16 The Mitral Valve



Francesco F. Faletra, Horia Muresian, Damián Sánchez-Quintana, and Farhood Saremi

16 The Mitral Valve



Introduction


Over the last two decades, echocardiography (two- and three-dimensional [2D and 3D] transthoracic [TTE] and transesophageal [TEE]), computed tomography (CT), and cardiac magnetic resonance (CMR), have become crucial in the modern diagnostic workflow and management of patients with cardiovascular disease. Each of these techniques has advantages and limitations. CT has been commonly used for evaluation of the integrity of the epicardial coronary arteries and CMR is primarily used to address the pathologies of the myocardium. Echocardiography has been the first-line imaging technique for evaluating morphology, size, and function of heart valves. However, CT and CMR can also provide added information in regard to the morphology and function of the cardiac valves. Furthermore, each of these studies can also explore surrounding structures not affected by the disease with an unprecedented quality and detail. In this chapter, we describe and illustrate the anatomy and variants of the mitral valve (MV) apparatus “revisited” by these techniques. To illustrate various anatomical aspects of MV, we use the imaging technique that best fits the described component/s. For instance, all three imaging techniques may visualize papillary muscles (PMs) but CT is the technique which best shows the anatomical details of the PMs. We also briefly describe common mitral pathologies.



Mitral Valve Apparatus


In 1972, Perloff and Roberts 1 first described the MV as a complex apparatus, which requires for maintaining a perfect competence, morphological integrity combined with a precise spatial and temporal coordination of the annulus, leaflets, chordae tendineae, and PMs (Fig. 16‑1). Any anatomical change or disruption of one or more of these components inevitably leads to mitral regurgitation.

Fig. 16.1 Three-chamber CT view of the heart showing components of the mitral valve apparatus. To maintain the functional integrity of the apparatus and the effective competence, fine spatial and temporal coordination and perfect anatomical integrity of all the components are required. The dotted line marks the virtual plane of the mitral annulus. Blood flow direction from inlet to outlet is shown by curved arrow. AML, anterior (aortic) mitral leaflets; LA, left atrium; LV, left ventricle; PML, posterior (mural) mitral leaflet; C, chorda tendineae; PM, papillary muscle; RV, right ventricle.


Mitral Annulus


Anatomical books describe the MV annulus as an interrupted fibrous ring, divided into anterior and posterior segments, which encircles the atrioventricular junction and support the mitral leaflets. Later Angelini et al 2 demonstrated that a continuous fibrous ring separating the atrial and ventricular myocardium is rather rare. Instead, the posterior segment of the annulus is often seen as a discontinuous fibrous arc interrupted by variable amount of atrial and ventricular myocardial fibers while the anterior segment is in fact a sheet-like structure made of dense connective tissue that connects the anterior mitral leaflet with the aortic interleaflet triangle. This area is called mitral–aortic fibrous continuity, also known as mitral–aortic curtain or intervalvular fibrosa (Fig. 16‑2).


Two nodules of dense fibrous tissue namely the “right and left fibrous trigons” near the commissures exist to reinforce the base of anterior leaflet. Finally, from a surgical point of view, the MV annulus is defined as the hinge line of attachment of mitral leaflets on the atrial and ventricular myocardium.


The MV annulus has a 3D saddle-shaped configuration with the highest points corresponding to the midpoints of aortic attachment of the mitral–aortic curtain and the lowest at the fibrous trigones. 3 This configuration concentrates the peak stress on the two fibrous trigons. 4 ,​ 5


Absence of a rigid fibrous ring makes the mitral annulus a deformable structure that changes in dimension and shape during the cardiac cycle. During early systole, the normal annulus contracts and its saddle-shaped configuration becomes more pronounced. These systolic changes are mainly due to the anteroposterior shortening while the bicommissural diameter remains unchanged (Fig. 16‑3).


Longitudinal shortening of the left ventricle with each contraction drags the MV annulus toward the apex. The resulting increase in the left atrial diameters facilitates its filling by increased systolic forward flow from the pulmonary veins. The opposite changes occur in diastole (Fig. 16‑4).


Finally, the posterior mitral annulus is close to important structures, including the left circumflex artery, the right coronary artery, the coronary sinus, and the bundle of His (posteromedially near the right fibrous trigone). The anatomical relationship of these structures with the hinge line of posterior annulus is important since the surgical suture during MV replacement occurs along the posterior hinge line and these vessels may be damaged (Fig. 16‑5).

