What are the etiologies of a mitral valve abscess?

Etiology of Mitral Valve Abscess

Mitral valve abscesses result from infective endocarditis, most commonly caused by Staphylococcus aureus in acute presentations and streptococcal species in subacute disease, with perivalvular extension occurring through direct spread of infection from valve leaflets into the annular tissue. 1

Primary Causative Organisms

Bacterial Pathogens by Clinical Context

Staphylococcal infections are the predominant cause of mitral valve abscesses, particularly in specific patient populations:

• Staphylococcus aureus causes acute endocarditis and is the leading pathogen in intravenous drug users, accounting for 80% of cases in this population 1, 2
• S. aureus can develop on previously normal valves and is associated with healthcare exposure, hemodialysis, diabetes mellitus, and intravascular devices 3
• Staphylococcus epidermidis is less common but occurs in subacute bacterial endocarditis, typically on abnormal valves 1

Streptococcal species cause subacute presentations:

• Streptococcal species are the most common organisms in subacute bacterial endocarditis, typically developing on abnormal valves after asymptomatic bacteremias from infected gums or the genitourinary/gastrointestinal tract 1
• The S. milleri/anginosus group must be distinguished due to their abscess-forming tendency 3
• Rare organisms like Streptococcus vestibularis have been reported causing mitral valve endocarditis with paravalvular abscess formation 4

Enterococcal infections represent 5-20% of all infective endocarditis cases:

• Enterococcus faecalis and E. faecium are the predominant species 5
• These organisms typically present in a subacute fashion rather than acute fulminant disease 5

Other organisms include:

• Group A hemolytic streptococci, pneumococci, and gonococci in acute endocarditis 1
• Fastidious Haemophilus species in subacute disease 1
• Fungal organisms, which are particularly associated with abscess formation 1

Anatomic and Pathophysiologic Mechanisms

Perivalvular Extension Leading to Abscess Formation

Mitral valve abscesses develop through periannular extension of infection:

• Perivalvular abscesses in mitral endocarditis are usually located posteriorly or laterally 1
• Most periannular infections involving the mitral area are associated with prosthetic mitral valves rather than native valves 1
• Abscess formation is a dynamic process that can progress to fistulous tracts, pseudoaneurysms, and complete disruption of cardiac structures 1

Specific pathologic mechanisms include:

• Neovascularization of the anterior mitral valve leaflet may predispose this territory to abscess and aneurysm formation 6
• Infection can spread from the valve leaflets to the annulus and then into surrounding myocardial tissue 7
• Mitral valve aneurysms represent another form of intravalvular abscess that has expanded 6

Risk Factors for Abscess Development

Prosthetic valve involvement is the strongest predictor:

• Perivalvular abscesses occur in 56-100% of prosthetic valve endocarditis cases 1
• The annulus, rather than the leaflet, is the usual primary site of infection in prosthetic valves 1

Native valve risk factors:

• Preexisting mitral valve disease (rheumatic disease, prolapse, mitral annular calcium) increases risk 8
• However, 67% of mitral valve endocarditis cases occur on previously normal valves 8
• Intravenous drug use is a major predisposing factor, present in 52% of cases with previously normal valves 8

Organism-specific factors:

• Coagulase-negative staphylococci (CoNS) are important risk factors for perivalvular complications 1
• Resistant or very virulent organisms often result in uncontrolled infection with abscess formation 1

Clinical Context and Predisposing Conditions

Healthcare-associated factors:

• Nosocomial infections, home-based nursing, IV therapy, or hospitalization within 90 days before endocarditis onset 3
• Chronic hemodialysis increases risk 3, 8

Patient-specific factors:

• Habitual alcoholism is present in a significant proportion of cases 8
• Immunosuppressive diseases predispose to infection 4
• Underlying structural abnormalities like bicuspid aortic valve (which can lead to secondary mitral involvement) 4

Critical Clinical Implications

Abscess formation portends poor prognosis:

• Patients with perivalvular abscess are at higher risk for embolization and death 1
• Extension of infection beyond the valve annulus predicts higher mortality rate, more frequent development of heart failure, and more frequent need for cardiac surgery 1
• Untreated endocarditis is almost always fatal 1

Most physicians believe that abscess formation is an indication for surgery regardless of the causative organism 1