During cardiac catheterization of a patient with mitral stenosis, the cardiologist measures the transmitral pressure gradient and calculates mitral valve area. Which is the most common site of obstruction in rheumatic mitral stenosis?

Question

Accepted Answer

C. Commissural fusion between anterior and posterior leaflets. ## Anatomic Pathology of Rheumatic Mitral Stenosis

**Key Point:** Commissural fusion (fusion of the junction between the anterior and posterior leaflets) is the hallmark and most common site of obstruction in rheumatic mitral stenosis.

### Morphologic Changes in Rheumatic MS

The sequence of pathologic changes in RHD-related mitral stenosis:

1. **Acute phase** — valve inflammation, edema, and fibrinoid necrosis (during acute rheumatic fever).
2. **Chronic phase** — fibrosis, hyalinization, and calcification of valve leaflets.
3. **Commissural fusion** — the two commissures (anterolateral and posteromedial) fuse, reducing the mitral orifice.
4. **Leaflet thickening** — the leaflets become rigid and immobile.
5. **Chordal shortening** — subvalvular apparatus becomes fibrotic and shortened.

**High-Yield:** The **commissures** are the primary sites of fusion in rheumatic MS. The anterior commissure (between anterior and posterior leaflets) is the most commonly and severely affected.

### Echocardiographic and Pathologic Correlation

| Feature | Rheumatic MS | Congenital MS | Calcific MS |
|---------|--------------|---------------|-------------|
| **Primary pathology** | Commissural fusion + leaflet thickening | Parachute valve, supravalvular ring | Annular calcification |
| **Leaflet mobility** | Reduced (doming in early diastole) | Variable | Restricted |
| **Chordal involvement** | Shortening, fusion | Abnormal insertion | Spared |
| **Commissures** | Fused (anterolateral > posteromedial) | May be normal | Normal |

**Clinical Pearl:** On transthoracic echocardiography, the "hockey stick" appearance of the anterior mitral leaflet (doming in early diastole due to commissural fusion) is pathognomonic for rheumatic mitral stenosis.

### Why Commissural Fusion Dominates

1. **Inflammatory target** — the commissures are the most inflamed regions during acute rheumatic fever.
2. **Fibrosis pattern** — post-inflammatory fibrosis preferentially bridges the commissural space.
3. **Progressive narrowing** — commissural fusion directly reduces the mitral orifice area, creating the stenotic lesion.
4. **Hemodynamic consequence** — fusion of both commissures can reduce valve area from normal (~4–6 cm²) to <1 cm² in severe stenosis.

**Mnemonic: "Commissures Fuse First" (CFF) — the anterolateral commissure fuses before the posteromedial, and both are the primary obstruction site in RHD.**

[cite:Robbins 10e Ch 12]

Answer

A. Thickening of the mitral annulus with calcification

Answer

B. Papillary muscle rupture with chordal elongation

Answer

D. Left atrial myxoma prolapsing into the mitral orifice

During cardiac catheterization of a patient with mitral stenosis, the cardiologist measures the transmitral pressure gradient and calculates mitral valve area. Which is the most common site of obstruction in rheumatic mitral stenosis?

Explanation

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