Fig. 16.2 Images obtained with three-dimensional transesophageal echocardiography in (a) systole and (b) diastole. A longitudinal cut of the aortic root (Ao) and left ventricle (LV) showing the ventricular site of the anterior mitral leaflet (AML) and the interleaflet triangle (ILT) between the aortic leaflets (L). The two dotted lines delimit a sheet-like connection between the AML and the ILT. This area is known with different names such as mitral–aortic fibrous continuity, mitral–aortic curtain, or intervalvular fibrosa.
Fig. 16.3 Three-dimensional transesophageal echocardiography showing the mitral valve and annulus from an atrial perspective in (a) diastole and in (b) systole. Images show that the annulus as a very dynamic and deformable structure, being nearly circular in diastole (a) and elliptical in systole (b). The change is primarily due to anteroposterior shortening (white line) while bicommissural diameter (red line) remains unchanged. Ao, aorta.
Fig. 16.4 Cardiac MR image in (a) diastole and in (b) systole showing longitudinal motion of the mitral annulus (dotted line) toward the apex due to longitudinal contraction of the left ventricle (LV). The resulting increase of the left atrium (LA) volume facilitates forward pulmonary venous flow.
Fig. 16.5 (a) CT images in long-axis view. The red square delimits the area of the posterior annulus hinge line, which is magnified in (b). The hinge line of the posterior leaflet is close to the posterior atrioventricular groove where the circumflex coronary artery (Cx) and the coronary sinus (CS) are located. Because of their proximity with the posterior annulus, the sutures may damage these vessels during mitral valve replacement. Ao, aorta; LV, left ventricle; LA, left atrium.


Mitral Leaflets


Two deep incisures called commissures are seen between the anterior and posterior leaflets. 6 The anterior leaflet has a triangular shape and covers approximately one-third of the entire annular circumference while the posterior leaflet has quadrangular shape and covers the remaining two-thirds. In most normal MVs, two additional incisures divide the posterior leaflet into three small segments or scallops (Fig. 16‑6 , Fig. 16‑7). To facilitate valve analysis (i.e., in the presence of mitral prolapse/flail) cardiac surgeons name the lateral scallop P1, the central scallop P2, and the medial scallop P3. Usually, the central scallop is the largest. Even though the free edge of the anterior leaflet has no identifiable incisures, the free margin of the anterior leaflet across the P1, P2, and P3 segments are named A1, A2, and A3, respectively. Occasionally, additional scallops occupy the commissural areas. These small scallops are called commissural scallops 7 (Fig. 16‑7).


From a ventricular perspective, both leaflets present two distinct zones: the rough zone and the clear zone (Fig. 16‑8). The rough zone covers the distal surface of both leaflets and receives the insertions of chordae tendineae assuming a corrugate surface. The clear zone covers the remaining ventricular surface and has a smooth appearance. From an atrial perspective, the rough zones correspond to the so-called coaptation surface, the area where leaflets juxtapose each other during systole. The width of this coaptation surface measures 8 to 10 mm. The redundancy of the coaptation surface provides a comfortable “valvular reserve” that assures an effective coaptation in presence of a certain degree of annulus dilation or leaflet tethering. Moreover, as the ventricular pressure rises, the leaflets mutually support each other along this zone. Since the vast majority of chordae insert within the coaptation surface, they may share the mechanical stress upon the leaflets (Fig. 16‑9).

