Endocarditis Pathology — Infective and Non-Infective

Mechanism of Bacterial/Fungal Adherence and Vegetation Formation

• Valvular predisposition: Pre-existing endothelial injury from turbulent flow (rheumatic heart disease, congenital lesions), valve replacement, or aging creates areas of denudation and fibrin deposition
• Bacterial adherence molecules: Organisms express adhesins (streptococcal fibronectin-binding proteins, staphylococcal clumping factor) that bind to exposed collagen, fibronectin, and fibrin on damaged endocardium
• Platelet aggregation and immune evasion: Bacteria induce platelet activation through tissue factor and von Willebrand factor interaction; platelets and fibrin entrap organisms, forming sterile thrombi that protect bacteria from antibiotics and phagocytes
• Biofilm formation: Organisms produce polysaccharide matrix and exopolysaccharides (especially in Streptococcus viridans, Staphylococcus aureus) creating a three-dimensional community resistant to antimicrobial penetration (reduced antibiotic efficacy by 100–1000-fold)

Valve-Specific Pathology and Destruction

• Progressive vegetation enlargement: Continuous bacterial proliferation and fibrin deposition create friable, crumbly masses (1–5 mm initially, potentially several centimeters) predominantly on the ventricular (atrial) surface of valve leaflets where blood flow impact is greatest
• Valve tissue invasion and necrosis: Direct bacterial penetration causes acute inflammation with neutrophilic infiltration, myxoid degeneration of valve substance, collagen disruption, and calcification; this compromises structural integrity leading to valve perforation, chordae tendineae rupture, and annular abscess formation
• Paradoxical immunologic phenomena: Circulating immune complexes (bacterial antigen-antibody) deposit in glomeruli, skin, and joints; polyclonal B-cell activation produces rheumatoid factor and cryoglobulins

Embolic Complications

• Septic emboli: Friable vegetation fragments embolize systemically, lodging in end arteries (brain, spleen, kidneys, coronary arteries) where they cause focal infections, microabscesses, and infarction; organisms within emboli seed distant tissues, perpetuating infection
• Sterile emboli: Small fibrin-platelet aggregates without organisms may also embolize, though less common in bacterial IE

Non-Infective Endocarditis Mechanisms

• Marantic endocarditis (cancer-associated): Circulating immune complexes and tissue factor-bearing cancer cells trigger platelet aggregation and fibrin deposition without infection; verrucae are small (<1 mm), sterile, occur on closure lines of valve leaflets, and resolve with cancer treatment
• Libman-Sacks endocarditis (SLE): Immune complex deposition and lupus anticoagulant create verrucae on both sides of valve leaflets (atrial and ventricular surfaces), a pathognomonic feature distinguishing it from IE; characterized by fibrinoid necrosis and lymphocytic infiltration
• Rheumatic heart disease: Organized inflammation with Aschoff bodies (pathognomonic granulomatous lesions with central fibrinoid necrosis and surrounding lymphocytes, plasma cells, and occasional Anitschkow cells) leads to chronic valve scarring and stenosis

Infective Causes

High-Risk Organisms

• Streptococcus viridans (alpha-hemolytic, oral commensal) — most common cause of native valve endocarditis (~50% of cases); enters via dental procedures, poor dentition, or hematogenous spread
• Staphylococcus aureus — leads in prosthetic valve endocarditis (especially early, <1 year post-implant) and IVDU-associated endocarditis; methicillin-resistant S. aureus (MRSA) is increasingly prevalent; notably rapid-onset, destructive with vegetation extending to annulus
• Enterococci — associated with genitourinary and gastrointestinal procedures; relatively penicillin-resistant requiring synergistic antibiotic regimens
• Streptococcus bovis/gallolyticus — colon malignancy association (50–60% of patients harbor colorectal pathology); warrants colonoscopic screening
• HACEK group (Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella) — fastidious, gram-negative, slow-growing organisms; associated with culture-negative endocarditis and delayed diagnosis
• Fungal organisms — Candida albicans, Aspergillus, Cryptococcus associated with immunocompromise, IVDU, cardiac surgery, or prosthetic valves; cause large, bland vegetations with minimal inflammation

Risk Factors for Infective Endocarditis

Pre-existing Cardiac Abnormalities

• Rheumatic heart disease — remaining leading cause globally; chronic turbulent flow on scarred, stenotic valves provides nidus
• Prosthetic valves — mechanical and bioprosthetic; early (0–2 months, surgical site infection) vs. late (>12 months, hematogenous seeding)
• Congenital heart disease — ventricular septal defects, patent ductus arteriosus, tetralogy of Fallot create turbulent jets
• Degenerative valve disease — calcific aortic stenosis, mitral valve prolapse with regurgitation (rare, <5% of IE cases, contrary to historical teaching)
• Intravenous catheters — central lines, dialysis fistulae, pacemaker/ICD leads

