How is the severity of mitral stenosis assessed on echocardiogram (echo)?

Assessment of Mitral Stenosis Severity on Echocardiography

Mitral valve area (MVA) measured by planimetry is the reference standard for determining mitral stenosis severity, with severe stenosis defined as MVA <1.0 cm², moderate stenosis as 1.0-1.5 cm², and mild stenosis as >1.5 cm². 1, 2

Primary Echocardiographic Parameters

Mitral Valve Area (MVA)

• Planimetry is the gold standard measurement and should be performed at the leaflet tips in a precisely oriented short-axis plane 1, 2, 3
• MVA <1.0 cm² = severe stenosis 1, 2
• MVA 1.0-1.5 cm² = moderate stenosis 2
• MVA >1.5 cm² = mild stenosis 2
• This measurement provides the most direct assessment of anatomic severity and should guide clinical decision-making 1, 3

Mean Transmitral Gradient

• Severe stenosis: >10 mmHg at rest 2
• Moderate stenosis: 5-10 mmHg 2
• Mild stenosis: <5 mmHg 2
• Important caveat: Gradients reflect hemodynamic consequences and are flow-dependent, so they can be misleading in low cardiac output states or atrial fibrillation 1, 3

Pulmonary Artery Systolic Pressure (PASP)

• Severe stenosis: >50 mmHg at rest 2
• Moderate stenosis: 30-50 mmHg 2
• Mild stenosis: <30 mmHg 2
• PASP provides prognostic information and reflects the hemodynamic burden of stenosis 1, 2

Alternative Measurement Techniques

Pressure Half-Time (PHT) Method

• Calculate MVA using the formula: MVA = 220/PHT 3
• Major limitations: Invalid immediately after percutaneous mitral commissurotomy, unreliable in elderly patients, and inaccurate in atrial fibrillation 3
• PHT <130 msec indicates good valve opening with excellent specificity 3
• Use cautiously as a semi-quantitative method only 3

Continuity Equation

• Provides accurate MVA estimation when properly performed 3
• Invalidated by: Concomitant aortic regurgitation, mitral regurgitation, or atrial fibrillation 3
• Should be used as a confirmatory method rather than primary measurement 3

PISA (Proximal Isovelocity Surface Area) Method

• Requires direct measurement of the angle between mitral leaflets 3
• Using a fixed angle of 100 degrees provides accurate MVA estimation 3
• Technically more challenging but useful when other methods are unreliable 3

Stress Echocardiography for Discordant Cases

Indications for Exercise Echo

• Asymptomatic patients with severe MS (MVA <1 cm²) to unmask symptoms 1, 2
• Symptomatic patients with non-severe MS (MVA >1.5 cm²) when symptoms seem disproportionate to resting measurements 1, 2
• Patients planning pregnancy or major surgery with MVA 1.0-1.5 cm² 1

Criteria for Hemodynamically Significant MS on Stress

• Mean gradient >15 mmHg during exercise 1, 2
• Mean gradient >18 mmHg during dobutamine infusion 1, 2
• PASP >60 mmHg during exercise 1, 2
• Early increase in PASP at low-level exercise correlates with higher rates of exercise-induced symptoms 1

Technical Considerations for Stress Echo

• Exercise echo is preferred over dobutamine for symptom assessment 1
• Acquire images at baseline and immediately post-exercise with treadmill 1
• Acquire images at baseline, low workload, and peak exercise with supine bicycle 1
• Use maximal sweep speed and minimal velocity scale for mitral valve continuous Doppler 1
• In atrial fibrillation, continue rate control medication to avoid early heart rate rise 1

Critical Pitfalls to Avoid

Measurement Errors

• Planimetry must be performed exactly at the leaflet tips, not at the base or mid-level of the valve 3
• Ensure proper plane orientation; oblique cuts overestimate valve area 3
• In degenerative mitral stenosis, transmitral gradient correlates poorly with MVA and may underestimate severity 4

Clinical Context Issues

• Body surface area indexing may be necessary in patients with large body habitus to avoid underestimating severity 2
• Concomitant mitral regurgitation increases flow across the valve, artificially elevating gradients 1, 3
• Associated aortic valve disease affects left ventricular filling and may confound gradient measurements 1, 3
• Low cardiac output states reduce gradients despite severe anatomic stenosis 1

Rhythm Considerations

• Atrial fibrillation causes beat-to-beat variation in gradients; average multiple beats (ideally 5-10 cycles) 3
• PHT method is particularly unreliable in atrial fibrillation 3

Role of Advanced Imaging

Transesophageal Echocardiography (TOE)

• Mandatory before percutaneous mitral commissurotomy to exclude left atrial thrombus 1, 2
• Indicated when transthoracic images are suboptimal 2
• Essential for assessing valve suitability for percutaneous intervention 2

3D Echocardiography

• Provides more accurate planimetry of valve area from en face views 2, 3
• Better assessment of commissural fusion and subvalvular apparatus 2
• Reduces operator dependence of planimetry measurements 3
• Comprehensive evaluation of valve morphology from "surgical view" 2

Valve Morphology Assessment (Wilkins Score)

Assess four components, each scored 1-4 points 5, 3:

• Leaflet mobility 5, 3
• Leaflet thickness 5, 3
• Degree and location of calcification 5, 3
• Subvalvular apparatus involvement 5, 3

Total score <8 with elastic symmetric commissures suggests suitability for percutaneous mitral commissurotomy 5, 3

Total score >10 or calcification of both commissures suggests valve replacement is needed 5, 3