Acquired Mitral Stenosis 

Acquired mitral stenosis (MS), or mitral valve stenosis, is virtually synonymous with rheumatic heart disease. In genetically susceptible individuals, rheumatic fever occurs as a complication of group A streptococcal infection. Other rare causes of acquired MS include carcinoid causes, systemic lupus erythematosus, rheumatoid arthritis, and some mucopolysaccharidoses. The underlying pathological process is a diffuse inflammation of connective tissue.

Not all group A streptococcal infections lead to rheumatic fever. Studies demonstrate that rheumatic fever follows infection of the upper respiratory tract and rarely, if ever, follows skin infection. Similarly, not all cases of streptococcal pharyngitis lead to rheumatic fever. In fact, only 2-3% of patients with untreated group A streptococcal pharyngitis develop this complication. Appropriate treatment of streptococcal pharyngitis prevents rheumatic fever.

Rheumatic heart disease primarily affects the mitral valve; mitral regurgitation (MR), or mitral valve regurgitation, is the initial hemodynamic consequence. Lesions of the mitral valve begin as deposits of fibrin and RBCs that form small verrucae along the borders of the mitral valve leaflets. When the inflammation subsides, the verrucae are replaced by fibrous tissue. Over at least several years, the individual may then develop fibrosis of the mitral ring; contracture of the mitral leaflets, chordae tendineae, and papillary muscles; and commisural adhesions that result in valve stenosis. Therefore, rheumatic heart disease is a lifelong and sometimes progressive disease.

Definition

Mitral valve stenosis results from a pathologic process that narrows the effective mitral valve orifice. Proper function of the mitral valve requires an intact mitral valve apparatus and satisfactory left ventricle (LV) function.

Anatomy

The mitral valve is the inlet valve to the LV. The normal mitral valve is a complex apparatus composed of an annulus and two leaflets that are attached to two papillary muscles by chordae tendineae. The papillary muscles arise from the walls of the LV and secure the chordae and mitral leaflets, preventing prolapse of the valve during ventricular systole.

Anatomy of subtypes

Mitral valve stenosis, such as is seen in rheumatic fever, occurs because of fibrous scarring of the valve leaflets with subsequent calcification, thereby decreasing the size of the effective valve orifice. Subvalvular and supravalvular MS are congenital anomalies (see Mitral Stenosis, Congenital).

The normal adult mitral valve orifice cross-sectional area is 4-6 cm². When reduced to 2 cm², hemodynamically significant MS occurs. At 1 cm², obstruction to blood flow into the LV becomes critical because a left atrial mean pressure of 25 mm Hg is necessary to maintain normal cardiac output. Elevated left atrial pressure is transmitted to the pulmonary veins and pulmonary capillaries. Congested bronchial veins encroach on small bronchioles and cause subsequent increase in airway resistance. In addition, elevated hydrostatic pressure in the capillaries forces fluid into the alveoli and interstitial space, producing pulmonary congestion.

As a compensating mechanism, pulmonary vasoconstriction develops, causing pulmonary hypertension. At this stage, the right ventricle (RV) faces an increased afterload, leading to RV hypertrophy. Over time, fixed pulmonary arterial hypertension may develop from medial hypertrophy and intimal thickening of the pulmonary arterioles. RV myocardial dysfunction may develop, resulting in tricuspid valve regurgitation. Severe MS results in decreased cardiac output. If reduction in cardiac output is critical, end organ failure with shock, metabolic acidosis, and renal and/or hepatic insufficiency can occur. In addition, RV failure provokes systemic venous congestion with development of hepatomegaly, ascites, and pedal edema.

Natural history

Patients may remain asymptomatic for many years as long as the MS is mild and not accompanied by more than mild MR. These patients, of course, are susceptible to further damage to the mitral valve with repeated group A streptococcal pharyngitis. For this reason, ongoing antibiotic prophylaxis is recommended.

By the second or third decade of life, calcium deposits further constrict the effective mitral orifice of the already damaged mitral valve. Once the effective valvular orifice decreases significantly, symptoms occur.

In developing countries, rheumatic MS manifests 10-30 years after the initial rheumatic insult to the mitral valve. In developed countries, this latent period may be as long as 50 years.

Frequency

United States

Acquired MS is exceedingly rare in the pediatric population in the United States. Acquired MS secondary to rheumatic fever remains the most common form of MS that occurs in adulthood. Current estimates indicate that the prevalence of rheumatic fever in the United States is less than 1 case per 100,000 people. A steady decline has been observed in the incidence of rheumatic fever and, thus, in acquired MS.

International

In some developing countries, such as India, the prevalence of rheumatic fever is 100-150 cases per 100,000 people. Following development of rheumatic heart disease, evidence of MS may develop as early as the teenage years, presumably because of a more aggressive initial attack and/or recurrent bouts of rheumatic fever (consequences of suboptimal or absent antibiotic prophylaxis). In some developing countries, the prevalence of rheumatic heart disease in children is 5-15 cases per 1000 people.

Mortality/Morbidity

If symptoms are absent or minimal, the overall 10-year survival rate of untreated patients with MS is 80%. Once symptoms develop, the mortality risk and disease progression increase substantially. In an unselected group of patients with MS of varying severity, 60% were alive after 10 years.

A significant risk of arterial embolization is observed in patients with atrial fibrillation. Atrial fibrillation has been attributed to left atrial distension, which creates conduction delay in the posterior left atrium on a line that runs vertically between the pulmonary veins.^[1] Increased inflammation is also thought to provoke atrial arrhythmias in these patients because patients with MS and atrial arrhythmias were found to have higher plasma levels of C-reactive protein.^{[2, 3]}

If congestive heart failure (CHF) develops, the prognosis is grim, with a 10-year survival rate of 15%.

Race

Genetic predisposition plays a significant role in occurrence of rheumatic fever after group A streptococcal infection. Family studies suggest that susceptibility to the disease involves a single recessive gene.

Sex

Rheumatic fever equally affects both sexes. However, in those who acquire rheumatic heart disease, MS is more common in women. Reasons for this are unknown.

A recent study found that, although MS is more frequent in women, mitral regurgitation is equally frequent in both sexes.^[4]

Age

Rheumatic fever is a disease of childhood, its incidence parallels that of streptococcal pharyngitis. MS usually arises in persons older than 15-20 years because the disease progresses to that stage over many years. This time interval is significantly shorter in developing countries.