Predisposing Factors for Bacteremia

• Poor dental hygiene, dental procedures (extraction, cleaning, endodontics) — highest incidence of transient bacteremia
• Intravenous drug use (IVDU) — tricuspid valve most commonly affected (right-sided endocarditis); recurrent bacteremia with skin flora (S. aureus, streptococci)
• Gastrointestinal/genitourinary procedures — especially in high-risk hosts
• Hemodialysis, immunocompromise, diabetes mellitus
• Nosocomial sources — healthcare-associated IE increasing (15–30% of modern cases)

Non-Infective Causes

• Systemic lupus erythematosus (SLE) — Libman-Sacks verrucae; antiphospholipid syndrome increases thrombotic risk
• Malignancy — marantic endocarditis classically associated with adenocarcinomas (lung, GI), lymphomas
• Rheumatoid arthritis, antiphospholipid antibody syndrome
• Chronic renal failure with uremia

Acute vs. Subacute Presentation

Acute Infective Endocarditis (days to weeks)

• High fever (often >38.5°C), shaking chills — reflects acute bacteremia and inflammatory response
• Septic appearance — tachycardia, hypotension (if severe sepsis or valve rupture)
• New or changing cardiac murmur (60–85%) — early diastolic decrescendo murmur (aortic regurgitation from vegetation-induced valve perforation or annular destruction), holosystolic murmur (mitral regurgitation from chordae rupture, vegetation bulk)
• Rapidly progressive heart failure — from acute aortic or mitral regurgitation due to valve perforation or vegetation mass obstruction
• Septic pulmonary emboli (in right-sided IE) — dyspnea, pleuritic chest pain, pulmonary infiltrates; caused by friable tricuspid vegetations embolizing to lung vasculature, creating septic microinfarcts and cavitary lesions
• Embolic phenomena — splenic infarction (causing left upper quadrant pain), renal infarcts (hematuria), stroke (septic emboli to middle cerebral artery distribution), mycotic aneurysms (from septic emboli lodging in vessel walls, causing pseudoaneurysm rupture and intracranial hemorrhage)

Subacute Infective Endocarditis (weeks to months)

• Low-grade fever, malaise, night sweats — mimics tuberculosis or lymphoma; reflects chronic bacteremia
• Weight loss, anemia — from chronic inflammation and hematologic sequelae
• Splenomegaly (25–60%) — due to septic splenic infarcts, abscess formation, or immune complex deposition
• New or worsening regurgitant murmurs
• Microembolic phenomena accumulating over time

Peripheral Stigmata of Endocarditis

• Osler nodes — painful nodules on fingertip/toe pads; caused by immune complex deposition (not septic emboli, despite historical confusion); present in ~10% of cases
• Janeway lesions — painless erythematous macules on palms/soles; represent septic microemboli with petechiae and microabscesses in dermis; less common than Osler nodes (5%)
• Splinter hemorrhages — linear hemorrhages under nails; caused by septic emboli or immune complex vasculitis (nonspecific; common in trauma)
• Petechiae — small red/purple spots on conjunctiva, oral mucosa, skin from microemboli and immune vasculitis; present in 10–40%
• Clubbing — late finding from chronic hypoxia and embolic phenomena

Laboratory and Diagnostic Correlates

• Anemia (80% of cases) — chronic disease, hemolysis from intravascular turbulence
• Elevated inflammatory markers — ESR (often markedly elevated, >100 mm/hr), CRP, leukocytosis (shift to left)
• Rheumatoid factor positive (50%) — from polyclonal B-cell activation
• Circulating immune complexes — demonstrable in ~90%, correlate with embolic manifestations
• Positive blood cultures — gold standard for diagnosis; typically grow gram-positive cocci or fastidious organisms; cultures negative in 5–15% (prior antibiotics, fastidious organisms, fungi)
• Electrocardiography — PR prolongation, AV blocks (indicate paravalvular abscess extension into conduction system, a surgical emergency marker)
• Echocardiography (TEE superior to TTE for prosthetic valves) — demonstrates oscillating verrucous masses on valve leaflets, valve perforations (discontinuity defects), chordae rupture, regurgitant jets on color Doppler, paravalvular abscesses (localized fluid collections at valve annulus)

Modified Duke Criteria (Clinical Diagnosis)

