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].

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].

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 [53–55]. 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 [50–56]. 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).