15: Mitral Valve and Left Atrial Anomalies


CHAPTER 15
Mitral Valve and Left Atrial Anomalies


James C. Nielsen and Daniela Y. Rafii


Hassenfeld Children’s Hospital at NYU Langone, New York, NY, USA


Definitions


In this chapter we focus on congenital structural abnormalities of the mitral valve. Acquired disease secondary to diseases like cardiomyopathy, rheumatic fever, and endocarditis are discussed in other chapters.


Mitral valve prolapse (MVP) occurs when there is systolic motion of a leaflet segment beyond the annular plane. Historically, the definition of MVP has varied considerably resulting in some diagnostic confusion [1]. Defining MVP as classic or nonclassic forms is supported by adult outcome studies [1,2]. Classic MVP is defined as systolic leaflet displacement of >2 mm and diastolic leaflet thickness exceeding 5 mm. Nonclassic MVP exists when the systolic displacement is >2 mm with leaflet thickness <5 mm. Borderline displacement <2 mm is considered a normal variant and is not associated with morbidity or progression [1].


Cleft mitral valve is present when the anterior mitral valve (MV) leaflet remains divided into two separate leaflet components, each of which attach to a separate papillary muscle group. Isolated cleft mitral valve, the subtype addressed in this chapter, is a cleft not associated with an ostium primum defect. Isolated cleft MV frequently coexists with other congenital heart lesions – the term “isolated” refers to the absence of an ostium primum defect.


Straddling mitral valve is defined as the presence of chordal attachments on both sides of the ventricular septum, and by definition can only occur in the presence of a ventricular septal defect (VSD) [35]. In a straddling MV the anterior leaflet is frequently divided [6], leading to a separate commissure and therefore a trileaflet MV; this arrangement, however, is not, strictly speaking, a cleft since chordae tendineae from two separate leaflets join a single (or closely spaced) papillary muscle group(s). It is appropriately termed a commissure.


Double‐orifice mitral valve is defined as two separate orifices each supported by its own chordae and papillary muscles.


Congenital mitral stenosis (MS) is defined as an abnormality at any level of the MV apparatus that results in restriction to diastolic filling. This includes: hypoplasia of the MV, MV arcade, parachute MV, and supravalvar mitral ring. Mitral regurgitation can be associated but stenosis is the main lesion.


Isolated congenital giant left atrium (GLA) is defined as left atrial enlargement out of proportion to the hemodynamic load on the left atrium in the absence of rheumatic heart disease. It is also frequently referred to as congenital left atrial aneurysm or left atrial appendage aneurysm [711].


Congenital left atrial appendage (LAA) stenosis is isolated stenosis of the LAA ostium. This differs from postoperative LAA stenosis which is not uncommonly seen after incomplete surgical ligation of the LAA in adults. Congenital LAA stenosis results in a relatively high‐velocity flow jet with to‐and‐fro flow at the level of the LAA ostium [911].


Incidence


Initial reports describing the prevalence of MVP showed it to be as high as 5–15%. The results of these studies were affected by selection bias and the lack of a clear definition of MVP [12]. Later studies in unselected individuals [1,13] report a much lower prevalence of 0.6–2.4%. The true incidence of isolated cleft MV, double‐orifice MV, and straddling MV are yet to be determined [14,15]. Pathologic studies from large congenital cardiac registries have reported isolated cleft MV in 41/3369 (1.2%) [15], double‐orifice MV in 28/2733 (1.0%) [16], and straddling MV in 8/2200 (0.4%) [3]. The increasing use of 3D imaging has increased the detection of previously undiagnosed isolated cleft MVs in adults. One recent study looked at over 600 adult patients with unexplained moderate or greater mitral regurgitation using 3D transesophageal echocardiography (TEE) and found 3.3% of these patients had undiagnosed isolated cleft [17]. Congenital MS is an equally rare condition that occurs in approximately 0.4% of patients with congenital heart disease [18]. Worldwide, the prevalence of acquired rheumatic MS exceeds that of congenital MS; however, in developed nations congenital MS is more common. Congenital GLA and congenital left atrial appendage stenosis are exceedingly rare, with <20 pediatric cases reported in the literature for each lesion [7,8,10,1928].


