Anatomy and histology of valves

Question 1

Recall the types of valves in the heart

Answer 1

Recall that there are two main types of valves in the heart: ^[a c/c question might be relevant]
- atrioventricular valves: i.e. the mitral and tricuspid valves. The AV valves are located between each atrium and ventricle, and prevent the backflow of blood into the atrium when the ventricle contracts. ^[as a general rule, valves promote unidirectional flow, prevent bidirectional flow]. Key features of the AV valves
- valve cusps that fill with blood during ventricular systole to prevent backflow into atria
- cusps tethered to chordae tendineae and papillary muscles, which contract to prevent prolapse of the valves and backward flow
Note: AV valves close in systole i.e. when ventricles contract
- semilunar valves: i.e. aortic and pulmonary valves, are located at the entrances to the pulmonary trunk and aorta, and close to prevent backflow of blood into ventricles when the ventricles relax. Key features of the semilunar valves include:
- valve cusps, similar to AV valves, which fill with blood to prevent backflow into ventricles
- openings into the coronary arteries (right and left aortic sinus) lie behind the valve cusp
Note: semilunar valves close in dystole i.e. when ventricles relax

Question 2

Describe the histology of the valves

Answer 2

There are several layers to the aortic valve:
- endothelium
- atrialis (the outer most layer of atrioventricular valve)
- spongiosa (made of loose connective tissue, has GAGs and PGs)
- fibrosa (made of dense connective tissue, collagen)

«The semilunar valves are divided into:

• Ventricularis- which is in direct contact with pulsatile blood and directed toward the ventricles. It is the most caudal layer.
• Spongiosa - which is between the ventricularis and fibrosa layers.
• Fibrosa - which is directed toward the outflow vessel lumen. This is the most cranial layer.

The atrioventricular valves have fibrosa and spongiosa layers as well, but there is an atrialis layer instead of a ventricularis layer. The layers of the atrioventricular valves are arranged such that:

• The atrialis layer is the most cranial and is directed toward the atrium.
• The spongiosa is in the middle.
• The fibrosa layer is the most caudal and directed toward the ventricles.

The valves are filled with an extracellular matrix that contains a mixture of proteoglycans in the spongiosa layer and collagen fibers in the fibrosa layer. These layers are encased in a sheath of endocardial endothelial cells interlaced with valve interstitial cells. Together, these cells have a homeostatic activity that aids in the daily function of the valve.» ^[kenhub]

The fibrosa and spongiosa are the two main layers

Question 3

Describe age-related valvular changes

Answer 3

• nodular dystrophic calcification of the valve, resulting in thickness
• stenosis
• deposition of hyaline cartilage, fibrotic change
• deposition of amyloid, associated with dystrophic changes

Question 4

Define and describe valvular heart disease

Answer 4

Valvular heart disease is caused by either damage or defect in one of the four heart valves.
Defects can be classified based on their origin and onset
- congenital ^[e.g. bicuspid aortic valve, predisposes to stenosis or occlusion]
- acquired: which can either be acute or chronic

Defects can also be classified based on their functional effects:
- valvular stenosis: which requires extra pressure on heart to push the blood, and results in hypertrophy of heart
- valvular regurgitation: in which blood flows back, resulting in dilatation

Question 5

Define and describe vegetations

Answer 5

Vegetations are abnormal growths, consisting of fibrin and platelets.
Vegetations are observed in infective endocarditis, rheumatic fever, Libman-Sacks endocarditis, and non-bacterial thrombotic endocarditis.
The appearance and locations of vegetations differs between conditions:
- rheumatic fever: typically small, found on edges of leaflets; patient typically has more symptoms or signs
- Libman-Sacks: small, ; patient has lupus and/or phospholipid antibody
- Bacterial (i.e. infective) endocarditis: generally large, Vegetations usually affect the left side of the heart, with the most common underlying lesions being mitral valve prolapse and degenerative mitral and aortic regurgitation; on edges of heart valves; more likely to occur on valves already deformed
- Marantic vegetations (or non-bacterial thrombotic endocarditis): usually small, ; patient has hypercoagulability, usually from cancer ^[unclear why blood is hypercoagulated?]

Question 6

Describe the histology of endocarditis

Answer 6

Valvular connective tissue appears busy and cellular due to inflammatory infiltrates

Below image compares aortic valve with endocarditis vs normal aortic valve.

In acute inflammation, neutrophils predominate.
Vegetations are composed of neutrophils, fibrin and organisms i,e, bacteria, which are Gram positive

In chronic endocarditis, plasma cells and neovascularisation can be seen in valve parenchyma.

It is also indicative of previous inflammatory insult, that is either the result of infectious endocarditis or rheumatic fever.

Question 7

Describe chronic rheumatic heart disease

Answer 7

Clues that indicate the appearance of rheumatic heart disease include:
- previous rheumatic heart disease
- low SES with poor access to health care
- mitral valves and/or aortic valves are abnormal
- stenotic mitral valves have a “fish mouth” appearance

• Other key features: Aschoff bodies (a collection of macrophages fibrinoid changes, lymphocytes), Anitschkow cells (macrophages with wavy, ‘caterpillar’ nuclei), and lymphocytes

Acute rheumatic fever is marked by Aschoff bodies, Anitschkow cells and lymphocytes, with 5 of the Jones clinical criteria

NOTE: bacterial endocarditis vegetations are not sterile a.k.a infective, but other vegetations typically are
Note 2: Molecular mimicry between streptococcal M protein and human proteins has been proposed as the triggering factor leading to autoimmunity in RF and RHD

Question 8

Describe mitral valve prolapse

Answer 8

results in increased hooding of the valve, and regurgitation.

Occurs in about 2-6% of the population.
Mitral valve is more common in connective tissue disorders e.g. Ehlers Danlos syndrome.

The consequences of valvular incompetence as a result of mitral valve prolapse include:
- asymptomatic murmur
- rupture with rapid-onset heart failure
- increased risk of sudden cardiac death
- stroke and other embolic events
- infective endocarditis
- arrhythmias and palpitations

Note: as a consequence? of mitral valve prolapse, myxomatous degeneration of mitral valve occurs.
Thickening of spongiosa due to increased GAGs and proteoglycans.

Question 9

Describe marantic endocarditis

Answer 9

• can increase risk of infarcts elsewhere e.g. cerebral