53. Pathology of the valvular disorders (inflammatory and degenerative ones)

Page created on November 29, 2018. Last updated on December 18, 2024 at 16:56

Valvular heart disease

Any disease of the cardiac valves results in either stenosis, insufficiency or both.

Stenosis is the failure of a valve to open completely. This obstructs the forward blood flow through valve. Stenosis almost always occurs due to a chronic process, like calcification or scarring of the valve.

Insufficiency is the failure of a valve to close completely. This allows blood to flow backwards during systole or diastole, a process called regurgitation. Regurgitation of the atrioventricular valves occurs during systole, while regurgitation of the semilunar valves occurs during diastole. This is due to the pressure gradients, i.e. during systole the pressure is higher in the ventricles than in the atria, while during diastole the pressure is higher in the pulmonary and systemic circulations than in the ventricles.

Valvular insufficiency usually occurs due to some problem with the valve cusps (like endocarditis), or disruption of supporting structures like chorda tendinae, papillary muscles or ventricular wall.

The most commonly affected valve is the mitral and aortic valves, but valvular heart disease doesn’t necessarily just target one valve. Flow through abnormal valves cause the heart sounds known as murmurs.

The following valvular heart diseases are most important:

• Mitral valve
  • Stenosis
    • Rheumatic valvular disease
    • Mitral valve calficication
  • Regurgitation
    • Myxomatous mitral valve
    • Dilated cardiomyopathy
• Aortic valve
  • Stenosis
    • Aortic valve calcification
    • Bicuspid aortic valve
  • Regurgitation
    • Nothing special
• Tricuspid valve
  • Rarely affected
• Pulmonary valve
  • Rarely affected

Rheumatic valvular disease

Rheumatic fever has clinical manifestations in many organs, and the manifestations in the heart are called rheumatic heart disease. Rheumatic heart disease is associated with inflammation of all parts of the heart, but especially the valves, which has spawned the expression rheumatic valvular disease. Mostly the mitral and aortic valves are affected.

Rheumatic fever is an inflammatory disease that occurs after a type A streptococcus pyogenes infection of the pharynx. The incidence of rheumatic fever has declined significantly after the introduction of antibiotics.

Rheumatic valve disease is the only cause of acquired mitral stenosis. Rheumatic fever has two phases, acute and chronic phase.

Acute rheumatic fever affects all layers of the heart, but causes fibrinoid necrosis, fibrin deposition and small vegetations on the valves. Inflammatory lesions called Aschoff bodies are present in the myocardium.

In chronic rheumatic fever does the healing fibrosis cause the cusps to thicken, fuse together and the chordae tendinae to thicken and fuse together as well. The Aschoff bodies are replaced by fibrous scars.

Mitral annular calcification

Dystrophic calcification of the mitral valve doesn’t primarily target the cusps like for the aortic valve, but instead the fibrous ring (annulus). It rarely causes stenosis or regurgitation. The valve can become ulcerated, causing thrombus formation with embolization, or function as a good place for bacteria to proliferate. This bacterial proliferation can be the beginning of infective endocarditis.

Myxomatous mitral valve

In this type of mitral valve disease will one or both mitral cusps become floppy and prolapse back into the left atrium during systole, causing mitral insufficiency.

It’s associated with Marfan’s syndrome, where the connective tissue isn’t as tough as in healthy people.

Most patients are asymptomatic. Because the cusps flow back into the left atrium will the chorda tendinae be elongated and can potentially rupture. Other complications include:

• Infective endocarditis
• Thrombosis with potential embolization
• Arrhythmias

Calcific aortic stenosis

Calcified aortic valve is the most common cause of aortic stenosis. It’s a type of dystrophic calcification. It’s associated with age, which, together with the high pressure-difference on both sides, causes a lot of wear and tear on the valve. It’s not uncommon to see in elderly patients. It’s often an incidental finding during autopsy.

In people with a congenital bicuspid aortic valve (instead of the normal tricuspid) the valve is more prone to calcification. In these people (1-2% of the population) calcification occurs earlier.

Calcification mechanically impairs the valve’s ability to open, reducing the normal area of the orifice from 4 cm² to 0.5 – 1 cm². The consequence is that the left ventricle hypertrophies to be able to get enough blood out. This predisposes the ventricle to ischaemic heart disease like angina and acute myocardial infarction.

Endocarditis

Inflammation of the endocardium is called endocarditis. Recall that the valves are covered by endothelium, so any inflammation of the valve is an endocarditis.

Endocarditis can be infective or non-infective. In both types will so-called vegetations grow on the valve. atrial side in atrioventricular valves and on the ventricular side in semilunar valves. Vegetations usually contain fibrin, WBCs, RBCs, platelets and possibly bacteria. The vegetations are sterile (don’t contain bacteria) in non-infective endocarditis.

The types of non-infective endocarditis are:

• Libman-Sacks endocarditis
  • Occurs in SLE
• Rheumatic valvular disease
  • Occurs in rheumatic fever
• Non-bacterial thrombotic endocarditis (marantic endocarditis)
  • Due to cachexia

Non-bacterial thrombotic endocarditis

NBTE for short has many small, sterile vegetations (as the name suggests). It usually affects healthy valves and not already damaged valves. They’re not destructive, meaning that they don’t cause damage to the cusps. However, they are still thrombogenic surfaces that can be the starting point for thromboembolisms.

NBTE is usually seen in:

• Cachectic cancer patients
• Patients with adenocarcinoma
• Acute promyelocytic leukaemia
• Extensive burns

Infective endocarditis

Infective endocarditis has a microbial origin, so the vegetations aren’t sterile (they do contain bacteria). This makes them destructive, with the potential to destroy the valves and chordae tendinae. The aortic and mitral valves are the most common sites of infection. The bacteria of the vegetations can spread into the underlying myocardium to produce an abscess cavity called ring abscess. This abscess can be a starting point for sepsis.

Also, non-sterile vegetations are more friable (crumbly) than sterile ones, so thrombi formed on them can more easily loosen to cause embolization. Bacteria can perforate valves, leading to heart failure.

We distinguish two types of infective endocarditis, the acute and sub-acute type.

Acute infective endocarditis affects healthy valves. Only highly virulent bacteria can infect healthy valves, like staphylococci, streptococci and enterococci. The acute form is therefore more dangerous than the sub-acute form. Indeed, more than 50% of cases lead to death despite antibiotic therapy.

Sub-acute infective endocarditis (or endocarditis lenta) affects already abnormal valves like valves artificial valves and valves damaged by dystrophic calcification. It’s caused by less virulent bacteria than the acute type, especially streptococcus viridans. This bacterium is normally found in the oral bacterial flora, so any patient that has diseased valves that undergoes dental procedures must receive prophylactic antibiotic treatment to prevent infection by s. viridans.

Artificial valves

Sometimes valves are so ruined (stenotic, insufficient) that they must be replaced by artificial valves. Many types exist, but common for all om them is that they are susceptible to infection. They’re also thrombogenic surfaces, so patients with artificial valves usually take anticoagulants, which can cause haemorrhages. Mechanical aortic valves can also cause RBCs haemolysis due to the high pressure the cells are exposed to when forced through the valve.