Heart vegetations are abnormal clumps of material that form on the heart’s valves. They’re made up of platelets, a clotting protein called fibrin, and often bacteria or other microorganisms. Most vegetations develop as a result of infective endocarditis, an infection of the heart’s inner lining, though some form without any infection at all. Vegetations range from microscopic clusters to growths large enough to interfere with valve function and break off into the bloodstream.
How Vegetations Form
The process typically starts with damage to the surface of a heart valve. This damage can come from turbulent blood flow, an existing valve problem, or a medical device. The body responds the way it does to any injury: platelets and fibrin rush to the site and form a small, sticky deposit. At this stage, the deposit is sterile.
The trouble begins when bacteria enter the bloodstream. This can happen during dental procedures, through skin infections, from intravenous drug use, or even from everyday activities like brushing teeth. Bacteria circulating in the blood land on that sticky deposit and take hold. Once attached, the bacteria multiply and trigger more clotting activity, more fibrin buildup, and inflammation. The growing mass of bacteria, platelets, and fibrin is the vegetation.
Some bacteria are aggressive enough to infect completely healthy valves. Staphylococcus aureus, for example, can bind directly to valve tissue, burrow inside cells, and evade the immune system while releasing substances that destroy surrounding tissue.
Infectious vs. Non-Infectious Vegetations
The vast majority of heart vegetations involve infection, but a condition called nonbacterial thrombotic endocarditis (sometimes called marantic endocarditis) produces vegetations without bacteria. These sterile vegetations are clumps of platelets and fibrin that form on previously healthy valves, most commonly the aortic and mitral valves. They’re associated with advanced cancers (particularly adenocarcinomas) and hypercoagulable states, conditions where the blood clots too easily. Though rare, these non-infectious vegetations carry their own risk of breaking off and causing strokes or organ damage.
Which Bacteria Are Most Common
Staphylococci and streptococci together account for roughly 80% of infective endocarditis cases, split nearly evenly between the two groups. Staphylococcus aureus tends to cause more aggressive, rapidly progressing infections. Viridans streptococci, which normally live in the mouth, cause a slower, more subtle form of the disease. Enterococci are another significant cause, often requiring longer treatment. Rarer culprits include a group of slow-growing bacteria collectively called HACEK organisms, certain fungi, and gram-negative bacteria.
How Vegetations Are Detected
Echocardiography, an ultrasound of the heart, is the primary tool for spotting vegetations. There are two types. A standard echocardiogram (performed by placing a probe on the chest) has a sensitivity of roughly 40% to 80% for detecting vegetations, meaning it misses a significant number of cases. A transesophageal echocardiogram, where the probe goes down the throat to get closer to the heart, detects vegetations in 86% to 100% of confirmed cases. For this reason, the throat-based approach is often used when suspicion is high but the chest ultrasound looks normal.
Doctors diagnose infective endocarditis using a standardized checklist called the Duke Criteria, updated in 2023. The criteria combine blood culture results, imaging findings, and clinical signs like fever, embolic events, and certain skin or eye changes. A vegetation visible on imaging counts as a major criterion. Positive blood cultures with typical organisms count as another. Meeting enough criteria leads to a definite or possible diagnosis.
Why Vegetations Are Dangerous
The biggest threat from a vegetation is embolism. Pieces of the vegetation can break off and travel through the bloodstream, lodging in distant organs and cutting off their blood supply. This happens in more than 50% of patients with infective endocarditis. When vegetations sit on the left side of the heart (the aortic or mitral valve), fragments can travel to the brain, spleen, kidneys, or liver. The brain and spleen are the most common destinations.
Stroke is among the most feared complications. Patients with mitral valve vegetations face a cerebral embolism rate of about 25%, compared to roughly 18% for those with aortic valve vegetations. What makes this especially tricky is that 20% to 50% of embolic events are completely silent, causing damage to the spleen or brain without producing obvious symptoms. About 20% of patients in large registries had splenic infarcts, and 5% of those progressed to abscesses.
Beyond embolism, vegetations can directly destroy valve tissue, causing severe leaking (regurgitation) that leads to heart failure. They can also spread infection into surrounding heart structures, forming abscesses in the tissue around the valve.
Vegetation Size and Treatment Decisions
Vegetations are generally classified as small (under 5 mm), medium (5 to 9 mm), or large (10 mm and above). Size matters enormously for treatment planning. A vegetation of 10 mm or larger on the left side of the heart is a strong predictor of embolic events and increased mortality, and it’s widely accepted as an indication for surgery. In studies, patients with vegetations over 10 mm had significantly more embolic events than those with smaller ones. Vegetations larger than 15 mm with high mobility often prompt early surgery regardless of other factors.
For right-sided vegetations, which are more common in people who inject drugs, the threshold is higher. Surgery is typically considered at 20 mm or above. Right-sided emboli travel to the lungs rather than the brain, which is generally less immediately dangerous but still serious.
How Treatment Works
Treatment for infected vegetations centers on prolonged intravenous antibiotics, typically lasting 2 to 6 weeks depending on the type of bacteria, how it responds to medication, and whether the infection involves a natural or prosthetic valve. Infections caused by enterococci generally require the longer end of that range, 4 to 6 weeks, because these bacteria are harder to kill. Blood cultures are repeated during treatment to confirm the antibiotics are working.
Surgery becomes necessary when antibiotics alone can’t control the infection, when the vegetation is large enough to pose a high embolism risk, when the valve is severely damaged and causing heart failure, or when abscesses have formed. The operation involves removing the infected tissue and either repairing or replacing the damaged valve.
Outlook and Survival
Infective endocarditis remains a serious diagnosis. In-hospital mortality reaches up to 30%, and the condition affects 3 to 10 people per 100,000 each year. Outcomes improve significantly when patients are managed by a specialized endocarditis team, with one-year mortality roughly cut in half in that setting. Early detection, appropriate antibiotic selection, and timely surgical decisions when needed are the factors that most influence survival. People with prosthetic valves, a history of prior endocarditis, congenital heart disease, or implanted cardiac devices face higher risk and need close monitoring if they develop unexplained fevers or bloodstream infections.