Mitral Stenosis

Mitral Stenosis: Etiology

Rheumatic heart disease is the overwhelming cause of mitral stenosis (MS) worldwide, producing commissural fusion, leaflet thickening, chordal fusion/shortening, and progressive restriction of the diastolic mitral orifice — often with associated mitral regurgitation and aortic valve involvement. In older patients in developed countries, severe mitral annular calcification (MAC) can extend onto the leaflet bases and produce a functional stenosis that is anatomically and hemodynamically distinct from rheumatic MS (calcific rather than commissural fusion). Congenital MS (for example, a parachute mitral valve) is rare and usually identified in childhood.

Severity Grading

Mitral valve area (MVA) is the primary severity parameter, supported by mean gradient and pulmonary artery systolic pressure (PASP), which rises secondary to chronically elevated left atrial pressure:

Within the severe category, an MVA <1.0 cm² defines "very severe" MS, a distinction used because prognosis and required follow-up intensity worsen further below this threshold (re-evaluation every 6–12 months, versus 1–2 years for MVA 1.0–1.5 cm²).

Measuring MVA

2D or 3D planimetry of the mitral orifice at the leaflet tips, in a true short-axis view at end-diastole, is the reference-standard method for MVA — it directly visualizes the anatomic orifice and does not depend on flow or loading conditions. It is technically demanding in heavily calcified valves due to acoustic shadowing, but is essential when other methods are unreliable.

The pressure half-time (PHT) method is the most widely used technique and is derived from the deceleration slope of the mitral inflow E-wave on continuous-wave Doppler:

MVA (cm²) = 220 ÷ PHT (ms)

PHT reflects how quickly the transmitral pressure gradient dissipates in diastole; a tighter valve empties the left atrium more slowly, prolonging PHT. PHT-derived MVA is less reliable in several common clinical scenarios and should be cross-checked against planimetry when present:

• Atrial fibrillation — beat-to-beat variability in diastolic filling time; average measurements over multiple cycles.
• Immediately after balloon valvuloplasty — acute compliance changes transiently alter the pressure decay independent of the new valve area.
• Coexisting significant aortic regurgitation — the AR jet raises LV diastolic pressure, shortening PHT and causing overestimation of MVA (falsely suggesting less severe MS than is truly present).

The Wilkins Score

The Wilkins score grades four echocardiographic features of the mitral apparatus, each scored 1 (normal/mild) to 4 (severe), for a total range of 4–16:

1. Leaflet mobility — from fully mobile with tip restriction only, to no or minimal diastolic leaflet movement.
2. Leaflet thickening — from near-normal (4–5 mm) to markedly thickened (>8–10 mm) throughout.
3. Calcification — from a single focal area of increased echo brightness to extensive calcification through most of the leaflet tissue.
4. Subvalvular thickening — from minimal chordal thickening just below the leaflets to extensive thickening and shortening of the chordal apparatus.

The total score predicts suitability for percutaneous balloon mitral commissurotomy (PBMC):

• ≤8: favorable anatomy — good PBMC outcome expected.
• 9–11: intermediate suitability.
• ≥12: unfavorable — surgical valve repair/replacement generally preferred over PBMC.

Significant commissural calcification, more than mild mitral regurgitation, or left atrial thrombus are additional factors that can disqualify an otherwise favorable Wilkins score from PBMC.

Associated Findings and Complications

Chronically elevated left atrial pressure from MS produces progressive left atrial enlargement, which in turn predisposes to atrial fibrillation and to left atrial appendage (LAA) thrombus or spontaneous echo contrast ("smoke"), best visualized by transesophageal echocardiography (TEE). Because the transthoracic exam cannot reliably exclude an LAA thrombus, TEE is required before PBMC or elective cardioversion to rule out thrombus that would be dislodged by the balloon catheter or by restoration of atrial mechanical contraction. Sustained elevation of left atrial and pulmonary venous pressure also drives pulmonary hypertension, tracked by PASP on the severity table above and by right heart size/function, and can progress to right ventricular failure if MS is left untreated. Because MS produces a fixed, low cardiac output that cannot augment with exertion, dyspnea on exertion is the dominant early symptom, often long before resting hemodynamics look severely abnormal.

Exercise Stress Testing for Latent Severe MS

When a patient's exertional symptoms seem disproportionate to a resting study showing only mild-to-moderate MS, exercise (treadmill or supine bicycle) stress echocardiography can unmask hemodynamically significant stenosis: the tachycardia and increased cardiac output of exercise shorten diastolic filling time and amplify the transmitral gradient and pulmonary pressures in a truly stenotic valve. A mean gradient >15 mmHg or a PASP >60 mmHg during exercise supports clinically significant MS and is a proposed indication for balloon mitral valvuloplasty even when resting parameters fall short of the severe threshold, mirroring the role of dobutamine stress echo in low-flow, low-gradient AS.

Key Testing Points

• MVA = 220 ÷ PHT is the core formula; planimetry is the reference standard, especially with atrial fibrillation or significant AR.
• Severe MS = MVA ≤1.5 cm²; very severe MS = MVA <1.0 cm².
• Mean gradient ≥10 mmHg at a normal heart rate indicates severe MS.
• Wilkins score ≤8 favors PBMC; ≥12 favors surgery.