the anterior leaflet and corresponds with the fibrous skeleton of the base of the heart, abutting the annuli of the aortic and tricuspid valves (Figure 34.1) [15]. The remaining two‐thirds of the annulus is composed of the posterior annulus and subtends the posterior leaflet.

The anterior and posterior leaflets can be divided into three sections according to the scallops present on the edge of the posterior leaflet. A1/P1 corresponds to the commissure (i.e., leaflet junction) abutting the anterolateral papillary muscle, A2/P2 to the central coaptation zone, and A3/P3 to the commissure abutting the posterior medial papillary muscle (Figure 34.2) [16]. The distal coaptation surfaces of the leaflets (i.e., the leaflet portions that oppose each other during systole) are thicker than the central portion of the leaflets and are often called the rough zone.

The leaflets attach to the papillary muscles via thin fibrous structures called chordae tendineae, which can be subdivided into first‐order, second‐order, and third‐order chordae. First‐order chordae attach to the rough zone of leaflets, while second‐order chordae insert onto the ventricular surface of each leaflet several millimeters from the free edge. This positioning allows second‐order chordae to reduce stress on the leaflet and prevent the central portion from billowing during systole. Third‐order chordae are sometimes muscular attachments that insert along the base of the leaflet and attach to the ventricular wall. They are more commonly found along the posterior leaflet. Although this three‐order classification system, which was originally coined by Tandler [17], remains the most common way to represent chordae, some prefer to classify chordae based on their attachments to the anatomic subdivisions of the valve, dividing chordae into rough zone chordae, cleft chordae, basal chordae, and commissural chordae (Figure 34.3) [18].

The chordae attach to two distinct papillary muscles, which attach to the anterolateral and posterior medial aspects of the left ventricular wall and lie beneath the corresponding commissures. At the commissures, first‐order chordal attachments from both the anterior and posterior leaflets attach to the single papillary muscle corresponding with that commissure (Figure 34.4) [15]. This relationship is important to note, as it defines the commissures in patients with complex valvar abnormalities such as mitral “clefts,” which can be difficult to distinguish from commissures.

As for vascular supply, the posterior medial papillary muscle is supplied by branches of the posterior descending coronary artery, the anterolateral papillary muscle by branches of the circumflex coronary artery, and leaflets and chordae by diffusion from blood in the ventricular cavity. The proper function of the mitral valve depends on the integrity of all these components, in addition to the supravalvar region and performance of the left ventricle.

Mitral Valve Pathology, Classification, and Analysis

Etiology of mitral valve dysfunction includes congenital malformations and acquired disease. Although similar pathologic findings may be present in acquired forms of mitral valve disease, this chapter will focus primarily on congenital malformations, namely congenital mitral valve stenosis and insufficiency. Moreover, it will focus on mitral valve disease associated with an adequate‐sized left ventricle, as conditions involving left ventricle hypoplasia are covered separately in Chapter 32.

Regardless of etiology, mitral valve pathology is best understood by segmental analysis according to anatomic location: supravalvar region, annulus, leaflets, and subvalvar apparatus. Combinations of lesions at multiple levels can be grouped into common constellations of mitral valve pathology, as described below. Components of leaflet, annular, and subvalvar pathology can contribute to either stenosis or insufficiency, while supravalvar pathology is mostly associated with stenosis (Table 34.1). It is important to remember that up to 70% of patients with mitral valve disease will have concomitant abnormalities of left heart structures that may need to be addressed [19]. It should also be noted that most congenital anomalies of the mitral valve will have elements of both stenosis and insufficiency.

Supravalvar Region

Stenosing supramitral membrane (or supravalvar mitral ring) is a fibrous ring above the level of the leaflets that presents as mitral stenosis (Figures 34.5 and 34.6). The membrane usually has a single central opening that may be eccentric in position. The size of the opening determines the degree of obstruction, and the position of the opening correlates well with the onset and degree of clinical symptoms. Stenosing supramitral membrane can also occur at the level of the annulus, in which the membrane extends onto the base of a leaflet and causes some degree of immobility (Figure 34.7). Since most membranes are circumferential, both leaflets are often affected by this fibrous tissue. There are rare cases in which the membrane can cause significant stenosis without involvement of the leaflets, despite being at the annular level.

Stenosing supramitral membrane should be distinguished from cor triatriatum, in which a membrane separates the pulmonary veins from the valve apparatus in the mid‐section of the atrium, essentially separating the atrium into two chambers. In cor triatriatum the left atrial appendage originates downstream from the membrane, whereas in stenosing supramitral membrane the atrial appendage is upstream from the membrane.