Fig. 16.6 Atrial (superior) views of the mitral valve in closed position showing the leaflets of the mitral valve and relation to the aortic valve. Ao, aorta; AL, aortic (anterior) leaflet; ML, mural (posterior) leaflet; L, left coronary sinus; N, nonfacing coronary sinus; LMA, left main artery; MPA, main pulmonary artery; R, right coronary sinus.
Fig. 16.7 Three-dimensional images of mitral valve from (a) atrial and (b) ventricular perspective. Two incisures (arrows) divide the posterior leaflet into P1, P2, and P3 scallops. The anterior leaflet has no incisure but the segments apposed P1, P2, and P3 take the name of A1, A2, and A3, respectively. Magnified image from a (c) medial and (d) lateral perspective showing the medial (MC) and lateral (LC) commissures.
Fig. 16.8 (a) Opened view of the left ventricular outflow tract (LVOT) showing the ventricular surface of the aortic leaflet (AL). (b) Opened view of the left ventricular inflow tract (LVIT) showing the atrial surface of the mitral valve. The aortic leaflet (AL) forms the posterior wall of the LVOT. The chordae arising from each papillary muscles (PM) insert on both the mural and aortic leaflets of the mitral valve. The boundary of the rough zone (RZ) is shown by dotted red line. Direction of blood flow is shown by yellow arrows. Yellow stars show approximate location of the fibrous trigones. (c) Inlet/outlet view of the left heart. The aortic leaflet of the mitral valve is dissected longitudinally and continued along the noncoronary sinus into the aorta. Ao, aorta; AL, aortic (anterior) leaflet; ML, mural (posterior) leaflet; L, left coronary sinus; N, nonfacing coronary sinus; LMA, left main artery; MPA, main pulmonary artery; PM, papillary muscle; R, right coronary sinus; RCA, right coronary sinus.
Fig. 16.9 CT image in long-axis view with focal magnification during (a, b) diastole and (c, d) systole. The anterior leaflet is divided into the rough (RZ) and clear (CZ) zones. The rough zone covers the distal surface of both leaflets and receives the insertions of chordae tendineae. The coaptation surface (arrows) in (d) is the area where leaflets juxtapose each other during systole. Ao, aorta; LV, left ventricle; LA, left atrium.


Chordae Tendineae


The chordae tendineae (tendinous chords) connect the PMs and the ventricular wall to MV leaflets. They arise from the tip of PMs or the ventricular wall and split in numerous branches interconnected to each other in a complex mesh that ensures balanced distribution of mechanical forces. 6 The chordae inserting on the lateral half of both leaflets arise from the anterolateral PM, while the chordae inserting on the medial half originate from the posteromedial PM. One classification divides the chordae into first, second, and third order based on their insertion point on the leaflets. The first-order chordae or marginal chordae are stiff and less extensible cord inserting on the free edges of leaflets (Fig. 16‑10 , Fig. 16‑11 , Fig. 16‑12). Commissural chordae and cleft chordae are subcategories of first-order chordae. Rupture of these chordae almost always causes leaflet prolapse and mitral regurgitation. The second-order chordate are larger, elastic, and fewer in number that insert on the ventricular surface of the anterior leaflet at the confine between the rough and clear zones and often include two “strut chordate.” Their function is to prevent the leaflets from eversion. Furthermore, with reducing the motion at the peripheral part of anterior leaflet, the strut cords help the central part of the anterior leaflet to remain mobile. Thus, in systole the anterior leaflet takes a concave shape toward the outflow tract, while in diastole it bulges toward the inflow tract. This configuration facilitates blood outflow into the aorta during systole or blood inflow into left ventricle during diastole. Moreover, strut cords provide a fibrous continuity between the leaflet and the ventricular wall, supporting the contraction of longitudinal fibers of left ventricle and maintain left ventricular geometry. 8 Second-order chordal transposition has been used to correct anterior MV leaflet prolapse. 9 The third-order chordae or basal chordae originate directly from the ventricular wall and insert only on the posterior leaflet. These chordae limit the motion of the posterior leaflet (Fig. 16‑10 ).

Fig. 16.10 Classification of the chordae tendineae. (a) Aerial surface of the AL. First-order (marginal) cords are essential for leaflet coaptation. (b) Aortic (ventricular) surface of the AL showing the second-order cords inserting on the body of anterior leaflet at the border between the rough and clear zones. Second-order cords of the rough zone cords are essential for maintaining leaflet geometry. Also shown in this view is the aortomitral fibrous curtain. (c) Third-order cords originate from the ventricular trabeculae and insert only on the mural leaflet. AL, aortic (anterior) leaflet; ML, mural (posterior) leaflet; L, left coronary sinus; N, nonfacing coronary sinus; PM, papillary muscle.
Fig. 16.11 Close-up views of the (a) atrial and (b) ventricular surfaces of the aortic leaflet of the mitral valve with detail of the chordae tendineae.
Fig. 16.12 (a, b) 2D transthoracic echocardiographic and (c, d) CT images in long-axis view with focal magnification. Arrows point at the strut chordae (SCs) and the marginal chordae (MC). The SCs are particularly thick and insert on the body of anterior leaflet at the border between the rough and clear zones. Ao, aorta; LV, left ventricle; LA, left atrium.

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Mar 16, 2021 | Posted by in CARDIOVASCULAR IMAGING | Comments Off on 16 The Mitral Valve
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