Etiology


In primary classic MV prolapse there is myxomatous degeneration of the valve without an identifiable connective tissue disorder. Familial inheritance can be present in an autosomal dominant fashion [29]. Secondary MV prolapse is histologically identical to primary, but is present with connective tissue disease (e.g., Marfan or Ehlers–Danlos syndromes). Isolated cleft MV with normally related great arteries and double‐orifice MV have in common abnormal development of the embryonic endocardial cushions and, therefore, can be seen with increased frequency in trisomy 21 [15,16]. The rarity of straddling MV [3] and its association with other complex anatomic lesions [6] makes it difficult to ascribe a specific etiology to straddling MV. Similarly, congenital MS typically involves disruption of several components of the valve apparatus, hypoplasia of additional left‐sided structures, and other complex cardiac lesions, which makes the determination of specific etiology equally challenging. In contrast to idiopathic giant right atrium (see Chapter 14) [30], which is by definition truly “isolated,” congenital GLA has most frequently been reported in the setting of some degree of MV disease [31]. The exact etiology of congenital GLA is obscure. Surgical and histologic examinations have demonstrated extremely thin atrial wall and fibrosis [8,21,22]. Authors have proposed fetal viral infection or focal developmental error leading to lack of myoblasts and loss of wall integrity [24,26]. Regarding congenital LAA ostial stenosis, some authors hypothesize that the presence of the ostial membrane is secondary to simple anatomic variation while others propose that it may be a variant of cor triatriatum [911].


Morphology and classification


Developmental considerations


The atrioventricular (AV) valve leaflets, chordae tendineae, and myotendinous junction are derived from the endocardial cushions, with some contribution of myocardial cells [3234]. The complex process of normal morphogenesis of the AV junction and AV valve is still incompletely understood [3537]. The rarity of straddling MV, double‐orifice MV, isolated MV, and congenital MS, together with their frequent association with other abnormalities, makes defining their developmental basis extremely challenging. Advancements in the understanding of the signaling pathways and epithelial to mesenchymal transition processes involved have provided insight into cardiac embryogenesis and the development of congenital MV pathology [38,39]. Formation of the AV valves involves complex intracellular signaling and interactions between different mesenchymal cell populations. Endocardial cells delaminate from the lower aspect of the endocardial cushions; the complex interaction of the mesenchymal cells, fusion of the cushions, and secretions from the extracellular matrix lead to the formation of mature and freely mobile MV leaflets leaving only the chordae tendineae attached to both leaflet and papillary myocardium [3539]. How interruption of this process leads to these complex anomalies is yet to be fully defined. For isolated cleft MV it is generally recognized that failure of fusion of the left ventricular (LV) aspect of the superior and inferior endocardial cushions results in a cleft in the anterior leaflet [36]. Of note, the portion of the endocardial cushions that participates in the final step of ventricular septation eventually leads to the formation of both the membranous septum and the anterior MV leaflet [3537], which fits with the anatomic observation that the cleft attachments in isolated cleft MV with normally related great arteries are always near the membranous septum [15]. Straddling MV in the setting of crossed AV inflows, superior–inferior ventricles, and right ventricular (RV) sinus hypoplasia is thought to be related to a post‐looping abnormal clockwise rotation (twisting) that brings the infundibular chamber in close proximity to the leftward aspect of the developing AV canal, allowing the MV to straddle into the infundibular chamber [4]. Congenital MS can develop from an abnormality of one or more of the MV components. The narrowing of the supravalvular area, annulus, and valve leaflets often extends distally with additional subvalvar obstruction from hypertrophied or misplaced papillary muscles and abnormal chordae. Several common anatomic subtypes have been described: MV arcade (typical congenital MS), parachute MV, supravalvar mitral ring, and hypoplastic MV [40]. By limiting flow to distal structures during embryologic development, MS may play a role in the hypoplasia or stenosis of downstream structures [41]. The LAA is derived from the left wall of the primary atrium. The smooth‐walled left atrial cavity develops from the outgrowth of the pulmonary veins after the development of the trabecular LAA. It is unclear what portion of development is interrupted causing the ostial membrane with congenital LAA stenosis [10].


