Differences Between Mitral and Aortic Stenosis
Mitral stenosis and aortic stenosis are fundamentally different valvular lesions with distinct pathophysiology, hemodynamic consequences, and clinical presentations that require different diagnostic and management approaches.
Anatomic Location and Pathophysiology
Aortic stenosis involves obstruction at the left ventricular outflow, creating a pressure overload on the left ventricle 1. The LV must generate significantly increased systolic pressure throughout ejection to propel blood across the narrowed aortic valve, resulting in a measurable LV-aortic pressure gradient 1. This pressure overload triggers concentric left ventricular hypertrophy where wall thickness increases while LV volumes remain the same or decrease, with myocytes adding sarcomeres in parallel 1.
Mitral stenosis creates obstruction between the left atrium and left ventricle, causing elevation of left atrial and pulmonary arterial systolic pressure 2. The pathophysiology centers on increased left atrial pressure rather than ventricular pressure overload. This results in pulmonary congestion and right heart consequences rather than the LV hypertrophy pattern seen in AS 2.
Etiology
Aortic Stenosis
- Degenerative/calcific disease (most common in Western countries): atherosclerotic disease with calcification, affecting normal trileaflet or bicuspid valves 3
- Bicuspid aortic valve: presents a decade earlier than trileaflet disease 3
- Rheumatic disease: uncommon in Western world, involves commissural fusion 3
The calcification pattern in degenerative AS is most prominent in the central and basal parts of each cusp, creating a stellate-shaped systolic orifice 4.
Mitral Stenosis
- Rheumatic MS accounts for 85% of cases, involving commissural fusion and thickening at leaflet tips 2
- Degenerative MS (12% of MS cases): increasingly prevalent with aging population, results in mitral annular calcification with valve thickening and chordal shortening 2
- Rheumatic MS presents in younger patients including during pregnancy, while degenerative MS is associated with increased age 2
Critical distinction: In rheumatic MS, commissural fusion creates a triangular systolic orifice, and the mitral valve is nearly always affected when rheumatic AS is present 4, 5.
Hemodynamic Consequences
Aortic Stenosis
- Creates increased LV afterload and systolic wall stress 1
- Develops concentric LV hypertrophy as compensatory mechanism 1
- Eventually leads to diastolic dysfunction, reduced coronary reserve, myocardial ischemia, and depressed contractility 3
- The ventricle becomes noncompliant due to pressure hypertrophy 6
Mitral Stenosis
- Causes elevated left atrial pressure and pulmonary arterial systolic pressure 2
- Results in low cardiac output state 4
- Can cause low flow, low gradient AS when both conditions coexist 4
- Leads to pulmonary hypertension and right heart dysfunction 7
Clinical Presentation and Natural History
Aortic Stenosis
Patients remain asymptomatic until late in disease course 3. Once symptoms develop:
- Angina: 5-year survival
- Syncope: 3-year survival
- Heart failure: 2-year survival
- Mortality rates after symptom onset: approximately 25% at 1 year, 50% at 2 years 3
Mitral Stenosis
- Presents earlier with symptoms due to pulmonary congestion
- More likely to cause atrial fibrillation due to chronic left atrial pressure elevation
- High thromboembolic risk with history of systemic embolism and dense spontaneous contrast in left atrium 7
Diagnostic Approach
Aortic Stenosis Assessment 4
Primary parameters:
- Peak jet velocity (severe: ≥4.0 m/s)
- Mean gradient (severe: ≥40 mmHg)
- Aortic valve area (severe: <1.0 cm²)
- Stroke volume index to define flow status (low flow: <35 mL/m²)
Key measurement: LVOT diameter measured inner edge to inner edge in mid-systole from parasternal long-axis view 4
Mitral Stenosis Assessment 7
Primary parameters:
- Valve area by planimetry is the reference measurement
- Mean transvalvular gradient (severe: ≥10 mmHg at rest or ≥15 mmHg with exercise)
- Mitral valve area (severe: <1.5 cm²)
- Pulmonary artery systolic pressure
Critical caveat: Assessment is more difficult with 2D TTE compared to AS 2.
Hemodynamic Interactions When Both Present
When mitral and aortic stenosis coexist, diagnostic complexity increases significantly 8, 9. Mitral stenosis causes low cardiac output, which can result in low flow, low gradient AS even when AS is truly severe 4. This creates a diagnostic challenge where:
- MS reduces transvalvular flow across the aortic valve
- AS gradients may be underestimated due to reduced flow from MS
- Nearly half of patients with significant MS (MVA ≤2.0 cm²) have "pseudo mitral stenosis" that improves after aortic valve replacement 9
- The other half have "true mitral stenosis" that persists and is associated with worse survival (adjusted HR 1.88) 9
Management Implications
Aortic Stenosis
- Intervention indicated when symptomatic with severe AS
- Transcatheter or surgical AVR based on surgical risk and anatomy 3
- No effective medical therapy to prevent progression 3
Mitral Stenosis
- Percutaneous mitral commissurotomy (PMC) is preferred for rheumatic MS with favorable anatomy 7
- PMC is unsuitable for degenerative MS (no commissural fusion); valve replacement should be considered 2
- Medical therapy focuses on rate control for atrial fibrillation and anticoagulation for thromboembolic risk 7
Combined Disease
When both moderate AS and MS are present, patients have a 5-year survival free of events of only 19% 10. The presence of heart failure symptoms (HR 2.32) strongly predicts clinical events 10.
For combined severe disease, multivalvular surgery carries increased morbidity and mortality but must be balanced against risk of second operation if one valve is neglected 8. The decision requires multidisciplinary heart team evaluation 8.
Common Pitfalls
- Underestimating AS severity in presence of MS due to low flow state 4, 9
- Overestimating mitral regurgitation in AS patients due to high LV driving pressure 4
- Assuming all MS will improve after AVR—only 49% of significant MS resolves (pseudo MS), while 51% persists (true MS) 9
- Missing rheumatic mitral involvement when rheumatic AS is present—the mitral valve is nearly always affected 4, 5
- Miscalculating AVA in AS due to LVOT diameter measurement errors—must measure inner edge to inner edge in mid-systole 4