Pathophysiology of Mitral Stenosis
Mitral stenosis results from pathologic narrowing of the mitral valve orifice, which obstructs blood flow from the left atrium to the left ventricle, causing progressive elevation of left atrial and pulmonary arterial pressures. 1, 2
Anatomic Substrate
The mitral valve apparatus consists of four interconnected structures that can each contribute to stenosis: the mitral annulus, the anterior and posterior leaflets, the chordae tendineae, and the papillary muscles. 2, 3 Abnormalities in any of these components can result in obstruction of left ventricular inflow. 1
Rheumatic Mitral Stenosis (85% of Cases)
Rheumatic MS involves commissural fusion with thickening at the leaflet tips, representing the dominant pathophysiologic mechanism worldwide. 1, 2, 3 The inflammatory process causes:
- Progressive commissural fusion that restricts leaflet separation 1
- Leaflet thickening concentrated at the tips 2
- Restriction of leaflet mobility through fibrosis 1
- Chordal shortening and fusion 4
This process typically presents in younger patients and is the consequence of prior rheumatic fever. 1
Degenerative Mitral Stenosis (12% of Cases)
Degenerative MS results from mitral annular calcification (MAC) with progressive calcium deposition, valve thickening, and chordal shortening—critically, without commissural fusion. 1, 2, 5 The pathophysiology differs fundamentally from rheumatic disease:
- Calcium deposits progressively in the mitral annulus, reducing annular area 1, 4
- The posterior mitral valve leaflet is affected first, with relative preservation of anterior leaflet mobility 1
- Valve thickening occurs at the base of leaflets rather than the tips 5
- Restriction of leaflet mobility develops without commissural involvement 1
This form is increasingly prevalent in aging populations in developed countries. 1, 4
Hemodynamic Consequences
The reduced mitral valve orifice area causes elevation of left atrial pressure and pulmonary artery systolic pressure, with severity determined by the valve area and transvalvular gradient. 2, 3 The hemodynamic cascade proceeds as follows:
- Normal mitral valve area is 4-6 cm²; symptoms typically begin when area falls below 2.5 cm² 6
- Severe stenosis (MVA ≤1.0 cm²) generates mean transmitral gradients ≥10 mmHg 2, 3
- Elevated left atrial pressure transmits retrograde to pulmonary veins and capillaries 3
- Chronic pulmonary venous hypertension leads to increased pulmonary vascular resistance 3
- Progressive pulmonary hypertension causes right ventricular dysfunction and tricuspid regurgitation 1
Clinical Pitfalls
Acute increases in cardiac output demands (exercise, pregnancy, fever, anemia, atrial fibrillation with rapid ventricular response) can precipitate acute pulmonary edema in patients with previously compensated stenosis. 3 The shortened diastolic filling time with tachycardia exacerbates the transvalvular gradient and left atrial pressure elevation. 3
Atrial fibrillation develops from chronic left atrial dilatation and further compromises hemodynamics by eliminating atrial contraction and reducing diastolic filling time. 3
Congenital Forms
Congenital mitral stenosis includes mitral annular hypoplasia, subvalvular stenosis, and supravalvular/intravalvular rings, often occurring as part of complex left-sided obstructive lesions (Shone complex). 1, 3 These patients require evaluation for other congenital abnormalities including ASD, VSD, and anomalous pulmonary venous connections. 1