Anatomy


Normal mitral valve anatomy


The MV apparatus is composed of the MV annulus, MV leaflets, chordae tendineae, and LV papillary muscles. Abnormalities at any of these levels can result in MV disease. The MV annulus is divided into anterior and posterior components. The anterior mitral annulus is in continuity with the aortic annulus through a region termed the aortomitral curtain which extends between the left and right fibrous trigones. This region is well supported and less prone to distortion and remodeling compared with the posterior mitral annulus which is adjacent to a rim of discontinuous fibrous tissue and fat. The annular shape is likened to a hyperbolic paraboloid or “saddle” (Figure 15.1) with the most superior aspect of the annulus located anteriorly and the most inferior aspect located laterally at the commissures (Figure 15.2). This morphology has been shown to reduce stress on the mitral leaflets during systole [42] and has implications for the design of annuloplasty rings [43,44].

Schematic illustration of an idealized saddle-shaped surface – hyperbolic paraboloid – which is concave downward in one direction and upward in a perpendicular direction.

Figure 15.1 Schematic of an idealized “saddle‐shaped” surface – hyperbolic paraboloid – which is concave downward in one direction (parallel to the yz plane) and upward in a perpendicular direction (parallel to the xz plane). The symbols a, b, and c denote constants that determine the shape of the structure.


Source: Levine RA, Triulzi MO, Harrigan P, Weyman AE. The relationship of mitral annular shape to the diagnosis of mitral valve prolapse. Circulation 1987;75(4):756. © 1987, Wolters Kluwer Health, Inc.

Image described by caption.

Figure 15.2 (a) Diagram of the model in vitro; with a planar annulus and leaflets curving downward toward the zone of coaptation. Adjacent cardiac structures are also shown. (b) Diagram of the model in vitro; with the leaflets and annulus shaped to lie on a saddle surface. (c) The model restructured with the leaflets concave toward the left ventricle, reflecting its systolic pressure, but not protruding above the anterior and posterior high points of the saddle‐shaped annulus. Ant, anterior; Ao, aorta; LA, left atrium; LV, left ventricle; Post, posterior.


Source: Levine RA, Triulzi MO, Harrigan P, Weyman AE. The relationship of mitral annular shape to the diagnosis of mitral valve prolapse. Circulation 1987;75(4):756. © 1987, Wolters Kluwer Health, Inc.


There are two MV leaflets, anterior and posterior. The anterior leaflet is sail‐shaped and attaches to about one‐third of the anteromedial portion of the MV annulus. The posterior leaflet is C‐shaped and hinges on the remaining posterolateral two‐thirds of the MV annulus. Each leaflet is further classified into three scallops, designated scallops 1, 2, and 3 (Figure 15.3). Scallop 1 is located laterally and the numbers ascend medially. Normally, the leaflet edges converge during ventricular systole along a 2–4 mm coaptation zone (zona coapta) at the leaflet tips. The chordae tendineae insert along the zona coapta and onto the LV papillary muscles. There are two major papillary muscles, the anterolateral and posteromedial papillary muscle, which provide attachments to the anterolateral and posteromedial half of both MV leaflets, respectively.


Mitral valve prolapse anatomy


Mitral valve prolapse results when a leaflet edge slips past the coaptation zone. During the cardiac cycle, annular movement is dependent on contraction of adjacent muscular structures and is threefold‐translational (toward and away from the apex), circumferential, and “folding” across the intercommissural axis [45]. The nonplanar shape of the mitral annulus is particularly important to appreciate when imaging and defining what constitutes MVP [2]. MVP usually results from excessive leaflet tissue (redundancy), myxomatous proliferation of the spongiosa, and elongation of the chordal apparatus [46,47]. These abnormalities lead to significant prolapse of part of a leaflet or the entire leaflet, resulting in a lack of coaptation resulting in mitral regurgitation and rarely chordal rupture.


