## Pathological Distinction Between Rheumatic Mitral Stenosis and Regurgitation ### Rheumatic Mitral Stenosis (MS) **Key Point:** Commissural fusion is the hallmark pathological feature of rheumatic MS. The two commissures (anterolateral and posteromedial) fuse, reducing the mitral orifice area to <2 cm². Leaflets become thick, fibrotic, and calcified, with reduced mobility. ### Rheumatic Mitral Regurgitation (MR) **Key Point:** In rheumatic MR, the leaflets undergo retraction and shortening due to fibrosis. Chordae tendineae shorten and thicken, preventing complete coaptation during systole. The leaflets remain mobile but fail to close completely. ### Comparison Table | Feature | Rheumatic MS | Rheumatic MR | |---------|-------------|-------------| | **Commissures** | Fused (pathognomonic) | Patent | | **Leaflet mobility** | Restricted | Reduced but present | | **Chordae** | May be shortened | Shortened and retracted | | **Leaflet coaptation** | Reduced orifice area | Incomplete closure | | **Echocardiographic sign** | Doming anterior leaflet | Systolic anterior motion of leaflet | **High-Yield:** Commissural fusion is UNIQUE to rheumatic MS and does NOT occur in rheumatic MR. This is the single best discriminating pathological feature. **Clinical Pearl:** A patient with rheumatic MS may develop MR if the stenotic valve is dilated by atrial fibrillation or if there is superimposed endocarditis, but the primary pathology remains commissural fusion. [cite:Robbins 10e Ch 12] 
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