Mitral stenosis (MS)

In practical terms, the only common cause of MS is rheumatic heart disease.

Much rarer causes include mitral annulus calcification (usually asymptomatic and more likely to be associated with MR, rarely stenosis), congenital (may be associated with congenital aortic stenosis or aortic coarctation), connective tissue disorders and infiltrations, systemic lupus erythematosus (SLE), rheumatoid arthritis, mucopolysaccharidoses (Hurler’s syndrome) and carcinoid.

Rheumatic fever is an autoimmune phenomenon caused by cross-reaction of antibodies to streptococcal bacterial antigens with antigens found on the heart. In its acute stages, rheumatic fever is associated with inflammation of all layers of the heart – endocardium (including that of the valves), myocardium and pericardium. MS does not occur at this stage, but many years later as a consequence of this initial inflammatory process. The MV leaflets progressively fuse, initially at the commissures and free edges, which become thickened and later calcified. The inflamed valve becomes progressively thickened, fibrosed and calcified. This restricts the opening and closing of the valve. The chordae may also become thickened, shortened and calcified, further restricting normal valve function. The leaflets shrink and become rigid. The size of the MV orifice reduces leading to MS, which restricts blood flow from LA to LV.

Remember that many years elapse between rheumatic fever and the clinical manifestations of MS but there may not be a clear clinical history of rheumatic fever in childhood. Some individuals may remember being placed on bedrest for many weeks, which was the often-favoured treatment for rheumatic fever.

The M-mode pattern changes in a predictable way (Fig. 2.4). The movement of the leaflets is more restricted, and the leaflet tips are fused, so the posterior leaflet is pulled towards the anterior leaflet rather than drifting away from it. In severe MS, there is often AF rather than sinus rhythm, and the 2nd peak of MV movement is lost. The calcified leaflets reflect ultrasound in a different pattern from normal leaflets due to their increased thickness, fibrosis and often calcification. Instead of a single echo reflection giving a sharp image of the leaflets, there is a reverberation with several echo reflections giving a fuzzy image. Calcified leaflets produce a stronger echo reflection.

Fig. 2.4 Mitral stenosis – severe.

On 2-D echo, the MV leaflets can be seen to be thickened and their movement restricted. Because of the fusion of the anterior and posterior leaflet tips, while the leaflet cusps may remain relatively mobile, there may be a characteristic ‘elbowing’ or ‘bent-knee’ appearance, particularly of the anterior MV leaflet (Figs 2.5, 2.6). This has also been likened to the bulging of a boat’s sail as it fills with wind. The LA also enlarges.

Fig. 2.5 Rheumatic mitral stenosis. (a) Parasternal long-axis view and (b) apical 4-chamber view. ‘Elbowing’ of the anterior mitral valve leaflet is shown (arrow).

Fig. 2.6 2-D echo in mitral stenosis.

The computer of the echo machine can calculate the area of the MV orifice after tracing around a frozen image on a parasternal short-axis view taken at the level of the MV leaflets in end-diastole. The normal leaflets in this view can be seen to open and close in a ‘fish-mouth’ pattern. In MS, the leaflet tips are calcified and opening is restricted with a reduced orifice size.

MV orifice area (Fig. 2.7) can also be measured using Doppler (Ch. 3).

Fig. 2.7 Rheumatic mitral stenosis. (a) Parasternal short-axis view at mitral level showing restricted orifice (arrow) and (b) valve orifice area calculated by computer software as 1.9 cm².

Criteria for diagnosis of severe MS (many derived from Doppler)

• Measured valve orifice area <1 cm²

• Mean pressure gradient >10 mmHg

• Pressure half-time >200 ms

• Pulmonary artery systolic pressure >35 mmHg.

A number of diseases produce other typical mitral M-mode patterns (Fig. 2.8):

• LA myxoma. This has a characteristic appearance. There are multiple echoes filling the space between the MV leaflets. There may initially be an echo-free zone which is filled with echo reflections as the myxoma prolapses through the MV from LA into LV. Other potential causes of a similar echo appearance are large MV vegetations, LA thrombus or aneurysm of the MV.