MV leaflet segment nomenclature provides consistency when describing the location of isolated prolapsing segments (Figure 15.3). In a large, adult, surgical series on MVP, isolated posterior and bileaflet prolapse were the more common patterns of mitral leaflet prolapse with a prevalence of 52% and 33%, respectively. The prevalence of isolated anterior leaflet prolapse was only 15% [48].

Schematic illustration of the nomenclature used to describe the anatomy of the mitral valve leaflets, segments, and commissures.

Figure 15.3 Nomenclature used to describe the anatomy of the mitral valve leaflets, segments, and commissures (surgical view from the left atrium). A1, lateral segment; A2, middle segment; A3, medial segment; P1, lateral scallop; P2, middle scallop; P3, medial scallop.


Source: Shanewise JS, Cheung AT, Aronson S, et al. ASE/SCA guidelines for performing a comprehensive intraoperative multiplane transesophageal echocardiography examination: recommendations of the American Society of Echocardiography Council for Intraoperative Echocardiography and the Society of Cardiovascular Anesthesiologists Task Force for Certification in Perioperative Transesophageal Echocardiography. J Am Soc Echocardiogr 1999;12(10):884–900. © 1999, Elsevier.


Isolated cleft mitral valve anatomy


Based on a large pathologic investigation, two separate categories of isolated cleft MV were identified [15]. When an isolated cleft MV coexists with an abnormal conotruncus the attachments of the cleft are typically inserted to the subarterial LV myocardium, in a more vertical location. This can lead to LV outflow tract obstruction [15,49,50]. In addition, an isolated MV with an abnormal conotruncus has a normal ventricular inlet/outlet ratio which differs it from an AV canal‐type cleft [51]. It also has no association with trisomy 21. In isolated cleft MV with normally related great arteries, the cleft attachments are more horizontal to the ventricular septum, a reduced inlet/outlet ratio is present, and trisomy 21 is more frequently associated [15,52]. Prior to this finding there had been debate about whether isolated cleft MV was related to AV canal defects [5355]. To further investigate this, the location of the cleft attachments, papillary muscle location, and inlet/outlet ratio have been further studied, producing, however, different conclusions [5056]. The recognition of two distinct groups (normally related great arteries and conotruncal abnormalites) was key to explaining the vertical location of the cleft attachment [15]. In isolated cleft with normal great artery relationship, the cleft attachments are inserted to the membranous septum (when intact) or to the crest of the ventricular septum when a VSD is present. Functionally, the attachment appears to migrate superiorly when the membranous septum is intact, resulting in a cleft location that is in between horizontal and vertical.


Straddling mitral valve anatomy


Straddling MV occurs when MV chordal attachments are present within the right ventricle in the presence of a VSD. Straddling is across the anterior (outlet) septum and differs from a straddling tricuspid valve which crosses the inlet septum [57]. The attachments of straddling MV leaflets are typically to a single papillary muscle or to closely spaced papillary muscle groups within the infundibulum either to the conal septum, the septal band, or the free wall. Straddling can involve the anterior leaflet or both leaflets. This lesion is always associated with a VSD and an abnormal conotruncus, either double‐outlet right ventricle or transposition of the great arteries [6]. When straddling MV is present in the setting of {S,D,L} or {S,L,D} segmental sets there are invariably crossed AV valve inflows, superior–inferior ventricles, and hypoplasia of the RV sinus [4]. Major straddling occurs when more than one‐half of the MV is related to the infundibulum. Typically, the MV orifice is adequately sized and rarely more than mildly incompetent. Outflow obstruction from the attachments is rare. Risk factors for survival are related to the presence of multiple or noncommitted (remote) ventricular septal defects [49].


