Mitral stenosis (MS) is one of those Step 1 “classic” valve lesions where the story, sound, and hemodynamics line up beautifully—if you know what to listen for. The exam writers love MS because it connects rheumatic fever, left atrial enlargement, atrial fibrillation, pulmonary hypertension, and a very testable diastolic murmur into one tight package.
Big Picture Definition (What is Mitral Stenosis?)
Mitral stenosis = narrowing of the mitral valve orifice → impaired LV filling during diastole.
- Normal mitral valve area: ~4–6 cm²
- Symptoms often begin when area falls below ~2 cm²
- Severe MS is often < 1 cm²
Classic cause (Step 1): Rheumatic heart disease → chronic scarring of the mitral valve.
First Aid Cross-References (Where it lives in your brain)
While edition layouts differ, MS is classically tied to these First Aid buckets:
- Cardiovascular: Valvular disorders (murmurs, maneuvers, timing)
- Cardiovascular: Rheumatic fever (post–Group A strep)
- Cardiovascular: Atrial fibrillation (LA dilation → AF → thromboembolism)
- Respiratory/Cardio: Pulmonary hypertension (late consequence)
Use FA’s murmur table + rheumatic fever page as your “two-page anchor.”
Pathophysiology (The Step 1 Chain Reaction)
1) Obstruction to LV filling
Stenotic mitral valve → blood backs up in the left atrium during diastole.
- ↑ Left atrial pressure
- ↓ LV end-diastolic volume → ↓ preload → ↓ cardiac output (fatigue, low exercise tolerance)
2) Left atrial enlargement (LAE)
Chronic pressure overload → LA dilation.
High-yield consequences:
- Atrial fibrillation (irregularly irregular pulse)
- Left atrial thrombus (esp. in the left atrial appendage) → systemic emboli (stroke)
3) Pulmonary venous congestion → pulmonary hypertension
Back pressure → ↑ pulmonary venous pressure → symptoms of congestion and eventually:
- Reactive pulmonary arteriolar vasoconstriction/remodeling → pulmonary HTN
- ↑ RV afterload → right-sided heart failure (late)
Etiologies (Know the classic + the zebras)
| Cause | High-yield clues |
|---|---|
| Rheumatic fever (most common worldwide) | History of strep throat; migratory polyarthritis, Sydenham chorea; commissural fusion, thickened leaflets |
| Mitral annular calcification (older adults) | Degenerative; may coexist with aortic stenosis |
| Congenital MS | Rare; pediatric presentation |
| Left atrial myxoma (mimic) | “Ball-valve” obstruction; positional symptoms; constitutional signs |
| Carcinoid (not typical MS) | Usually right-sided valves (tricuspid/pulmonic) due to serotonin |
Step 1 default answer: rheumatic heart disease unless the vignette screams otherwise.
Clinical Presentation (What patients actually feel)
Symptoms
Think pulmonary congestion + low forward flow:
- Dyspnea on exertion, orthopnea, PND
- Fatigue (↓ CO)
- Palpitations (AF)
- Hemoptysis (from pulmonary venous HTN/rupture of bronchial veins)
- Hoarseness from Ortner syndrome (LA enlargement compresses the recurrent laryngeal nerve) — very testable association
Physical exam (the money findings)
- Loud S1 (still-mobile but stiff valve snaps shut loudly early on)
- Opening snap after S2 (earlier snap = more severe MS)
- Low-pitched, rumbling mid-diastolic murmur best at the apex
- Often heard best with the bell in left lateral decubitus
- If pulmonary HTN develops: loud P2, RV heave, signs of right HF
Auscultation + Maneuvers (Step-friendly)
Timing and location
- Diastolic murmur at the apex = think mitral stenosis.
Maneuvers
Most murmur maneuvers are more famous for regurg lesions and HCM, but for MS:
- Exercise or anything that increases flow across the valve can accentuate diastolic murmurs.
- Left lateral decubitus brings the apex closer to the chest wall → easier to hear.
Severity clue (commonly tested)
- Shorter S2 → opening snap interval = more severe MS
Because higher LA pressure forces the valve open sooner in diastole.
Key Hemodynamics (What changes in pressures/volumes?)
What increases?
