Causes of Psoas Abscess
Psoas abscesses arise from two main pathways: hematogenous spread (primary) or direct extension from adjacent infections (secondary), with the secondary form now being more common and accounting for approximately 55% of cases.
Primary vs. Secondary Classification
Primary Psoas Abscess (45% of cases)
Primary psoas abscesses develop through hematogenous seeding from distant infection sites 1. This occurs predominantly in:
- Elderly patients
- Severely malnourished individuals
- Patients with chronic diseases (diabetes, immunosuppression)
- Immunocompromised hosts (HIV infection, steroid therapy, systemic lupus erythematosus) 2
The causative organisms in primary cases include:
- Staphylococcus aureus (approximately 90% of pyomyositis cases, including community-acquired MRSA) 3, 4
- Mycobacterium tuberculosis (36% in some series, particularly in endemic areas) 1
- Group A streptococci
- Streptococcus pneumoniae
- Gram-negative enteric bacteria 5
Secondary Psoas Abscess (55% of cases)
Secondary abscesses result from direct extension of adjacent infections 1. The specific sources include:
Gastrointestinal Sources
- Crohn's disease (most common underlying condition in several series) 6, 2
- May be the first manifestation of Crohn's disease in up to 24% of cases (11 of 46 reported patients)
- Gastrointestinal symptoms may be completely absent 6
- Appendicitis with retroperitoneal extension
- Diverticulitis
- Colonic perforation or malignancy
Musculoskeletal Sources
- Tuberculous spondylitis (Pott's disease) - the classic historical cause 4, 1
- Nontuberculous lumbar osteomyelitis or diskitis 2
- Sacroiliac arthritis 1
Genitourinary Sources
- Pyelonephritis
- Perinephric abscess
- Urinary tract infections with retroperitoneal spread
Causative Organisms by Source
For secondary abscesses, the microbiology reflects the source:
- Tuberculous etiology: Remains frequent (36-92.8% in some series, particularly in endemic regions) 7, 1
- Pyogenic bacteria (7.2% in tuberculous-endemic areas, higher elsewhere):
Critical Clinical Pitfall
A psoas abscess that fails to improve or relapses after successful treatment of presumed staphylococcal infection should prompt immediate evaluation for tuberculosis, particularly in immunocompromised patients 4. The indolent course of tuberculous psoas abscess can be masked by concomitant pyogenic infection, leading to delayed diagnosis and severe morbidity.
Risk Factor Summary
The changing epidemiology shows:
- Shift from tuberculous to pyogenic causes in non-endemic areas
- Increasing recognition in temperate climates (previously called "tropical pyomyositis") 5
- Higher incidence in patients with:
- Diabetes mellitus
- HIV infection
- Intravenous drug abuse
- Chronic steroid use
- Inflammatory bowel disease (especially Crohn's)
- Advanced age with multiple comorbidities 2
Blood cultures are positive in only 5-30% of cases, emphasizing the importance of image-guided aspiration for microbiological diagnosis 5, 3.