Double‐orifice mitral valve anatomy


In its most frequent form, double‐orifice MV exists with two discrete orifices, each with its own chordal support and papillary muscles. The orifices are frequently divided by a fibrous ridge, which is actually normal MV leaflet tissue [58,59]. This separation can either be limited to the tips of the respective orifices, sparing the more basal aspects, or extend along the entire length of the leaflets [58]. Another much less common anatomic variation is a muscular separation of the orifices associated with papillary muscle abnormality [58]. The size of the papillary muscles varies directly with the size of the orifice that they subtend. At one end of the spectrum is a chordal ring, a tiny anterolateral orifice without papillary support inserting directly into the LV wall. More typically, the valves are relatively equal in size. When double‐orifice MV is present with an AV canal defect, it is often characterized by a smaller posteromedial orifice. Double‐orifice MV is more prevalent in partial AV canal defects (64%) versus complete AV canal defects (34%) or transitional AV canal defects (2%) [60].


Mitral stenosis anatomy


Mitral valve arcade, also referred to as typical congenital MS in the literature, involves thickened and rolled MV leaflets, shortened chordae tendineae with the absence of interchordal spaces, and underdeveloped papillary muscles which may be closely spaced. This constellation of findings results in direct attachment of the MV leaflets to the papillary muscles. The chordae tendineae may be completely absent and there may be additional abnormal fibrous tissue bridges between the papillary muscles [32,61,62]. This may present with severe neonatal MS or mitral regurgitation. Reports of MV arcade presenting later in life have also been described in the literature [62]. Congenital MS has been referred to as “symmetric” [63] in the literature, implying equal distribution of chordae to each papillary muscle and equal papillary muscle size. This is in contrast to “asymmetric” MS, referring to asymmetric papillary muscle location, size, and distribution of chordal attachments (found in approximately 30% of a large cross‐sectional study) [64]. Asymmetric MS is a spectrum of parachute MV with the extreme form being true parachute MV with all chordae inserting into a single papillary muscle (typically the posteromedial) [40,65].


Congenital MS is associated with other obstructive left‐sided lesions including pulmonary vein stenosis, supravalvar and valvar aortic stenosis, parachute MV, supravalvar mitral ring, subaortic membrane, and coarctation of the aorta; the last four lesions together were originally described by Shone et al. [18], and are often referred to as Shone complex. Supravalvar mitral ring is an important subtype of MS characterized by a membranous tissue growth from the left atrial side of the mitral leaflets that results in variable degrees of stenosis. Two types of congenital mitral rings exist. The more common intramitral ring consists of a thin membrane adherent to the atrial surface of the MV leaflets, and it is more commonly associated with Shone complex. The supramitral ring is a shelf‐like membrane at or just above the mitral annulus, and this less common form of ring is usually associated with a normal subvalvular apparatus [66]. Congenital mitral ring can be progressive and is optimally treated surgically [63,64,66].


Congenital giant left atrium anatomy


The morphology of congenital GLA is fairly consistent [7,8,67], with a globally enlarged left atrial cavity and more focal aneurysmal enlargement of the atrial wall adjacent to the appendage. Typically the atrial cavity extends apically along the anterior and lateral LV wall (Figure 15.4). The mass effect can lead to compression of the LV free wall, right atrium, and adjacent thoracic structures, including the esophagus, left bronchus, and recurrent laryngeal nerve.


Congenital left atrial appendage stenosis anatomy


The LAA is often described as a finger‐like projection from the main body of the left atrium. There is considerable variability from individual to individual in the size, shape, and location of the LAA in relationship to surrounding cardiac structures [68]. Normally, the flow in the LAA is biphasic, and normal velocities range from 40 to 80 cm/s. With congenital LAA, there is a discrete narrowing of the left atrial ostium resulting in a relatively higher velocity to‐and‐fro jet, often seen on Doppler color mapping, with velocities reported as high as 290 cm/s (see Figure 15.15).