Major Criteria

• Positive blood cultures — typical organism for IE (e.g., S. viridans, S. aureus, S. bovis) from two separate cultures; or organism consistent with IE from persistently positive cultures
• Echocardiographic findings — oscillating intracardiac mass on valve, vegetation, prosthetic valve dehiscence with regurgitation, new regurgitation jet on Doppler, paravalvular abscess
• Cardiac imaging — PET/CT showing abnormal uptake at valve sites or prosthetic hardware

Minor Criteria

• Predisposition (IVDU, prosthetic valve, congenital disease)
• Fever (>38°C)
• Vascular phenomena (major artery emboli, mycotic aneurysms, splenic infarcts)
• Immunologic phenomena (Osler nodes, Roth spots, rheumatoid factor, glomerulonephritis)
• Microbiologic findings — positive but not meeting major criteria

Histopathological Findings

Acute Endocarditis

• Vegetations — friable, crumbly masses (tan-brown, ~1–5 mm, sometimes extensive) localized on ventricular (atrial) surface of valve leaflets or at chordae insertions
• Acute suppurative inflammation — neutrophilic infiltrate in valve substance, surrounding myocardium (endocardium), and extending into valve annulus
• Bacterial colonies — visible within vegetation core and in inflammatory infiltrate; Gram stain and culture essential
• Valve necrosis — loss of collagenous architecture, myxoid degeneration, focal areas of fibrinoid necrosis
• Paravalvular abscess — localized collection of suppurative material extending through valve annulus into myocardium; indicates valve instability and conduction abnormalities; surgical indication
• Septic thrombi — fibrin-platelet-bacterial thrombi in small vessels; distinguish from sterile fibrin-platelet thrombi of marantic endocarditis by presence of organisms

Chronic/Healed Endocarditis

• Organized vegetation — fibrosis, neovascularization, calcification
• Valve deformity — stenosis, regurgitation, perforation (defects in leaflet substance)
• Annular fibrosis

Non-Infective Endocarditis

• Small verrucae (<1 mm) on both sides of valve leaflets (atrial and ventricular surfaces) — pathognomonic distribution distinguishing from IE
• Fibrinoid necrosis of valve substance
• Lymphocytic infiltration without suppuration
• Hematoxylin bodies (SLE-specific nuclear remnants)

• Small sterile verrucae (1–2 mm, nondestructive) along closure line of valve
• Absence of inflammation
• Microscopic confirmation of sterility (no organisms on Gram/culture)

Gross Pathology Appearance

Infective Endocarditis Vegetations

• Location: Predominately on ventricular surface of leaflets, extending from closure line toward apex
• Appearance: Tan-brown, crumbly, irregular masses; larger vegetations may bridge leaflets or extend to chordae tendineae
• Valve destruction: Perforations (typically irregular defects 2–10 mm), cusp retraction, chordal shortening/rupture, **papillary muscle necr

Immediate steps

• Blood cultures before antibiotics: the American Heart Association infective endocarditis scientific statement advises at least three sets from separate venipuncture sites; in the septic or hemodynamically unstable patient, draw them rapidly and start empiric therapy without prolonged delay.
• Hemodynamic stabilization: acute severe aortic or mitral regurgitation causes cardiogenic pulmonary edema that does not respond to medical therapy — this is a surgical problem, not a diuretic problem.

Empiric antibiotics (bactericidal, IV, prolonged)

• Glycopeptide: vancomycin covers MRSA and most streptococci/enterococci; per the 2020 IDSA/ASHP consensus, dose to a 24-hour AUC/MIC of 400–600, not to a trough.
• Third-generation cephalosporin: ceftriaxone covers streptococci and the HACEK organisms.
• Fourth-generation cephalosporin: cefepime adds antipseudomonal gram-negative coverage when the host or syndrome (healthcare exposure, injection drug use, neutropenia) suggests it.
• Prosthetic valve staphylococcal regimens: vancomycin plus gentamicin for the first two weeks, with rifampin — added per AHA once bacteremia has cleared, since rifampin monotherapy exposure to a high organism burden selects resistance — to penetrate biofilm on the sewing ring.

Pathogen-directed therapy (AHA)

• Penicillin-susceptible viridans streptococci: penicillin G or ceftriaxone, typically 4 weeks on native valve.
• MSSA: an antistaphylococcal beta-lactam (nafcillin, or cefazolin) is superior to vancomycin — do not leave a patient with MSSA on vancomycin.
• MRSA: vancomycin or daptomycin.
• **Penicillin-susceptible *Enterococcus faecalis***: ampicillin plus ceftriaxone (dual beta-lactam saturating complementary penicillin-binding proteins) or ampicillin plus gentamicin (cell-wall agent plus aminoglycoside synergy) — cell-wall agents alone are only inhibitory against enterococci. E. faecium is usually ampicillin-resistant and requires vancomycin, daptomycin, or linezolid.
• Fungal: amphotericin-based therapy plus surgery; medical cure is rare.

