L05: Valvular Heart Disease

The heart contains four valves that maintain unidirectional blood flow: the mitral valve (between left atrium and left ventricle, two leaflets, attached by chordae tendineae to papillary muscles), the tricuspid valve (right atrium to right ventricle, three leaflets), the aortic valve (left ventricle to aorta, three semilunar cusps nestled in sinuses of Valsalva), and the pulmonary valve (right ventricle to pulmonary trunk, three cusps). Valve disease takes two fundamental forms: stenosis (narrowing preventing complete opening, causing pressure overload upstream and compensatory hypertrophy) and regurgitation (incomplete closure causing volume overload and dilation).

Calcific aortic stenosis is the most common valvular lesion in developed countries, predominantly affecting elderly individuals. Lipid accumulation and calcification in the leaflets — a process resembling atherosclerosis — progressively restrict leaflet mobility and narrow the orifice. Risk factors mirror those of atherosclerosis: hypertension, hyperlipidaemia, diabetes, and smoking. A bicuspid aortic valve (present in approximately 1–2% of the population) is subject to accelerated turbulent stress and calcifies decades earlier than a tricuspid aortic valve.

Mitral valve prolapse (MVP), also called myxomatous mitral valve disease, is the most common valvular lesion in Western countries (2–3% prevalence). The mitral leaflets are thickened, rubbery, and redundant due to deposition of glycosaminoglycans (myxomatous change) in the spongiosa layer, weakening leaflet and chordal architecture. Leaflets prolapse (billow) into the left atrium during systole. Most cases are clinically insignificant, but a minority progress to mitral regurgitation requiring repair.

Rheumatic heart disease (RHD) results from rheumatic fever, an autoimmune sequela of pharyngeal infection with Group A β-haemolytic Streptococcus (GAS). Streptococcal M proteins share structural homology with cardiac proteins (molecular mimicry), and cross-reactive antibodies and T-cells attack the heart. The resulting pancarditis involves the endocardium, myocardium, and pericardium. Characteristic histological features of acute rheumatic carditis include Aschoff bodies: focal granulomatous lesions containing Aschoff giant cells and Anitschkow cells (also called caterpillar cells — activated macrophages with a distinctive undulating nucleus). On the endocardium, rows of small sterile thrombotic vegetations (verrucae) form along the lines of valve closure due to surface damage. The pericarditis produces a fibrinous exudate giving a "bread-and-butter" appearance macroscopically.

Repeated episodes of rheumatic fever cause progressive fibrosis of valve leaflets and fusion of commissures. The mitral valve is most affected (mitral stenosis), producing the classic "fish-mouth" or "button-hole" deformity on gross examination. Patients develop left atrial dilation, atrial fibrillation, and pulmonary hypertension.

Infective endocarditis (IE) is caused by microbial colonisation of heart valves or mural endocardium. Bacteria adhere to endothelial lesions or to previously damaged valve tissue and proliferate within a protective matrix of platelets and fibrin, forming bulky, destructive, irregular vegetations. Subacute IE is caused by organisms of low virulence (viridans streptococci, Streptococcus bovis, HACEK organisms) on previously damaged valves; presentation is indolent with systemic features of prolonged infection. Acute IE is caused by virulent organisms (Staphylococcus aureus predominantly) that can destroy normal valves rapidly. Complications include valve destruction and regurgitation, systemic embolisation of infected vegetation fragments (causing septic infarcts in brain, kidney, spleen), metastatic abscesses, and immune complex glomerulonephritis (from persistent bacteraemia).