Annulus

Hypoplasia or enlargement of the annulus is common in patients with mitral valve pathology. Annular dilation results in mitral insufficiency, whereas hypoplasia is associated with mitral stenosis. Patients with Marfan syndrome or other connective tissue disorders are particularly prone to dynamic enlargement of the mitral annulus, eventually leading to distraction of central coaptation and valvar insufficiency. Patients with rheumatic heart disease may also experience annular remodeling and resultant mitral insufficiency due to chronic inflammatory changes [20]. In hypoplastic annulus, the extent of and approach to pathology largely depend on the integrity of the left ventricle. Often in the presence of a ventricular septal defect, a hypoplastic annulus can be associated with a functionally adequate left ventricle. However, if an inadequate left ventricle is present, the patient must be treated as having single‐ventricle physiology.

Valve Leaflets

Valve insufficiency may occur through a mitral cleft (i.e., a split in the valve apparatus), when the leaflet segments are subtended by chordae that travel to separate papillary muscles. This arrangement of chordal structures and papillary muscles causes the leaflet portions to distract away from each other during systole and allow regurgitation. This is in contrast to a commissure, in which adjacent leaflets contain chordae that attach to the ipsilateral papillary muscle and therefore coapt appropriately during systole. Although not all clefts exhibit regurgitation early in life, with growth of the heart and dilation of the left ventricle regurgitation becomes more likely. Cleft in the anterior leaflet region A2 is commonly seen in patients with endocardial cushion defects, but may occur in the absence of atrial or ventricular septal defects [21]. Atypical location of cleft in the anterior or posterior leaflet may occur in patients with heterotaxy syndrome [22].

Insufficient length of one or both leaflets (i.e., leaflet deficiency) may also lead to poor central coaptation and result in valve insufficiency. Etiology of leaflet deficiency may be either congenital or acquired (e.g., rheumatic disease). Leaflet deficiency must be distinguished from leaflet immobility, which often occurs in the setting of leaflet thickening. Leaflet thickening can occur due to multiple processes, including chronic valve insufficiency and subsequent leaflet scarring, atrial endocardial fibroelastosis extension, and extension of supravalvar membrane onto the leaflet itself. The specific etiology of the thickened leaflet may not be apparent by imaging or direct visualization. Leaflet mobility may also be reduced in the setting of tethering second‐order chordal attachments beneath the leaflet itself (see the subvalvar apparatus section below).

Double‐orifice mitral valve refers to a valve with two separate orifices, each supported by its own chordal and papillary muscle system (Figure 34.8). It is most commonly seen in patients with endocardial cushion defects [23]. In such patients with common AV canal defects, the dominant orifice is typically the anterior orifice, with the posterior orifice being much smaller. The bridge of tissue separating the two orifices is frequently mobile. This abnormality is rarely associated with significant insufficiency or stenosis, although patients with an additional cleft in the anterior orifice may have insufficiency through the cleft.

Subvalvar Apparatus

The term “parachute” mitral valve has been used to describe the congenital abnormality in which all chordal attachments from both anterior and posterior leaflets coalesce onto a single papillary muscle within the left ventricular cavity [24]. A related abnormality is the finding of closely spaced papillary muscles. Presence of a single papillary muscle or closely spaced papillary muscles results in impaired excursion of leaflet tips, thus resulting in stenosis.

In patients with either a single papillary muscle or two distinct papillary muscles, fusion of these structures to the left ventricular free wall results in tethering of leaflet tips, resulting sometimes in both mitral stenosis and insufficiency. Frequently, papillary muscles are thickened, and attachments to the free wall may exist as distinct bridging muscle or complete fusion. By echocardiography, this may sometimes mistakenly appear as basally displaced papillary muscle.

Leaflet mobility may be impaired by abnormally shortened chordae, with the most extreme case involving direct attachment of the leaflet tips to the papillary muscles. Extensive tethering of second‐order chordal attachments due to hypertrophied or foreshortened second‐order chordae can lead to leaflet immobility. Hypertrophied third‐order chordal attachments to the left ventricular wall may occur concomitantly with papillary muscle fusion and restrict leaflet motion during systole and diastole as well. These abnormalities frequently coexist with shortened first‐order chordae. As one can imagine, chordal shortening is particularly damaging for patients with hypertrophied, immobile papillary muscles. Of note is that chordal shortening can contribute to both stenosis by limiting opening of the valve leaflets and regurgitation by preventing leaflet tips from coaptating centrally during systole.

A distinct form of chordal abnormality has been described as the “arcade,” “funnel,” or “hammock” mitral valve (Figure 34.9). This terminology describes obliteration of interchordal spaces by the presence of a membrane that extends from the leaflet tips to the level of the papillary muscles. The interchordal membrane has the appearance and consistency of leaflet tissue, thus creating an appearance of a “hammock” by echocardiography and direct visualization. The inlet to the left ventricle is typically a defect in the membrane, which may only be several millimeters in diameter and eccentrically located.

