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Mitral stenosis


Mitral stenosis (MS)
Causes 

The most common underlying cause of MS is prior rheumatic fever
occurring, on average, 20 years before presentation of mitral stenosis. 

Rheumatic valve disease is characterized by fibrous thickening and calcification of the valve leaflets, fusion of  the commissures (the borders where the leaflets meet), shortening and thickening of the chordae tendineae.
 Other more rare etiologies of MS include calcification of  the mitral annulus that extends onto the leafets, infective endocarditis with large vegetations obstructing the valve orifice, and rare congenital mitral stenosis (parachute mitral valve, supravalvular mitral ring). In parachute mitral valve, there is a single papillary muscle to which chordae to both leaflets attach. It results in mitral stenosis or mitral regurgitation.
Some systemic diseases can cause valvular fibrosis and stenosis (carcinoid, systemic lupus erythematosus, rheumatoid arthritis, healed endocarditis, mucopolysaccharidosis. 
Mitral valvular fibrosis and stenosis can also be caused by prior anorectic drug use.

Pathophysiology of mitral stenosis (MS)
In the normal heart, the mitral valve opens in early diastole  and blood flows freely rom the left atrium (LA) into the left ventricle (LV). Normally the pressure difference between these two chambers in diastole is negligible (the LA and the LV have almost the same diastolic pressure). The normal cross sectional area of the mitral valve orifice is 4-6 cm2
 In MS, there is obstruction to blood flow across the mitral valve. This produces an abnormal pressure gradient (pressure difference)
between the LA and LV, resulting in an icreased  left atrial (LA) pressure. Hemodynamic changes (a rise in transvalvular pressure gradient) begin when the cross-sectional area of  the valve,  is reduced to less than 2 
cm2.
In MS the high LA pressure is transmitted retrograde to the pulmonary circulation, resulting in increased pulmonary venous and capillary pressures. The elevated pressure in the pulmonary vasculature may cause transudation of  plasma into the lung interstitium and alveoli. This causes symptoms of heart failure, such as exertional dyspnea and paroxysmal nocturnal dyspnea, or orthopnea. 
In MS, chronic pressure overload of the LA (i.e. the chronically elevated LA pressure) leads to left atrial dilatation. This stretches the atrial conduction fibers and may adversely affect intra-atrial conduction of electric impulses, resulting in atrial fibrillation.
Atrial fibrillation due to the increased heart rate leads to a shortened diastolic period. The contribution of atrial systole to diastolic filling is also lost. These factors worsen the patient's clinical condition, because they result in a further elevation in atrial and pulmonary venous pressures and in a diminished cardiac output. 