- Left atrial pressure ↑
- Pulmonary capillary wedge pressure (PCWP) ↑ (reflects LA pressure)
- Over time: pulmonary artery pressure ↑ (pulmonary HTN)
What decreases?
- LV preload ↓ (less filling)
- Cardiac output ↓ (especially with exertion)
Step-style “pressure tracing” pearl
If you’re shown a wedge tracing with big waves:
- MS classically causes elevated LA pressures; AF can change wave patterns (loss of organized atrial contraction affects the a-wave).
Diagnosis (What confirms it?)
Echocardiography is first-line
Transthoracic echo (TTE):
- Measures mitral valve area, transmitral gradient
- Assesses LA size, pulmonary pressures
- Looks for valve morphology (rheumatic thickening, commissural fusion)
Transesophageal echo (TEE):
- Useful to detect LA thrombus, especially before cardioversion in AF.
ECG
- Atrial fibrillation
- Signs of LA enlargement (classically “P mitrale” if in sinus rhythm: broad, notched P waves)
CXR
- LA enlargement
- Pulmonary vascular congestion/edema
- Prominent pulmonary arteries if pulmonary HTN
Treatment (Step 1 level + clinically relevant)
General principles
- Treat congestion
- Diuretics for pulmonary edema symptoms (symptomatic relief)
- Control rate/rhythm in AF
- Rate control (e.g., beta-blocker, non-DHP CCB depending on context)
- Prevent stroke
- Anticoagulation when indicated (MS + AF is a big deal)
Anticoagulation high-yield nuance
- Atrial fibrillation due to moderate-to-severe mitral stenosis is considered “valvular AF” clinically.
- Classically tested management: warfarin (not DOACs) for MS-associated AF.
Definitive interventions
- Percutaneous balloon mitral valvotomy (good for pliable rheumatic valves without significant MR)
- Surgical valve repair/replacement if not a candidate for balloon valvotomy or if severe disease with complications
High-Yield Associations (Exam writers love these)
Rheumatic fever → MS (the classic)
- Group A strep → immune-mediated damage (molecular mimicry)
- Chronic rheumatic heart disease:
- Commissural fusion
- Thickened leaflets/chordae
Atrial fibrillation + thromboembolism
- LA dilation predisposes to AF
- AF + MS = high stroke risk → anticoagulate
Ortner syndrome
- Hoarseness due to recurrent laryngeal nerve compression from enlarged LA
Pregnancy can unmask MS
- Increased blood volume/cardiac output → worsened transmitral gradient → dyspnea
Infective endocarditis?
- Less classically emphasized for pure MS than for regurgitant lesions, but damaged valves can still be at risk depending on context. Don’t over-anchor here—Step questions more often link MS to rheumatic fever and AF.
“MS vs MR vs AS vs AR” Quick Differentiation Table
| Lesion | Timing | Best heard | Classic sound | Key complication |
|---|---|---|---|---|
| Mitral stenosis | Diastolic | Apex | Opening snap + rumble | AF, LA thrombus, pulmonary HTN |
| Mitral regurgitation | Systolic | Apex → axilla | Blowing holosystolic | LV dilation, HF |
| Aortic stenosis | Systolic | RUSB → carotids | Crescendo-decrescendo | Syncope, angina, HF |
| Aortic regurgitation | Diastolic | LSB | Decrescendo + wide PP | LV dilation |
USMLE-Style Vignette Patterns (If you see this, think MS)
- Immigrant patient or older adult with history suggesting rheumatic fever, now with:
- Dyspnea, hemoptysis, palpitations
- Irregularly irregular rhythm
- Opening snap + diastolic rumble at apex
- Echo shows enlarged LA and reduced mitral valve area
- Complication question: stroke from LA thrombus, or pulmonary hypertension → right HF
Rapid-Fire High-Yield Facts (Last-minute review)
- MS is diastolic and best at the apex.
- Opening snap is a hallmark; earlier snap = worse stenosis.
- Most classic cause: rheumatic heart disease.
- Leads to LA dilation → AF → thromboembolism.
- PCWP increases (reflects LA pressure).
- Treat symptoms with diuretics, control AF, anticoagulate (warfarin for moderate-to-severe MS with AF), and consider balloon valvotomy.