Elongation of chordae or papillary muscles can cause prolapse of subtended leaflet segments and result in mitral insufficiency. Patients with Marfan syndrome or other connective tissue disorders in particular can present with severe leaflet prolapse. Regurgitation through a cleft may also present as prolapse of that leaflet segment. Flail leaflet associated with papillary muscle or chordal rupture is uncommon in congenital mitral valve disease, but may occur in the setting of ischemia or endocarditis.

In certain patients with a ventricular septal defect, the base of a papillary muscle may originate from the right ventricle, a condition commonly called “straddling mitral valve.” This finding is more common in patients with complex congenital anomalies such as superior–inferior ventricles and L‐transposition of the great arteries. Although this finding is rarely associated with mitral insufficiency or stenosis, the anomalous attachments may need to be addressed at the time of cardiac repair [25].

Congenital Mitral Regurgitation

Most forms of congenital mitral regurgitation share valvar phenotypes with congenital mitral stenosis. One exception is annular enlargement, which leads to malcoaptation in the central portion of the valve. However, annular enlargement is rarely the only abnormality, so simple annular reduction is often insufficient for treatment. Abnormalities of the subvalvar apparatus tether the anterior and posterior leaflets in positions of malcoaptation, and chronic insufficiency results in further left atrial and annular dilation.

Pathophysiology and Clinical Presentation

Both mitral stenosis and regurgitation lead to left atrial and pulmonary venous hypertension in the presence of intact atrial septum due to compensation for increased pressure and volume loads. Specifically, left atrial dilatation occurs in patients with mitral regurgitation to accommodate for increased blood volume, whereas left atrial fibrosis and thickening occur in patients with mitral stenosis to accommodate for an increased pressure load.

In the presence of a large atrial septal defect, a significant gradient across the mitral valve may not be detected due to shunting. Moreover, since mitral stenosis is commonly associated with bicuspid aortic valve and coarctation, it is not uncommon for mitral stenosis to manifest following repair of a simple coarctation, as improvement in cardiac output and flow across the mitral valve following repair may unmask mitral stenosis. Valve morphology and leaflet mobility must therefore be examined carefully to detect the presence of underlying mitral valve pathology.

Neonates and infants with mitral valve disease may be completely asymptomatic at rest. If symptomatic, they often present with difficulty feeding and less commonly signs of tachypnea. Chronic cough or gastrointestinal symptoms such as chronic emesis may be signs of heart failure from left atrial hypertension and may contribute to poor feeding and growth. A neonate with severe mitral stenosis may present with circulatory failure upon ductal closure, necessitating institution of prostaglandins to maintain cardiac output. In the current era, most patients are diagnosed in utero and present for evaluation in the early postnatal period. Older children with severe mitral stenosis or regurgitation may present with easy fatigability, decreased exercise tolerance, or classic left‐sided heart failure symptoms such as orthopnea, dyspnea on exertion, or paroxysmal nocturnal dyspnea.

Not infrequently, a patient who is asymptomatic presents with echocardiographic evidence for pulmonary artery or right ventricular hypertension. With prolonged pulmonary hypertension, right ventricular hypertrophy and failure may ensue. The time course for development of irreversible pulmonary hypertension in patients with left atrial hypertension is unknown, but prolonged medical management for patients with confirmed pulmonary artery hypertension secondary to left atrial hypertension increases the risk of developing irreversible pulmonary vascular disease, and may affect candidacy for alternative management strategies such as single‐ventricle palliation or transplantation.

Imaging and Studies

Radiographic evidence for left atrial hypertension includes the presence of cardiomegaly with left atrial enlargement and prominent interstitial lung markings with increased pulmonary vascularity. In particularly severe cases, the left main‐stem bronchus may be elevated and the left lower lobe collapsed. It is important to note, however, that in patients with congenital mitral stenosis, endocardial fibroelastosis may limit left atrial dilatation, thus resulting in a radiograph that does not demonstrate cardiomegaly. Potential electrocardiographic findings include left atrial dilatation (large second hump in P wave in lead II and deep negative deflection in biphasic P wave in lead V1) and right ventricular hypertrophy (right axis deviation, dominant R wave in V1, and dominant S wave in V5–6).

Although radiography and electrocardiography can be helpful, two‐dimensional and three‐dimensional echocardiography is essential for the diagnosis and characterization of mitral valve pathology (Figure 34.11). Initial diagnosis is typically determined by two‐dimensional echocardiography and Doppler interrogation. The critical views include parasternal long‐axis view, four‐chamber view, and short‐axis sweeps through the mitral apparatus. Three‐dimensional echocardiography provides synthesis of various components of the valve detected by two‐dimensional echocardiography and is essential to the surgical planning.

The distinct abnormalities of the supravalvar membrane, annular dimension, leaflet morphology, and subvalvar apparatus should be systematically interrogated by echocardiography. On the parasternal long‐axis views, the length of the anterior and posterior leaflets must be determined, and evaluation of mechanisms for malcoaptation must be determined by careful sweeps through the valve.

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