CHAPTER 50: MITRAL STENOSIS

ETIOLOGY

Worldwide, most cases of mitral stenosis (MS) are caused by rheumatic heart disease (RHD) (Fig. 50–1).¹ However, rheumatic fever has become quite rare in developed nations and so too has MS. 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. Degenerative calcific MS may be confused with rheumatic MS. Although the incidence of degenerative calcific MS increases in the very elderly, the MS is most often mild to moderate, and does not require intervention.

RHD remains prevalent in developing countries. Using echocardiographic screening, the prevalence of RHD ranged from 20 to 30 per 1000 school children.^3,4 This leads to a large pool of rheumatic MS. Forty percent of patients with RHD have isolated MS, but only 60% of these patients report a past history of rheumatic fever.^4,5

Rheumatic MS is a progressive disease. The time interval between rheumatic fever and the clinical appearance of MS varies considerably among various countries. In developed countries, the disease appears to take two to four decades before symptoms appear.^5,6,7,8 Thus, presentation in the west is in the fifth to sixth decades of life.^{5,6,7,8,9,10,11} In developing countries, the disease may progress much more rapidly, leading to symptoms by the age of 20 years (juvenile MS), often within 5 years of the initial attack of RF.¹² This is likely a result of recurrences of RF (either clinical or subclinical). The mean age of symptom onset in developing countries is between the third and fourth decades.^11,13 Progression from mild to severe disability takes a decade.^6,8 Once symptoms develop, the prognosis becomes poor if left untreated. Among 271 symptomatic patients,⁷ the 10-year survival of patients in functional class II, III, and IV was 69%, 33%, and 0%, respectively. Only half of the patients in functional class II remained alive at 20 years, with none among class III patients. Overall, the mortality was 70% over an average period of 11 years.⁷

There is a progressive reduction in the valve area by an average of 0.09 cm²...