What is a psoas abscess, particularly in adults with underlying medical conditions such as diabetes or immunocompromised status?

Psoas Abscess Definition

A psoas abscess is a collection of pus within the iliopsoas muscle compartment located in the retroperitoneal space, which can occur as a primary infection from hematogenous or lymphatic spread (typically monomicrobial, most commonly Staphylococcus aureus) or as a secondary infection from direct extension of adjacent structures such as the spine, gastrointestinal tract, or genitourinary system (often polymicrobial with enteric and anaerobic organisms). 1, 2, 3

Anatomic Location and Pathophysiology

• The psoas abscess develops within the iliopsoas muscle compartment, which comprises the psoas and iliacus muscles situated in the extraperitoneal/retroperitoneal space 3
• The infection can extend from its origin through the iliopsoas muscle and potentially reach the inguinal region 4
• The retroperitoneal location makes clinical detection challenging and often delays diagnosis 5, 6

Classification: Primary vs. Secondary

Primary Psoas Abscess

• Primary abscesses result from hematogenous or lymphatic seeding from a distant infection site, occurring without an identifiable adjacent source 2, 3
• Staphylococcus aureus is the causative organism in approximately 90% of primary cases 2, 7
• Other less common pathogens include Streptococcus species (Group A, S. pneumoniae, S. agalactiae), and gram-negative enteric bacilli 2, 7
• Primary abscesses are typically monomicrobial 3

Secondary Psoas Abscess

• Secondary abscesses develop from direct extension of infection from adjacent anatomic structures including the spine (vertebral osteomyelitis, spondylitis), hip joint (septic arthritis, osteomyelitis), gastrointestinal tract (Crohn's disease, perforated colon neoplasm, diverticulitis), aorta, or genitourinary tract (pyelonefritis, obstructive uropathy) 1, 2, 4, 6
• Gram-negative enteric organisms (E. coli, Klebsiella, Enterobacter) are the predominant pathogens in secondary abscesses 2, 7
• Secondary infections can be either monomicrobial or polymicrobial, often including enteric and anaerobic organisms 3, 6
• Mycobacterium tuberculosis remains a significant cause of secondary psoas abscess, representing 36% of cases in some series 6

High-Risk Populations

• Immunocompromised patients including those with HIV infection, diabetes mellitus, hematologic malignancies, or neutropenia have substantially elevated risk 1, 2
• Diabetes mellitus is the leading underlying disease associated with psoas abscess formation 7
• Patients with gastrointestinal or genitourinary malignancies causing obstruction are at increased risk 2
• Males are affected more frequently than females, with a male predominance of approximately 80% 4
• The typical age range is 20-60 years, with a mean age of approximately 40-50 years 5, 4, 7

Clinical Presentation Characteristics

• Fever, lower back pain, flank pain, and hip pain are the most common presenting symptoms, though the classic triad of fever, flank pain, and limitation of hip movement (psoas sign) is present in only a minority of patients 5, 7
• The clinical presentation is often nonspecific and prolonged, with symptom duration exceeding 30 days in 64% of cases before diagnosis 6
• Fever is present in only 36-82% of cases, making its absence unreliable for excluding the diagnosis 5, 6
• A palpable mass may be detected on physical examination 5
• Leukocytosis is the most common laboratory finding, though laboratory tests may not accurately reflect disease severity in immunocompromised patients 8, 5
• Septic shock can be the initial presentation in severe cases, occurring in approximately 18-20% of patients 5, 4