Cardiac (highest mortality)

• Acute valvular regurgitation with heart failure: vegetation-induced leaflet perforation or chordal rupture creates a sudden regurgitant volume into a non-compliant, non-dilated chamber — flash pulmonary edema, a soft or short murmur despite severe regurgitation. Emergency; the ACC/AHA 2020 valve guideline calls for urgent surgery rather than medical temporizing.
• Paravalvular/annular abscess: contiguous spread through the annulus into the septum destroys the conduction system. The signal is new PR prolongation or AV block on serial ECG. Emergency — obtain TEE and consult surgery.
• Prosthetic valve dehiscence: rocking prosthesis with a new paravalvular jet; emergency.
• Pericarditis or purulent pericardial effusion from abscess rupture.

Embolic

• Stroke and mycotic aneurysm: friable vegetation fragments lodge in cerebral vessels; septic arteritis weakens the wall, and rupture produces subarachnoid or intraparenchymal hemorrhage. Emergency; hemorrhage generally precludes anticoagulation and delays cardiopulmonary bypass.
• Splenic, renal, or coronary infarction/abscess: persistent fever on appropriate antibiotics with left upper quadrant pain suggests splenic abscess requiring drainage.
• Septic pulmonary emboli in right-sided disease: nodular, cavitating peripheral infiltrates.

Immunologic

• Immune-complex glomerulonephritis: hematuria with RBC casts and low complement; resolves with cure of infection.

Treatment-related

• Vancomycin nephrotoxicity, amplified by concurrent piperacillin-tazobactam; monitor renal function and AUC.
• Aminoglycoside nephrotoxicity and irreversible ototoxicity/vestibulotoxicity.
• Daptomycin: CPK elevation/myopathy and eosinophilic pneumonia.
• Rifampin: hepatotoxicity and potent CYP induction (warfarin, antiretrovirals).
• Beta-lactams: interstitial nephritis, neutropenia with prolonged courses, C. difficile colitis; long-term catheter access adds line infection and thrombosis.

• ***Streptococcus gallolyticus* (formerly S. bovis) bacteremia or endocarditis → colonoscopy** is the single best next step; the association with colorectal neoplasia is the classic tested link.
• Fever plus a new regurgitant murmur → blood cultures first, then echocardiography. TTE is the initial study; TEE is required for prosthetic valves, suspected abscess, or a negative TTE with high clinical suspicion.
• Osler nodes are Ouchy — painful, pulp of digits, immune-complex mediated. Janeway lesions are painless macules on palms/soles from septic microemboli. Roth spots are retinal hemorrhages with pale centers.
• New or progressive PR prolongation/AV block in a patient with IE means paravalvular abscess until proven otherwise — get TEE and call surgery. This is the conduction pearl examiners love.
• Libman-Sacks vegetations sit on both surfaces of the leaflet; marantic (NBTE) vegetations sit on the closure line and are small and sterile; IE vegetations are bulky, friable, and destructive. Marantic disease is classically linked to mucin-secreting adenocarcinoma (pancreas, GI) and to Trousseau syndrome.
• **IV drug use → tricuspid valve, S. aureus, septic pulmonary emboli with cavitary nodules and often no murmur**. Right-sided (tricuspid) disease predominates in people who inject drugs, though left-sided involvement occurs in a substantial minority and is increasingly reported; across all-comers with IE, left-sided valves are the more commonly affected.
• MSSA is treated with nafcillin or cefazolin, not vancomycin — vancomycin is inferior for methicillin-susceptible strains. And vancomycin is followed by AUC/MIC 400–600 (2020 IDSA/ASHP), not a 15–20 mcg/mL trough.
• Enterococcal pearl: E. faecalis → ampicillin plus ceftriaxone (or ampicillin plus gentamicin); E. faecium is typically ampicillin-resistant and needs vancomycin, daptomycin, or linezolid.
• Common distractors: mitral valve prolapse no longer warrants antibiotic prophylaxis (AHA restricts prophylaxis to prosthetic valves, prior IE, certain congenital lesions, and transplant valvulopathy); culture-negative endocarditis should prompt serologies for Coxiella burnetii and Bartonella, not simply broader empiric antibiotics.