In 1966, Barlow and Bosman described a constellation of clinical findings consisting of nonejection systolic clicks and a late systolic murmur, T-wave abnormalities, and systolic aneurysmal billowing of the posterior mitral leaflet into the left atrium. Since then, in areas without rheumatic heart disease, mitral valve prolapse (MVP) has been portrayed as the most common form of valvular heart disease. It is characterized by pathologic anatomic and physiologic changes in the mitral valve apparatus affecting mitral leaflet motion and function.
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.
Early Presentation of Mitral Valve Prolapse
Most patients with primary MVP remain asymptomatic. The diagnosis is often made by a routine cardiac auscultation or by an echocardiography performed for other reasons. The diagnosis of MVP is sometimes considered in patients who have thoracic skeletal abnormalities reflecting suboptimal connective tissue: the most common of these are scoliosis, pectus excavatum, straightened thoracic spine, and narrowed anteroposterior diameter of the chest.
Some patients with primary MVP become symptomatic without significant mitral valve dysfunction. Chest discomfort, anxiety, fatigue, atypical dyspnea with exercise, at rest and nocturnal atypical palpitations, and orthostatic and neuropsychiatric symptoms that are not correlated with the mitral valve function, are described as MVP syndrome (MVPS). The cause of these latter symptoms in MVP syndrome is unknown, but an association between a dysfunction of the autonomous nervous system and MVP is suggested. MVP may be complicated by more serious events such as infective endocarditis, thromboembolic events, atrial and ventricular arrhythmia, and rarely by syncope and sudden cardiac death.
On physical examination, MVP is characterized by an apical mid- or late systolic click, at least 140 ms after the first heart sound, after the beginning of the carotid pulse upstroke; the click can be intermittent and may be aggravated by maneuvers such as squatting or leaning forward. It seems to be caused by the sudden systolic tensing of the mitral valve apparatus as the leaflets billow into the left atrium. Any maneuver that decreases left ventricular volume, such as a Valsalva maneuver, sudden standing, inhalation of amyl nitrate, tachycardia, or augmentation of contractility, results in an earlier occurrence of prolapse during systole. In contrast, when left ventricular volume is augmented, such as during a sudden change from standing to supine position, leg raising, squatting, maximal isometric exercise, decreased contractility, and expiration, the click is delayed. The sensitivity of a click for diagnosis of MVP is low (19%) but its specificity is high: in only 1.5% of cases, can a midsystolic or late systolic click be heard in the absence of MVP.
MVP is often associated with mitral valve regurgitation. Therefore, in one-third of the patients, the midsystolic click is followed by a typical apical late systolic heart murmur.
The ECG is often normal in patients with MVP. The most common abnormality is the presence of ST-T wave depression or T-wave inversion in the inferior leads. Exercise testing is frequently falsely positive with ST-T wave depression, especially in women, even with normal coronary arteries.
The two-dimensional transthoracic or transesophageal echocardiography is the easiest diagnostic tool to confirm the diagnosis of MVP. Two-dimensional views display the leaflets and the annulus of the mitral valve, but the images must be interpreted in the context of the three-dimensional saddle-like shape of the valve. The nonplanar saddle shape of the normal mitral leaflets can give the appearance of prolapse in certain echocardiographic views. The echocardiographic criteria used for the diagnosis of a classic MVP are a dislocation greater than 2 mm to the left atrium of at least one of the mitral valve leaflets during systole and a thickening greater than or equal to 5 mm of the prolapsing valve leaflet during diastole. Dislocation is referred by a hypothetical line through the insertion points of the anterior and the posterior mitral valve leaflet in parasternal and apical long-axis views ( Fig. 33.3 ).
The current two- and three-dimensional transthoracic and transesophageal echocardiography machines generate exquisite images, allowing one to clearly identify the mechanism of mitral regurgitation and to differentiate Barlow’s disease from fibroelastic deficiency. In patients who require a surgical intervention, the possibilities of reconstructive surgery can be better estimated, allowing the patient to be treated by the best technique done by the best surgeon.
The most typical MVP is characterized by important mitral valve regurgitation, significant enlargement of the mitral valve leaflets and annulus, elongation of the chordal apparatus, and loss of leaflet apposition. At the other end of the spectrum, patients with mild bowing and normal-appearing leaflets should be considered as normal variants because their risk of adverse events probably does not differ from that in the general population.
Management of Mitral Valve Prolapse
Most patients with MVP require no treatment. Because there is no cure for MVP, management should concentrate on adequate patient guidance, relief of symptoms, and avoidance of complications ( Table 33.1 ).
