Mitral Valve Prolapse, Mitral Regurgitation

Anatomy of the Mitral Valve

The mitral valve apparatus consists of an annulus, cusps, chordae tendineae, and papillary muscles. The shape of the mitral valve annulus is saddle-like. The mitral valve is functionally bicuspid, but embryologically comprises four cusps. Two cusps are large (the anterior or aortic cusp and the posterior or mural cusp) and two are small commissural cusps ( Fig. 33.1 ). In a normal mitral valve, these commissures are never complete. The posterior leaflet is the widest around the annulus and is divided into three scallops: P1, P2, and P3. The opposing sections of the anterior leaflet are designated A1, A2, and A3. The chordae tendineae can be divided into three groups. The first two groups originate from or near the apices of the papillary muscles ( Fig. 33.2 ). The first-order chordae insert into the extreme edge of the valve. The second-order chordae insert on the ventricular surface of the cusps. The third-order chordae originate from the ventricular wall much nearer the origin of the cusps. These chordae often form bands or foldlike structures, which may contain muscle. Usually there are two papillary muscles (anteriolateral and posteromedial), which have bifid apices; each receives chordae from both major mitral valve cusps.

Definition, Etiology, and Pathology of Mitral Valve Prolapse

The term billowing valve refers to superior motion of the body of the leaflet. The longer anterior leaflet normally exhibits mild billowing during ventricular systole. Billowing at the base of the leaflet may be considered abnormal when it exceeds 2 mm above the annular plane in a long-axis view (∼130 degrees in the midesophageal plane by transesophageal echocardiography) and 5 mm in the four-chamber view (0 degree midesophageal plane). The three scallops of the posterior leaflet have shorter height (length) and normally do not exhibit billowing.

Prolapse is defined as the systolic billowing of the free edge of the mitral valve leaflets into the atrium superior to the annular plane, with or without associated regurgitation. In a flail valve, the edge of the leaflet projects toward the atrium, typically after chordal or papillary muscle rupture, of abnormal elongation of the chordae, excess tissue. It is typically associated with severe mitral regurgitation.

Many conditions may affect components of the mitral valve apparatus and cause secondary prolapse, such as coronary artery disease, rheumatic disease, various cardiomyopathies, and trauma with elongation or rupture of mitral chordae resulting in a flail leaflet. MVP due to primary disorders of connective tissue such as Marfan syndrome is described in another chapter. Usually a primary disorder of the mitral valve leaflets exists, associated with specific pathologic changes that cause redundancy of the valve leaflets and their prolapse into the left atrium during systole.

Surgeons differentiate two different forms of degenerative mitral valve disease: Barlow’s disease and fibroelastic deficiency. Barlow’s disease is a more generalized form of valve degeneration and has a myxoid appearance of the whole valve with excess tissue and a dilated annulus. In fibroelastic deficiency, thickening is restricted to the prolapsed area(s) and the remaining valve tissue is more transparent, not thickened, without excess tissue, and the annulus may or may not be dilated.

The exact etiology of primary MVP is unknown. Individuals with MVP are usually of a slender body habitus indicating higher rates of linear growth, suggesting that the connective tissue is of lesser quality and gives less resistance to linear growth. This condition is observed in its most extreme form in Marfan syndrome. MVP might result from a mild imbalance of the growth dynamics of the mitral valve apparatus, especially between the leaflets, the chordae tendineae, and the rest of the heart. Such imbalance may be transient with complete disappearance of MVP in some patients. In many patients, an abnormal metabolism of collagen associated with an overproduction of mucopolysaccharides results in thickening of one or both mitral valve leaflets and a redundancy of the mitral valve leaflet(s) area. Indeed, the characteristic microscopic feature of primary MVP is a marked proliferation of the spongiosa, the myxomatous connective tissue between the atrialis and the fibrosa or ventricularis that supports the leaflet. In secondary MVP, no occurrence of myxomatous proliferation of the spongiosa is found.

When the leaflets become grossly abnormal and redundant with increasing quantities of myxoid stroma, they may prolapse. In addition, regions of endothelial disruption occur and become possible sites of thrombus formation or endocarditis. Even the mitral valve annulus and the chordae tendineae can be affected by a myxomatous proliferation, resulting in chordal rupture and worsening of a preexisting mitral valve regurgitation. Myxomatous changes in the annulus can cause annular dilatation and calcification, contributing to the severity of the mitral valve regurgitation.

Prevalence of Mitral Valve Prolapse

Primary MVP is the most frequently diagnosed cardiac valvular abnormality in the developed world, the most frequent cause of significant mitral valve regurgitation, and the most common substrate for mitral valve endocarditis. MVP appears to exhibit a strong hereditary component transmitted as an autosomal trait. When using strict criteria and adequate diagnostic tools, a prevalence of 2.4% without preponderance in age or gender is observed.

Primary MVP occurs most often as an isolated valve dysfunction, but can be associated with connective tissue diseases such as Marfan syndrome, Ehlers-Danlos syndrome, osteogenesis imperfecta, and muscular dystrophy. In addition, MVP seems to be associated with congenital cardiac abnormalities such as Ebstein malformation of the tricuspid valve, secundum-type atrial septal defect, and Holt-Oram syndrome.