Endocarditis: Pathology review

Two people came into the cardiology ward.

One of them was 25 year old Darren, who came in with a fever, chills and fatigue.

On the clinical examination, his fingernails had splinter hemorrhages and the palm of his hands had some erythematous flat lesions.

There were also some track marks on his forearm.

The other one is 75 year old Anna, who also had a fever and the same splinter hemorrhages and erythematous flat lesions that we previously saw.

On auscultation, a heart murmur was heard.

On her history, she said she was at the dentist 2 weeks ago.

Okay, so both people likely have endocarditis, or inflammation of the inner layer of the heart.

Remember that the heart’s wall is made up of three layers, the epicardium being the outermost layer, then the myocardium, and the endocardium, which is the layer that gets inflamed.

The inflammation can affect the heart valves, the mural endocardium or even prosthetic valves!

The most common cases of endocarditis are due to a microbial infection, and this is called infective endocarditis but in some cases, endocarditis can also be non-infective.

For non-infective endocarditis, the first step is usually damage to the endocardium.

Damage exposes the underlying collagen and tissue factor, which causes platelets and fibrin to adhere, which forms tiny blood clots.

This is called Nonbacterial Thrombotic Endocarditis or NBTE.

Tiny clots and fibrin can develop into vegetation, especially on the heart valves which damages them and makes it harder for them to open or close.

Although the exact cause of NBTE is unknown, it’s thought that a proinflammatory state where cytokines levels are elevated can increase clot formation.

This can happen with hypercoagulable states, like when there’s a malignancy, especially pancreatic adenocarcinoma.

Another situation where NBTE can happen is with systemic lupus erythematosus,

This is an autoimmune disease involving antigen-antibody complexes, and in this case they settle in the endocardium and cause inflammation, leading to a particular type of endocarditis, called Libman-Sacks endocarditis.

Remember that it’s associated with large vegetations, sometimes described as verrucous vegetations since they look like warts.

Now, infective endocarditis occurs when pathogens find their way to the endocardium, typically from the heart valves.

Every day, there are opportunities for pathogens like bacteria and fungi to get into the bloodstream, but this is not a problem because they are usually few in number and can easily be cleared by our immune system.

However, sometimes a larger quantity of microbes can get into the bloodstream, like if a person has an obvious open wound or an abscess, during a dental or surgical procedure, or use of infected needles.

These microbes can float around in the blood for long enough to reach the heart.

A particular site that’s prone to infections is the heart valve which is supplied by tiny blood vessels.

Now, most often the valves on the left side are affected, the most common being the mitral valve and than the aortic valve.

This is sometimes due to predisposing conditions, with the most common one in high- and middle-income countries being mitral valve prolapse and, less often, bicuspid aortic valves.

So on these valves bacterial colonies, clots and fibrin, can also form vegetations.

They usually present on the mitral or aortic valve, but what’s special here is that they can present on either surface of the valve, though most commonly, on the undersurface.

Let’s now go over the microbes that can cause endocarditis. Viridans Streptococci , especially Streptococcus sanguinis, is the most common cause.

They have low virulence, are found in the mouth, and they typically only affect valves that have had some previous damage, so think older patients or those with a history of heart valve disease after a dental procedure.

That’s because S. sanguinis uses special molecules on its surface, called dextrans, that bind to fibrin-platelet aggregates on damaged heart valves.

This usually results in small vegetations which don’t destroy the valve.

Staphylococcus aureus, on the other hand, is a highly virulent bacteria that can be found on the skin, and it can infect damaged valves.

They are often introduced via surgical procedures, wounds, or intravenous drug use, and often affects the tricuspid valve.

S aureus causes large vegetations that can destroy the valves.

Next we have Staphylococcus epidermidis.

A high yield fact is that this bacteria loves foreign prosthetic material, like prosthetic heart valves and this could be your best clue on a test.

This bacteria is usually introduced into the body at the time of heart valve surgery and it literally creates an extracellular matrix around itself called biofilm which allows it to stick around on the valve.

Another common point of entry into the body is through an infected intravenous catheter.

Enterococcus is a part of the normal urogenital flora.

But following genitourinary catheterization or surgery, it can escape into the bloodstream and go on to cause enterococcal endocarditis on either damaged or healthy valves.

Another bacterial species is Streptococcus gallolyticus, previously known as Streptococcus bovis which is normally found in the gut flora.

But, when there’s colorectal bleeding, like with colorectal cancer, these gut bacteria can migrate across the gut lining and into the bloodstream, which can develop into endocarditis.

A high yield fact to remember is that in case of S gallolyticus endocarditis, we need to do a colonoscopy to look for colorectal cancer.