Etiology

Rheumatic fever leading to rheumatic heart disease is the main cause of mitral stenosis.

Rarer causes of mitral stenosis include congenital deformity of the valve, the carcinoid syndrome, use of ergot and/or serotonergic drugs such as fenfluramine, SLE, mitral annular calcification (MAC) due to aging, and amyloidosis.

It is thought that some M antigens are held in common by the heart and some strains of group A streptococci.[6][7] Following infection by the organism (usually manifested as pharyngitis), the inflammatory response against the bacterium leads also to attack of the heart. This concept has, however, been challenged. There is evidence that, rather than immunological cross-reactivity giving rise to tissue damage, the streptococcal M protein may bind directly to the valvular collagen, inciting an inflammatory response.[8] While endocardium, myocardium, and pericardium may all be involved, it is the endocardium containing the heart valves that receives the most inflammatory damage. During an attack of acute rheumatic fever the mitral valve becomes thickened and retracted leading to mitral regurgitation. However, years later, fusion of the mitral leaflet commissures and thickening of the leaflets and subvalvular apparatus cause mitral stenosis.[9] It is still unclear whether this remote damage stems from hemodynamic stress on a previously damaged valve or is caused by ongoing inflammation from a smoldering rheumatic process.[10]

As world populations age, MAC, a nonrheumatic build up of calcium in the mitral annulus, is another important cause of mitral stenosis. Here, the leaflets themselves are usually normal and stenosis is caused by encroachment of calcium on the mitral orifice.

Pathophysiology

The normal mitral valve has an orifice area of about 4 cm² that permits free flow of blood from the left atrium into the left ventricle during diastole. As the valve orifice becomes reduced in mitral stenosis, flow between left atrium and left ventricle is progressively impeded and pressure in the left atrium remains higher than that of the left ventricle.[9]

By restricting flow, mitral stenosis results in 2 primary pathophysiologic consequences:

  • Increased left atrial pressure is referred to the lungs, where it leads to congestion and the symptoms associated with it

  • The restricted orifice limits filling of the left ventricle, thereby limiting cardiac output.

Thus, although left ventricular contractility is usually normal, the pathophysiologic effects of mitral stenosis produce a syndrome mimicking left heart failure.

As left atrial pressure increases, a pressure overload encumbers the right ventricle, the chamber responsible for filling the left ventricle. With progression of the disease, pulmonary vasoconstriction adds to right ventricular pressure overload, and severe pulmonary hypertension may ensue.

Whether left ventricular contractility is affected in mitral stenosis is still in debate.[11][12][13] Ejection performance is reduced in about one third of patients. In most patients this is secondary to increased afterload induced by reflexive vasoconstriction and is compounded by reduced preload from reduced left ventricular filling.[14] In such cases the ejection performance returns to normal following relief of the mitral stenosis.[15] However, in developing countries where the rheumatic process seems very aggressive, myocardial injury and impaired contractility may be present.[12]

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