Bacterial Endocarditis
Bacterial endocarditis is an infection of the heart's inner lining (endocardium) that primarily affects the heart valves, characterized by the formation of vegetations composed of platelets, fibrin, microorganisms, and inflammatory infiltrate. 1
Definition and Pathophysiology
Bacterial endocarditis, more broadly termed infective endocarditis (IE), is an endovascular microbial infection of:
- Heart valves (most common)
- Intracardiac structures facing the bloodstream
- Large intrathoracic vessels
- Intracardiac foreign bodies/prosthetic material 2
The characteristic pathological lesion is a vegetation, although destruction, ulceration, or abscess formation may develop as the infection progresses 2. These vegetations consist of:
- Platelets
- Fibrin
- Microorganisms
- Inflammatory cells 1
Epidemiology
- The incidence of IE remains stable globally but with changing epidemiological patterns 1
- Causes approximately 1.58 million disability-adjusted life years worldwide 1
- Peak incidence of 14.5 episodes/100,000 person-years in patients aged 70-80 years 1
- Epidemiological shifts include:
- Decreased cases related to rheumatic heart disease in developed countries
- Increased cases due to Staphylococcus aureus (now the most common pathogen in industrialized countries)
- Increased cases in older patients with degenerative valve disease 1
Risk Factors
High-risk cardiac conditions:
- Prosthetic heart valves (most significant risk factor)
- Previous history of IE
- Complex congenital heart disease
- Surgically constructed systemic or pulmonary conduits 1
Moderate-risk cardiac conditions:
- Acquired valvular disease
- Mitral valve prolapse with regurgitation
- Non-cyanotic congenital heart disease
- Hypertrophic cardiomyopathy
- Bicuspid aortic valve 1
Other risk factors:
- Intravenous drug use (1.5-3.3 cases per 1000 person-years)
- Advanced age
- Conditions promoting non-bacterial thrombotic vegetation
- Compromised immune system 2, 1
Microbiology
The most common causative organisms include:
Staphylococci:
- Staphylococcus aureus (now the leading cause in developed countries)
- Coagulase-negative staphylococci (especially in prosthetic valve endocarditis)
Streptococci:
- Viridans group streptococci (common in community-acquired native valve endocarditis)
- Streptococcus bovis (associated with gastrointestinal pathology)
- Nutritionally variant streptococci (Abiotrophia and Granulicatella species)
Enterococci
HACEK organisms (slow-growing gram-negative bacteria)
Clinical Presentation
Common clinical manifestations include:
- Fever (present in up to 90% of cases)
- New regurgitant heart murmur or change in pre-existing murmur
- Embolic phenomena (stroke, pulmonary embolism, splenic/renal infarcts)
- Immunological phenomena:
- Roth spots (retinal hemorrhages with pale centers)
- Janeway lesions (painless hemorrhagic macules on palms/soles)
- Osler nodes (painful nodules on finger/toe pads)
- Glomerulonephritis 1
Diagnosis
Modified Duke Criteria
Diagnosis is based on the modified Duke criteria, which include:
Major criteria:
- Positive blood cultures for typical microorganisms
- Echocardiographic evidence of endocardial involvement
Minor criteria:
- Predisposing heart condition
- Fever
- Vascular phenomena
- Immunological phenomena
- Microbiological evidence not meeting major criteria 1
Diagnostic Testing
Blood cultures: Cornerstone of diagnosis
- Three separate samples taken 30 minutes apart
- Positive in 85-90% of cases
- Should be obtained before starting antibiotics 1
Echocardiography:
Treatment
Treatment requires prolonged antimicrobial therapy with high doses to penetrate vegetations 1:
For viridans group streptococci and S. bovis (penicillin-susceptible):
- Penicillin G 12-18 million U/day IV in 6 divided doses for 4 weeks, OR
- Ceftriaxone 2g/day IV for 4 weeks 2, 1
For enterococci:
- Penicillin or ampicillin plus gentamicin for 4-6 weeks 3, 4
- Vancomycin for penicillin-resistant strains 3
For staphylococcal infections:
- Native valve (methicillin-susceptible): Nafcillin, oxacillin, or cefazolin for 4-6 weeks
- Prosthetic valve: Triple therapy with rifampin, gentamicin, and either nafcillin/oxacillin/cefazolin (methicillin-susceptible) or vancomycin (methicillin-resistant) 3
For culture-negative endocarditis:
- Empiric therapy based on clinical presentation and epidemiology
- Consider unusual pathogens (Bartonella, Coxiella, Brucella) 2
Complications
Major complications include:
- Congestive heart failure (most common serious complication and leading cause of death)
- Periannular abscesses (42-85% of cases)
- Systemic embolization (22-50% of cases)
- Neurological complications (20-40% of patients)
- Mycotic aneurysms
- Splenic abscess 5, 6
Surgical Indications
Approximately 50% of patients require surgery 1. Indications include:
Emergency (immediate surgery):
- Severe heart failure or cardiogenic shock due to valve dysfunction
Urgent (within days):
- Persistent heart failure
- Uncontrolled infection (abscess, false aneurysm, fistula)
- Large vegetations (>10mm) with embolic events
Elective:
- IE due to fungi or multiresistant organisms
- Recurrent emboli despite appropriate antibiotic therapy 1
Prevention
Antibiotic prophylaxis is recommended for high-risk patients undergoing dental procedures with bleeding risk:
- Amoxicillin 2g orally 1 hour before procedure
- Clindamycin 600mg orally 1 hour before procedure (for penicillin-allergic patients) 1
Current guidelines no longer recommend prophylaxis for gastrointestinal or genitourinary procedures in most patients, though some evidence suggests continued benefit in elderly, cancer, and immunocompromised patients 7.
Prognosis
If untreated, bacterial endocarditis is universally fatal. Even with appropriate treatment, in-hospital mortality remains high (15-20%), particularly with certain pathogens (S. aureus), prosthetic valve involvement, or complications such as heart failure 2, 1.