Pathophysiology


The pathophysiology associated with either abnormal MV development (cleft, arcade, straddling, parachute, or double orifice) or abnormal MV apparatus function (MVP) is predominantly a function of the competence of the valve (regurgitation) and the adequacy of the flow orifice (stenosis). An additional factor specific to isolated cleft MV with abnormal conotruncus and straddling MV is the location of the attachments relative to the potential pathway from the left ventricle to the outflow and whether this pathway is adequate for a biventricular repair [49,69,70].

Photo depicts apical four-chamber view demonstrating the massively dilated left atrial appendage in a patient with giant left atrium.

Figure 15.4 Apical four‐chamber view demonstrating the massively dilated left atrial appendage in a patient with “giant left atrium.” An, aneurysm; LA, left atrium; RA, right atrium.


Severe mitral regurgitation can result in heart failure, failure to thrive, and pulmonary hypertension. If symptomatic, surgical repair or replacement is indicated. Chronic asymptomatic mitral regurgitation can be clinically well tolerated with minimal symptoms. The indications for surgical MV surgery are related to the exact mechanism, the integrity of the MV apparatus, the severity of MR, age, and left ventricle size. Preservation of long‐term LV function is the rationale behind surgical intervention in asymptomatic patients with severe mitral regurgitation [48]. Adult criteria for intervention have been published and guidelines are available but no specific pediatric guidelines exist. Adult asymptomatic patients with a lower ejection fraction (30–60%) and higher end‐systolic dimension by M‐mode (>40 mm) are considered for MV repair or replacement [71]. For asymptomatic patients with preserved LV ejection fraction and a LV end‐systolic dimension <40 mm, surgical repair can be considered if the likelihood of successful and durable surgical repair without residual mitral regurgitation is greater than 95% with an expected mortality rate <1%. Acute chordal or papillary muscle rupture, resulting in a flail leaflet, and acute mitral regurgitation is poorly tolerated, producing acute left atrial and pulmonary artery hypertension. In general, flail MV involving the posterior leaflet is more amenable to surgical repair.


Isolated cleft MV frequently results in mitral regurgitation due to the lack of an adequate coaptation zone along the cleft. The outcome following MV plasty, which basically involves suture plasty of the cleft to improve coaptation, is a trade‐off between improved coaptation and potential inadequacy of the flow orifice, but overall good mid‐term outcomes are reported [72,73].


Double‐orifice MV without associated lesions occurs rarely (7% of all double‐orifice MV cases) [59], which is likely an underestimate because when double‐orifice MV is present in isolation, the valve typically is functioning well and is clinically silent. The pathophysiology is generally not related to dysfunction of the double‐orifice MV, but more to the associated lesions – frequently AV canal defects or obstructive left‐sided lesions [15,59,60]. In double‐orifice MV in the presence of an AV canal defect, the regurgitation is often located at the posteromedial orifice. Surgical management of double‐orifice left AV valve with AV canal defects involves (partial) closure of the accessory orifice if possible; this requires modification to the surgical closure of the cleft [60].


The hemodynamic burden of MS, and therefore symptoms, depends upon the flow orifice size and cardiac output. As the heart rate increases, diastole is shortened more than systole and the mean gradient across the valve increases, even at a fixed cardiac output. To maintain a given cardiac output, a greater mean pressure gradient is required. For severe MS, a small increase in flow rate necessitates a large increase in the pressure gradient [74]. The elevated left atrial pressure results in left atrial dilation, elevated pulmonary venous pressures, pulmonary edema, and dyspnea. Pulmonary hypertension in moderate or severe MS is secondary to transmission of pressure to the pulmonary bed, reactive vasoconstriction, and vessel wall changes [75]. Severe pulmonary hypertension can in turn lead to RV dysfunction and tricuspid regurgitation. Left atrial dilation is a powerful substrate for atrial fibrillation, which increases the risk of thromboembolic events.


Imaging

Oct 30, 2022 | Posted by in EQUINE MEDICINE | Comments Off on 15: Mitral Valve and Left Atrial Anomalies

Full access? Get Clinical Tree

Get Clinical Tree app for offline access