Definition – Infective endocarditis is a microbial infection of the heart valves or mural endocardium that leads to the formation of vegetations composed of thrombotic debris and organisms, often associated with destruction of the underlying cardiac tissue
Infective endocarditis is classified on clinical grounds into acute and subacute forms indicating the severity of disease which depends upon the virulence of infecting organism
Acute infective endocarditis-
Infection of previously normal heart valve by highly virulent organism (staphylococcus aureus) that rapidly produces necrotizing and destructive lesions
Death ensue with in days to weeks despite appropriate treatment with antibiotics and surgery
Sub acute infective endocarditis
Infection by organisms of lower virulence (Streptococci viridans) that causes insidious infections of deformed valves with over all less destruction
Disease has course of weeks to months and cure can be achieved with antibiotics
Etiopathogenesis
Organisms causing are
50% to 60% of cases affecting previously damaged or otherwise abnormal valves – Streptococcus viridans (normal component of oral cavity flora)
20% to 30% of cases affecting healthy or deformed valves – Staphylococus aureus
IE in IV drug abusers – Staphylococcus aureus
Prosthetic valve endocarditis is caused most commonly by – Coagulase negative staphylococci (Staphylococcus epidermidis)
Other bacterial causes include Enterococci and HACEK group
H – Haemophilus
A- Actinobacillus
C – Cardiobacterium
E – Eikenella
K – Kingella
Other agents causing endocarditis include – Gram negative bacilli and fungi
In 10% of cases – no organism is identified (“Culture negative” endocarditis)
Causes for the culture negative endocarditis are
Prior antibiotic therapy
Difficulty in isolating the offending agent
Organism is deeply embedded in enlarged vegetation that it cannot be released into blood
Predisposing factors
3 main types of predisposing factors leading to bacterial endocarditis are
Conditions that causes seeding of microorganisms into the blood (Bacteremia or Fungemia)
Underlying heart disease
Impaired host defenses
Source of infection may be
Periodontal infections and dental procedures
Contaminated needle shared by IV drug abusers
Infections of genitourinary tract during procedures like catheterization, cystoscopy, and obstetrical procedures during normal delivery or abortions
Infections and surgeries of bowel and biliary tract
Skin infections like boils, carbuncles and abscesses
Respiratory tract infections
Cardiac catheterization and surgery for valve replacement
Commonly associated underlying heart diseases are
Chronic rheumatic valvular disease (50% of cases)
Congenital heart diseases (20% of cases) like
VSD, PDA, Sub aortic stenosis, pulmonary stenosis, bicuspid aortic valve and coarctation of aorta
Cytotoxic therapy for cancer patients and transplant patients
Deficient functions of neutrophils and macrophages
Pathogenesis
Bacteria from blood stream in any of the above mentioned routes are implanted on the cardiac valves or mural endocardium as they have surface adhesion molecules which mediate their adherence to endocardium
Conditions predispose to implantation are –
Previously damaged valves from diseases like RHD, congenital heart disease and prosthetic valves
Hemodynamic stress
These conditions causes damage to endothelium on valves, favoring the formation of platelet-fibrin thrombi which get infected from circulating bacteria where they proliferate and form vegetations
Vegetations can embolize and as their embolic fragments contain virulent organisms, abscesses develop where they lodge, leading to sequelae such as septic infarcts or mycotic aneurysms
Vegetations of subacute endocarditis are associated with less valvular destruction than acute endocarditis
Reference
Vinay kumar, Abul K.Abbas, Nelson Fausto, Jon C. Aster. Robbins and Cotran Pathologic basis of disease. 8th edition
