What is an Iliopsoas Abscess?
An iliopsoas abscess is a collection of pus within the iliopsoas muscle compartment, typically classified as either primary (arising from hematogenous spread of bacteria without an identifiable source) or secondary (resulting from direct extension from adjacent infected structures such as the spine, gastrointestinal tract, or genitourinary system). 1, 2
Classification and Etiology
Primary Iliopsoas Abscess
- Occurs in approximately 20-30% of cases and results from hematogenous seeding of the iliopsoas muscle without an identifiable adjacent source 1, 3
- Staphylococcus aureus is the predominant pathogen in primary abscesses, isolated in approximately 70% of cases 1, 3, 4
- Intravenous drug users and HIV-positive patients are at particularly high risk, with up to 86% of primary cases occurring in IV drug users 3
Secondary Iliopsoas Abscess
- Represents 70-80% of all iliopsoas abscesses and arises from direct extension of infection from adjacent structures 1, 2
- Spondylodiscitis (spinal infection) is the leading cause, accounting for approximately 38-52% of secondary cases 1, 5
- Other sources include gastrointestinal pathology (Crohn's disease, appendicitis, diverticulitis), genitourinary infections, and trauma 2, 3
- Secondary abscesses typically harbor polymicrobial enteric flora rather than S. aureus 3, 4
Clinical Presentation
Classic Triad (Present in Minority of Cases)
- Only 33% of patients present with the classic triad of fever, flank/hip pain, and limp, making diagnosis challenging 2
- The vague and nonspecific presentation frequently leads to diagnostic delays, with median time to diagnosis of 3 days 2
Common Clinical Features
- Fever occurs in approximately 60% of patients on admission 2
- Pain in the flank, hip, or abdomen is the most consistent symptom 3, 4
- Nearly all patients demonstrate anemia and elevated inflammatory markers (CRP, leukocytosis) 2
- Left unilateral involvement occurs in approximately 66% of cases 1
Diagnostic Approach
Imaging Modality of Choice
- Computed tomography (CT) is the preferred diagnostic modality, used in 87% of cases, providing superior visualization of the abscess and potential underlying causes 1, 2
- MRI can be useful when evaluating for iliopsoas bursitis or soft tissue abnormalities in the hip region 6
- A high index of clinical suspicion is essential given the nonspecific presentation 2, 4
Critical Pitfall to Avoid
The vague and atypical presentation of iliopsoas abscess frequently results in delayed diagnosis, which significantly increases morbidity and mortality. 2, 5 Any patient presenting with unexplained fever combined with flank, hip, or abdominal pain—particularly if they are immunocompromised, IV drug users, or have risk factors for spinal infection—should prompt consideration of this diagnosis and urgent CT imaging 2, 3.