Mitral Valve Anatomy

The human heart has 4 valves. The mitral valve connects the left atrium (LA) and the left ventricle (LV). The mitral valve opens during diastole to allow the blood flow from the LA to the LV. During ventricular systole, the mitral valve closes and prevents backflow to the LA. The normal function of the mitral valve depends on its 6 components, which are (1) the left atrial wall, (2) the annulus, (3) the leaflets, (4) the chordae tendineae, (5) the papillary muscles, and (6) the left ventricular wall (see the image below). ^[1]

Any congenital or acquired disorder of individual components can disturb the finely coordinated mechanisms of the mitral valve and result in an incompetent valve. ^[1] This article summarizes basic gross anatomy, microscopic anatomy, and pathophysiologic variants of the mitral valve disorders.

The mitral apparatus is composed of the left atrial wall, the annulus, the leaflets, the chordae tendineae, the papillary muscles, and the left ventricular wall. ^{[1, 2]} The valve is located obliquely behind the aortic valve.

Left atrial wall

The left atrial myocardium extends over the proximal portion of the posterior leaflet. Thus, left atrial enlargement can result in mitral regurgitation by affecting the posterior leaflet. The anterior leaflet is not affected, because of its attachment to the root of the aorta. ^[1]

Mitral annulus

The mitral annulus is a fibrous ring that connects with the leaflets. It is not a continuous ring around the mitral orifice ^[3] and appears to be more D-shaped, rather than circular as prosthetic valves are (see the image below). ^[2]

The straight border of the annulus is posterior to the aortic valve. The aortic valve is located between the ventricular septum and the mitral valve. ^[2] The annulus functions as a sphincter that contracts and reduces the surface area of the valve during systole to ensure complete closure of the leaflets. Thus, annular dilatation of the mitral valve causes poor leaflet apposition, which results in mitral regurgitation.

Mitral valve leaflets

Harken et al have described the mitral valve as a continuous veil inserted around the circumference of the mitral orifice. ^[4] The free edges of the leaflets have several indentations. Two of these indentations, the anterolateral and posteromedial commissures, divide the leaflets into anterior and posterior (see the first image below). These commissures can be accurately identified by the insertions of the commissural chordae tendineae into the leaflets (see the second image below). ^{[5, 6]}

Normally, the leaflets are thin, pliable, translucent, and soft. ^[2] Each leaflet has an atrial and a ventricular surface.

Anterior leaflet

The anterior leaflet is located posterior to the aortic root and is also anchored to the aortic root, unlike the posterior leaflet. Accordingly, it is also known as the aortic, septal, greater, or anteromedial leaflet. ^{[6, 7]} The anterior leaflet is large and semicircular in shape. It has a free edge with few or no indentations. ^[6] The 2 zones on the anterior leaflet are referred to as rough and clear zones, according to the chordae tendineae insertion. These 2 zones are separated by a prominent ridge on the atrial surface of the leaflet, which is the line of the leaflet closure. The prominent ridge is located approximately 1 cm from the free edge of the anterior leaflet.

Distal to the ridge is a rough zone that has a crescentic shape. ^[6] During systole or mitral valve closure, the rough zone of the anterior leaflet will appose to the rough zone of the posterior leaflet. ^[6] The rough zone is thick and has chordae tendineae insertions on the ventricular surface. Therefore, it appears to be opaque on transillumination. Conversely, the clear zone is defined as clear on transillumination and has no chordae tendineae insertion. It is located between the rough zone and the annulus.

Posterior leaflet

The posterior leaflet is also known as the ventricular, mural, smaller, or posterolateral leaflet. ^{[6, 7]} The posterior leaflet is the section of the mitral valve that is located posterior to the 2 commissural areas. ^[6] It has a wider attachment to the annulus than the anterior leaflet. It is divided into 3 scallops by 2 indentations or clefts. The middle scallop is larger than the other 2 (the anterolateral and posteromedial commissural scallops). The 3 zones on the posterior leaflets are referred to as rough, clear, and basal zones, according to the chordae tendineae insertion.

