What are the causes of psoas abscess?

Causes of Psoas Abscess

Psoas abscesses are classified as either primary (hematogenous spread from distant occult sites) or secondary (direct extension from adjacent structures), with secondary causes now predominating in contemporary practice.

Primary Psoas Abscess

Primary psoas abscess results from hematogenous seeding from a distant, often occult infectious source without an identifiable adjacent focus. 1, 2

Key predisposing conditions include:

• Diabetes mellitus - a major risk factor for hematogenous bacterial seeding 1
• Intravenous drug use - introduces bacteria directly into bloodstream 1
• Immunocompromised states - including malignancy (prostate carcinoma documented) 1
• Hematoma formation - particularly in patients with bleeding disorders like hemophilia A, which can become secondarily infected 1

The causative organism in primary psoas abscess is most commonly Staphylococcus aureus spread hematogenously. 2

Secondary Psoas Abscess

Secondary psoas abscess results from direct extension of infection from adjacent structures and now represents the majority of cases in modern surgical practice. 3, 2

Spinal/Vertebral Sources

The spine is an increasingly important primary source of infection for secondary psoas abscess, particularly with more frequent invasive spinal procedures and resurgent tuberculosis. 4

• Vertebral osteomyelitis/spondylodiscitis - five of seven secondary cases in one series 1
• Tuberculous spondylitis (Pott's disease) - the classic historical cause, now re-emerging 4, 1
• Post-spinal surgery infections - including implant infections after spinal instrumentation 4
• Spinal epidural abscess - can extend into psoas muscle 5

Gastrointestinal Sources

Gastrointestinal pathology is the most common cause of secondary psoas abscess in the literature, though this varies by geographic region. 3, 2

• Crohn's disease - the most frequently reported gastrointestinal cause, with psoas abscess being the first manifestation in 11 of 46 reported patients 6
• Appendicitis - documented in two of seven secondary cases 3
• Sigmoid diverticulitis - through direct perforation or fistula formation 3
• Colorectal carcinoma - both primary and metastatic disease 3
• Colitis - various inflammatory conditions 1

Notably, gastrointestinal symptoms may be completely absent when psoas abscess is the presenting feature of Crohn's disease. 6

Genitourinary Sources

• Urinary tract infections - can extend retroperitoneally to involve psoas muscle 1
• Renal/perinephric infections - through direct extension 2

Clinical Pitfalls and Diagnostic Considerations

Diagnosis is frequently delayed because psoas abscess mimics other conditions including arthritis, joint infection, or urologic/abdominal disorders. 4

The spine as a primary infectious source must always be included in the differential diagnosis, especially in patients with:

• Back or hip pain with fever 4
• History of spinal surgery 4
• Psoas spasm and tender mass 3

When psoas abscess complicates Crohn's disease, gastrointestinal symptoms may be minimal or absent, making this a particularly challenging diagnosis. 6

Exclude underlying Crohn's disease in all patients presenting with psoas abscess, especially if recurrent, through detailed history assessing for inflammatory bowel disease symptoms. 7