Most cases of mitral stenosis (MS) are caused by rheumatic heart disease (Fig. 78–1).1 However, rheumatic fever has become quite rare in developed nations and so too has MS become rare. Indeed, most MS in the United States occurs in patients who have emigrated here from countries where rheumatic fever is still commonplace. Why rheumatic fever has waned in developed nations is unclear. Although antibiotic use almost certainly plays a role,2 the decline in disease incidence began before antibiotics were widely available, suggesting that socioeconomic factors also play a key role in the disease process. In addition, the organism responsible (group A Streptococcus) itself may have mutated to a less rheumatologic agent.
The typical fish mouth appearance of rheumatic mitral stenosis is shown. Reproduced with permission from Otto.1
Although it is clear that rheumatic fever causes the disease, the exact mechanisms are still controversial. It is generally agreed that rheumatic fever occurs after infection with group A Streptococcus.3-5 The bodily defense against the organism attacks "M" protein antigens shared by the bacterium and the heart in some patients.4 Thus there is an inflammatory response that leads to cardiac damage, potentially of all three cardiac layers, the pericardium, myocardium, and endocardium. However, it is the endocardium, from which the cardiac valves are derived, that is most severely affected. Although all four valves may become damaged, the mitral valve is virtually always affected, but the reasons for this propensity are unclear. Perhaps greater mechanical stress on the mitral valve causes the inflammatory process to be manifested more severely there than on other valves.
The initial attack causes inflammation, thickening, and retraction of the mitral leaflets, usually causing mild mitral regurgitation, which may disappear as the attack subsides. Why MS develops later is not entirely clear, but at least 3 factors contribute to the process: sex, the severity of carditis in the first attack, and the number of subsequent attacks. Mitral stenosis is primarily a disease of women, with a 3:1 female preponderance. If after the initial attack there is little evidence of valvulopathy and no subsequent attacks occur, the chance that the patient will develop severe MS later in life is probably less than 5%.6 Subsequent attacks can be prevented by faithful adherence to antibiotic prophylaxis. However, what pathologic processes occur between the initial attack of acute rheumatic fever and eventual development of MS (when it does occur) are uncertain. At the time of surgery for MS there are active Aschoff nodules (the pathognomonic lesion of rheumatic fever) in the left atrial appendages of many patients, suggesting that a smoldering rheumatic process persists years after the last acute attack.7 Alternatively, it may be that after the initial lesion is created by rheumatic fever, hemodynamic stress on the valve may lead to continued inflammation and scarring. ...