The rough zone is defined in the posterior leaflet. It is distal to the ridge of the line of the leaflet closure. It is broadest at the distal part of the scallops and tapers toward the clefts or indentations between the scallops. ^[6] Like that of the anterior leaflet, the clear zone of the posterior leaflet is clear on transillumination and has no chordae tendineae insertion. It is located in the middle part of the posterior leaflet, between the rough zone and the basal zone. The basal zone is located between the clear zone and the mitral valve annulus and has the insertion of basal chordae tendineae. This zone is only seen in the posterior leaflet and is best visualized on the middle scallop (because most of the basal chordae insert into this scallop).

Chordae tendineae

The chordae tendineae are small fibrous strings that originate either from the apical portion of the papillary muscles or directly from the ventricular wall and insert into the valve leaflets or the muscle. These 2 types are called true chordae tendineae and false chordae tendineae, respectively. This article will discuss only true chordae tendineae.

Commissural chordae

Commissural chordae are the chordae that insert into the interleaflet or commissural areas located at the junction of the anterior and posterior leaflets. ^[8] Two types of commissural chordae exist. Posteromedial commissural chordae insert into the posteromedial commissural area; anterolateral commissural chordae insert into the anterolateral commissural area. Most of the main stems of the commissural chordae point toward the center of the commissural areas. ^[8]

Leaflet chordae

The leaflet chordae are the chordae that insert into the anterior or posterior leaflets. Two types of chordae tendineae are connected to the anterior leaflet. The first is rough zone chordae, which insert into the distal portion of the anterior leaflet known as the rough zone. The second is strut chordae, which are the chordae that branch before inserting into the anterior leaflet.

The posterior leaflet has 3 types of chordae tendineae. The first is rough zone chordae, which are the same as the rough zone chordae of the anterior leaflet. The second is basal chordae, a type unique to the posterior leaflet; these insert into the basal zone of the posterior leaflet, which is located between the clear zone and the mitral valve annulus. Unlike the anterior leaflet, the posterior leaflet does not have strut chordae. The third type of chordae on the posterior leaflet is cleft chordae; these insert into the clefts or indentations of the posterior leaflet, which divide the posterior leaflet into 3 scallops.

Papillary muscles and left ventricular wall

These 2 structures represent the muscular components of the mitral apparatus. The papillary muscles normally arise from the apex and middle third of the left ventricular wall. ^[8] The anterolateral papillary muscle is normally larger than the posteromedial papillary muscle and is supplied by the left anterior descending artery or the left circumflex artery. The posteromedial papillary muscle is supplied by the right coronary artery. Extreme fusion of papillary muscle can result into mitral stenosis. On the other hand, rupture of a papillary muscle, usually the complication of acute myocardial infarction, will result in acute mitral regurgitation.

The 3 layers of the ventricular wall are the endocardium, the myocardium, and the epicardium. The endocardium consists of a simple squamous endothelium and a thin subendothelial tissue. The myocardium consists of cardiac muscle fibers. The epicardium consists of a simple squamous mesothelium and subepicardial tissue. There is a layer of dense fibrous connective tissue, called the annulus fibrosus, located between the atrium and ventricle. The mitral valve connects the left atrium (LA) and the left ventricle (LV). The mitral valve leaflets are composed of an outer layer of endocardium and a dense connective tissue core, which is continuous with the annulus fibrosus.

Mitral regurgitation

Mitral regurgitation (depicted in the video below) is characterized by the reversal of blood flow from the left ventricle (LV) to the left atrium (LA). The presentation of mitral regurgitation varies and largely depends on etiology, severity, and onset (see Mitral Regurgitation).

Mitral stenosis

Mitral stenosis is characterized by a narrowing of the left ventricular inflow tract at the level of the mitral valve resulting from a structural abnormality of the mitral valve apparatus. The most common cause is rheumatic heart disease (see Mitral Stenosis).

Mitral valve prolapse

Mitral valve prolapse is the most common valvular abnormality, affecting 2-6% of the population of the United States. It is the most common cause of isolated mitral regurgitation in the United States (see Mitral Valve Prolapse). Classic mitral valve prolapse is defined as greater than 2 mm superior displacement of the mitral leaflets into the left atrium during systole, with a leaflet thickness of at least 5 mm as revealed by transthoracic echocardiography (parasternal long-axis view).