Auscultatory findings in MS 
The opening snap is an early diastolic sound of short duration, and it is considered as the most characteristic auscultatory finding of mitral stenosis (MS). You can hear the opening snap near the cardiac apex, but it is more easily heard along the lower left sternal border. However, as the disease progresses and the valve becomes more calcified and immobile, the opening snap may be lost. Also the first heart sound (S1), which is usually accentuated (loud) in MS, for the same reason can become softer at a later stage of the disease. The murmur of MS is a low-pitched rumbling mid-diastolic murmur. It is best heard with the bell of the stethoscope with the patient in the left lateral decubitus position. Presystolic accentuation of the murmur can be present if the patient is in sinus rhythm. Auscultation after a brief period of exercise usually accentuates the murmur of MS, because exercise increases the transvalvular gradient, due to the increased cardiac output and heart rate.
 The  length of the murmur correlates better with the severity of MS than the loudness. MS is more severe when the murmur is longer and when the time interval from the second heart sound (S2) to the opening snap is short.
ECG
The electrocardiogram (ECG) in MS, if the rhythm is sinus, shows left atrial enlargement. Atrial fibrillation may be present (it is common in MS). If  pulmonary hypertension has developed, then there is also ECG evidence of right ventricular hypertrophy.
 Echocardiography in MS 
It shows structural abnormalities of the valve (in rheumatic MS  mitral leaflets are thickened with abnormal fusion of  their commissures). Echocardiography also shows restricted separation
of the valve leaflets and doming of leaflets during diastole.
 Left atrial enlargement is also present.
The mitral valve area (MVA) can be measured directly from the parasternal short axis view at the level of the tips of the mitral valve. Optimal positioning of the echocardiographic view, in order to obtain this measurement, is done by first obtaining a parasternal long-axis view and placing the mitral valve orifice in the center of the scan plane. The transducer is then rotated 90° to obtain the short-axis view. Measurements are obtained at the tips of the mitral leaflets. Three-dimensional echocardiography can provide a more accurate determination of the mitral valve area (MVA).
MVA can also be calculated from Doppler velocity measurements (the diastolic pressure half time). The pressure half time (PHT) is the time it takes for the pressure gradient across the valve to fall to one-half the starting value. (This is equal to the time for the velocity of the mitral E wave to decrease to 70% of peak velocity). The mitral inflow E wave is used in this calculation.
 MVA (in cm2) = 220/PHT.
Current guidelines define clinically important, severe, MS as a valve area ≤1.5cm2  , because this valve area is typically accompanied by left atrial enlargement and elevated pulmonary artery systolic pressure. A valve area ≤1.0 cm2 is termed "very severe" MS.
The transvalvular mean gradient (assessed by means of tracing mitral inflow continuous wave doppler signal) provides an estimate of stenosis severity. In mild stenosis : mean gradient < 5 mm Hg Moderate stenosis : mean gradient between 5 and 10 mm Hg.
 Severe MS : mean gradient > 10 mm Hg. 
Treatment of MS
 Symptoms due to vascular congestion can be improved by restriction of salt intake and diuretic therapy.
Heart rate slowing agents, such as beta-blockers or nondihydropyridine calcium channel blockers (diltiazem or verapamil), increase diastolic left ventricular filling time and so they there decrease symptoms with exercise. These drugs, or digoxin, are also used to slow the ventricular rate in patients with rapid atrial brillation. Anticoagulant therapy to prevent thromboembolism is indicated  in MS patients with atrial fibrillation, or an identied LA thrombus, or a prior embolic event.
Percutaneous or surgical valve interventions are the only treatments that alter the natural history of severe MS. They are indicated in patients with severe (see above for the echocardiographic criteria of MS severity), symptomatic MS. Percutaneous mitral balloon 
 valvuloplasty is the treatment of  choice in appropriately selected patients (those without advanced anatomic deformity of  the valve, and without moderate or severe mitral regurgitation, or left atrial thrombus). Transesophageal echocardiography (TEE) is indicated to exclude LA thrombus prior to valvuloplasty.
Percutaneous mitral valvuloplasty (PMV) is also indicated for asymptomatic patients with severe MS (valve area  ≤1.5cm2) , who have pulmonary hypertension (pulmonary artery systolic pressure > 50 mm Hg at rest or > 60 mm Hg with exercise) if valve morphology is favorable for PMV, in the absence of left atrial thrombus or moderate to severe mitral regurgitation.
In patients with severe MS causing symptoms, not suitable for percutaneous valvuloplasty, surgical treatment is indicated. This is true for patients with severe subvalvular disease or severe valvular calcification, or concomitant mitral regurgitation (moderate or severe). Surgical treatment choices include: 
Open mitral valvotomy: It involves direct visualization of the mitral valve (with cardiopulmonary bypass), debridement of calcium, and splitting of fused commissures and chordae.
 Mitral valve replacement.with a prosthetic valve is often required, when there is extensive fibrosis and calcification or concomitant moderate to severe mitral regurgitation.

LINKS : 
Mitral stenosis and echocardiography. A good video by 123sonography
https://www.youtube.com/watch?v=MQ5UyGoYhZ8


An echocardiogram of a patient with mitral stenosis (by dr Maged Al Ali)
https://www.youtube.com/watch?v=9yPfTxBAq3s