Excerpt from Mitral Stenosis

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Background

Mitral stenosis (MS) is a narrowing of the inlet valve into the left ventricle that prevents proper opening during diastolic filling. Patients with mitral stenosis typically have mitral valve leaflets that are thickened, commissures that are fused, and/or chordae tendineae that are thickened and shortened.

The most common cause of mitral stenosis is rheumatic fever (RF). Approximately 40% of patients with rheumatic heart disease have isolated mitral stenosis. Conversely, rheumatic involvement is present in 99% of stenotic mitral valves excised at the time of mitral valve replacement. Rheumatic fever however, is exceedingly rare nowadays. It has been more than 10 years since the Centers for Disease Control and Prevention (CDC) discontinued mandatory reporting of acute rheumatic fever. Studies in the 1950s during an epidemic on a military base demonstrated a 3% incidence of RF in adults with streptococcal pharyngitis not treated with antibiotics. Studies in children during the same period demonstrated an incidence of only 0.3%.

After the initial episode of rheumatic fever, a latency period of 20-40 years occurs until the onset of symptoms. The natural history of mitral stenosis is typically progressive, with a slow and stable course early on, followed by progressive acceleration in the later years.

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Pathophysiology

The serum of patients with rheumatic fever contains antibodies to the type 5 streptococcal M protein, which cross-reacts with myocardial tissue. Pathologic examination of the mitral valve at this time reveals proliferation of fibroblasts and macrophages.

Subsequent disease may occur as a consequence of the healing of the rheumatic process, subclinical repetitive rheumatic insults, chronic rheumatic activity, or progressive hemodynamic stresses on the traumatized valve, similar to that of aortic stenosis. The plethora of postulated mechanisms for this pathologic evolution may account for the fact that some patients experience a chronic stable disease, whereas others have an accelerated course necessitating early surgical intervention.

The normal area of the mitral valve orifice is 4-5 cm², which effectively creates a common chamber between the left atrium and the left ventricle in diastole. In early diastole, a small and brief pressure gradient is present, but, during most of the filling period, the pressures in the 2 chambers are equal. Narrowing of the valve area to less than 2.5 cm² impedes the free flow of blood and causes a build up of left atrial pressure (LAP) to promote normal transmitral flow volume. Symptoms typically do not begin until valve area reaches 1.5 cm².¹

Critical mitral stenosis occurs when the opening is reduced to 1 cm². At this stage, a LAP of 25 mm Hg is required to maintain a normal cardiac output. With progressive stenosis, critical flow restriction reduces left ventricular output. The increase in LAP also enlarges the left atrium and raises pulmonary venous and capillary pressures. The resulting pulmonary congestion and reduced cardiac output can mimic primary left ventricular failure, but left ventricular contractility is normal in most cases of mitral stenosis. As the disease evolves, chronic elevation of LAP eventually leads to pulmonary hypertension, tricuspid and pulmonary valve incompetence, and secondary right heart failure.

Frequency

United States

The prevalence of mitral stenosis has decreased because of the decline in RF. Antibiotic treatment of strep throat may be partially responsible for the decline. Early trials demonstrated that penicillin treatment of streptococcal pharyngitis is effective in preventing acute RF (relative risk [RR], 0.28; number needed to treat [NNT], 63) in the samples studied. However, the incidence of acute RF in these trials was much higher in both treated and control groups than is seen today; the reported incidence per population was approximately 60 times greater in 1965 than in 1994 and the NNT today would be close to 4000.

Although the true incidence of disease is not known, about 1500 balloon mitral valvotomies were performed in the United States in 2004. This number serves as a rough index of severe disease prevalence.

International

Both RF and mitral stenosis remain common in developing nations, and progression of mitral stenosis tends to be more rapid in these areas. A recent echocardiographic study of school children in Mozambique and Cambodia demonstrated an incidence of rheumatic heart disease of 30.4 cases per 1000 and 21.5 cases per 1000, respectively.²

Mortality/Morbidity

Overall, the 10-year survival rate of untreated patients with mitral stenosis is 50-60%, depending on symptoms at presentation.

Without surgical intervention, the 20-year mortality rate is 85%.

In untreated patients, the causes of death are as follows:

• Progressive heart failure in 60-70%
• Systemic embolism in 20-30%, pulmonary embolism in 10%
• Infection in 1-5%

Race

The attack rate for patients born and raised in the United States is low. In areas where streptoccocal infections are more prevalent (including skin infections), the rate of rheumatic fever, and thus mitral stenosis, is much greater. Residents of developing countries are at the highest risk.

Sex

Although the attack rate for rheumatic fever is roughly equal among genders, mitral stenosis is 2-3 times more common in women than in men.

Age

Age at onset of symptoms depends on geography; where patients in developing countries may become symptomatic in their late teens or early twenties, patients in more developed countries have a mean age of presentation in the fifth or sixth decade of life.³

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