• Hypertrophic cardiomyopathy (HCM). In diastole, the valve may be normal, but in systole the entire MV apparatus moves anteriorly producing a characteristic bulge touching the interventricular septum. This is called systolic anterior motion (SAM) of the MV.

• MV prolapse. This may be asymptomatic or cause varying degrees of MR. Either anterior or posterior valve leaflets may prolapse into the LA cavity in late systole. This produces an audible click and a late systolic murmur.

• Flail posterior leaflet. This may occur as a result of chordal rupture (due to degeneration) or to papillary muscle dysfunction. The posterior leaflet shows erratic movement, rather than the normal ‘W’ pattern.

• Aortic regurgitation (AR). The regurgitant jet passes during diastole along the anterior MV leaflet, causing fluttering vibration of the leaflet and restricting its normal pattern of movement. With increasing severity of AR, the MV is more restricted and there may be ‘functional’ MS (with an anatomically normal MV) giving rise to the diastolic Austin Flint murmur.

Fig. 2.8 Mitral valve M-mode patterns.

Mitral regurgitation (MR)

This is leakage of blood through the MV from LV into LA during ventricular systole. It ranges from very mild to very severe, when the majority of the LV volume empties into the LA rather than into the aorta with each cardiac cycle. A small amount of MR occurs during the closure of many normal MVs – in some series in up to one-third of normal hearts.

In MR, there are changes in:

Echo may show:

Echo assessment of severity of MR

While the diagnosis of MR may be easy (Fig. 2.9), the echo assessment of severity can be difficult. A balance must be made of all the echo information. The severity relates to the regurgitant fraction, which depends on:

• The size of the regurgitant orifice

• The length of time for which it remains open

• The systolic pressure difference between LV and LA across the valve

• The distensibility of the LA.

Fig. 2.9 Causes of mitral regurgitation.

The features of severe chronic MR are those of:

1. Volume overload of the LV – dilatation with hyperdynamic movement

2. Volume overload of the LA – dilatation

3. Large regurgitant volume – broad jet extending far into the LA

4. Abnormal valve function.

M-mode shows LV dimensions are increased, as is velocity of motion of the posterior wall and interventricular septum (IVS). The LA is enlarged. There may be features of an underlying cause of MR, e.g. multiple echoes suggesting vegetations due to endocarditis, MV prolapse or flail posterior leaflet.

2-D echo helps to suggest an underlying cause and assess its consequences. The parasternal long- and short-axis views and apical 4-chamber views are the most helpful and may show:

1. LV abnormality – dilatation causing annular stretching and ‘functional’ MR, regional wall motion abnormality due to MI or ischaemia, volume-overloaded LV

2. Leaflet abnormalities – rheumatic leaflets, vegetations due to endocarditis, prolapse, flail leaflet

3. Chordae – rupture, thickening, shortening, calcification, vegetations

4. Papillary muscles – rupture, hypertrophy, scarring, calcification.

Doppler echo features of severe MR (Fig. 2.10):

1. Wide jet. The width of the MR jet at the level of the leaflet tips (broad colour flow signal) correlates with severity (a wider jet represents more severe MR).

2. Jet fills a large area of LA. The extent to which the MR jet fills the LA cavity is also an indication. The area of colour in the LA depends on the machine settings and is controversial. However, an area >8 cm² is likely to be severe, <4 cm² likely to be mild.

3. Systolic flow reversal in pulmonary veins. The jet extends to the pulmonary veins. This can be seen on colour flow mapping and may also cause retrograde flow (LA to lungs) detected by pulsed wave Doppler with the sample volume in one of the pulmonary veins.

4. Dense signal on continuous Doppler. The intensity of the jet is greater with more severe MR since more red cells reflect ultrasound.

5. Raised pulmonary artery (PA) pressure. This is estimated by Doppler from tricuspid regurgitation (TR) (Ch. 3).

Fig. 2.10 Severe mitral regurgitation. (a) Colour flow mapping shows a broad jet filling the left atrium and extending into pulmonary veins (arrow). (b) Continuous wave Doppler.

It is important to know that severe acute

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