37 Mitral Valve Prolapse

Lee R. Goldberg, Park W. Willis, IV

Mitral valve prolapse (MVP) is the most common form of congenital heart disease in adults, affecting 4% to 5% of the U.S. population. MVP refers to the superior and posterior displacement of one or both mitral leaflets into the left atrium during systole. Although MVP is usually benign, important complications, including infective endocarditis and severe mitral regurgitation, can occur.

MVP can be classified as primary, secondary, or functional. Primary MVP occurs in the absence of connective tissue disease. Myxomatous degeneration of the tricuspid valve may be present, and the aortic and pulmonic valves are sometimes involved. Primary MVP can be associated with skeletal abnormalities, von Willebrand’s disease, and hypomastia. Secondary MVP occurs in the presence of a known connective tissue disorder, such as Marfan’s syndrome, Ehlers-Danlos syndrome, adult polycystic kidney disease, osteogenesis imperfecta, and pseudoxanthoma elasticum. The pathologic changes of the mitral valve apparatus are identical to those found in primary MVP. In functional MVP the mitral valve is anatomically normal, but both superior and posterior displacement of the valve can occur secondary to other cardiac conditions. Causes of functional MVP include a dilated mitral annulus and ischemic papillary muscle dysfunction. In hypertrophic cardiomyopathy, the left ventricular (LV) cavity may be too small to accommodate the mitral valve, causing functional MVP. In atrial septal defect, left-to-right shunting and right ventricular chamber dilation secondary to volume overload can result in a small left ventricle and functional MVP.

Etiology and Pathogenesis

The etiology of MVP is unknown. There is a familial form of MVP in which transmission occurs in an autosomal-dominant fashion, with variable penetrance. However, there is no single, or group of, genetic abnormalities thought to be responsible for MVP. Pathologic findings in primary and secondary MVP often involve the mitral leaflets and the chordae tendineae. Typical gross pathologic findings include thickened, redundant mitral leaflets and elongated chordae tendineae.

Although both the anterior and posterior mitral leaflets may be affected, the middle scallop of the posterior leaflet is most commonly involved in the myxomatous process. Histologic examination of the mitral valve leaflets reveals interruption of collagen bundles and an accumulation of acid mucopolysaccharides within the spongiosa layer.

Clinical Presentation

The clinical presentation of MVP is highly variable. Most patients with MVP are asymptomatic. The most common complaint is atypical chest pain. Other nonspecific symptoms associated with MVP include palpitations, dizziness, dyspnea, anxiety, numbness, and tingling. There is ongoing controversy about whether these complaints are caused by MVP, as was initially reported, or are a coincidental finding. It is quite possible that the original descriptions of MVP were probably influenced by selection bias. Several subsequent studies have not shown a truly increased frequency of symptoms such as chest pain, dyspnea, or dizziness.

MVP is usually discovered incidentally on routine physical examination, and cardiac auscultation is the key to making the clinical diagnosis. One or more nonejection systolic clicks in midsystole or late systole with or without a late systolic murmur are characteristic of MVP. The systolic click, or clicks, heard in MVP are thought to originate from the chordae tendineae snapping as the mitral leaflets bow into the left atrium. Multiple systolic clicks can cause a scratching sound occasionally likened to a pericardial friction rub. Mitral regurgitation at the time of leaflet prolapse results in late systolic murmur. This murmur typically has a crescendo contour and envelops the second heart sound (S₂). It is commonly preceded by a nonejection click but may occur alone. A late systolic murmur usually indicates mild mitral regurgitation; with more significant mitral regurgitation, the murmur can be pansystolic and a click may not be audible. Although late systolic accentuation is often preserved, the murmur can become indistinguishable from the murmur related to mitral regurgitation from other causes. In the case of posterior leaflet prolapse, the mitral regurgitant flow is commonly directed anteriorly toward the aortic root and the murmur may be transmitted along the left sternal border and to the aortic area. With anterior leaflet prolapse, the murmur radiates to the left axilla and back.

MVP is a dynamic, load-dependent phenomenon, and the most sensitive and specific physical diagnostic criteria are based on characteristic postural changes in auscultatory findings. A complete examination with the patient in the supine, standing, and sitting positions is required to alter hemodynamics and ventricular loading conditions and thus detect the characteristic findings with the highest degree of accuracy. The postural auscultatory changes are related primarily to changes in LV volume, augmented by alterations in heart rate and myocardial contractility. Generally, measures that decrease LV volume produce earlier and more prominent systolic prolapse of the mitral leaflets, causing the systolic click and murmur to move closer to the first heart sound (S₁) (Fig. 37-1).

Figure 37-1 Auscultation in mitral valve prolapse.

C, click; SM, systolic murmur.

Differential Diagnosis

The differential diagnosis of a nonejection systolic click includes ejection sounds arising from the aortic and pulmonic valves, splitting of S₁ or S₂, and clicks arising from nonvalvular structures such as an atrial septal aneurysm, pericardial sounds, and clicks heard in patients with a pneumothorax. Aortic and pulmonic ejection sounds are high-frequency sounds that occur in early systole. An aortic ejection sound is best heard with the diaphragm of the stethoscope and simulates wide splitting of S₁. Though audible over the entire precordium, these sounds are usually loudest at the mitral area, where the S₁–ejection click sequence is commonly mistaken for a fourth heart sound followed by S₁. Pulmonary ejection sounds can be difficult to differentiate from the splitting of S₁, but the characteristic loud and “clicky” quality of these sounds, exaggerated in the expiratory phase of respiration and disappearing on inspiration, is a reliable diagnostic feature. Ejection clicks are not perceptibly affected by altering preload with postural changes. Because the ejection clicks occur as the semilunar valve opens, they precede the carotid upstroke, whereas the nonejection clicks of MVP occur afterward. A midsystolic click can occur with an atrial septal aneurysm. In this circumstance there is no associated late systolic murmur. Clicking pneumothorax can mimic MVP, but extra sounds do not show a consistent relationship to the cardiac cycle and can also occur in diastole. For this reason, a prolonged period of continuous cardiac auscultation is useful in diagnosis.

Diagnostic Approach

When the diagnosis of MVP is made by cardiac auscultation, transthoracic echocardiography can be useful to confirm the physical findings. Both two-dimensional and M-mode techniques are sensitive in detecting MVP. Echocardiography provides additional information, including the degree of leaflet prolapse, the severity of the mitral regurgitation, and the thickness of the mitral leaflets (Fig. 37-2

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