U01.11.023 Psoas abscess

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Learning Objectives

  • Identify the anatomical compartments involved in a psoas abscess.
  • Differentiate between the primary microbiology (S. aureus vs. M. tuberculosis).
  • Recognize high-yield clinical signs (Psoas sign) and risk factors (Crohn disease).
  • Analyze imaging findings and first-line treatment protocols.

1. Pathophysiology and Spread

A psoas abscess is a collection of pus within the iliopsoas muscle group. Because the psoas muscle originates from the T12–L5 vertebrae and inserts into the lesser trochanter, it provides a direct “conduit” for infection to travel from the spine to the groin.

  • Hematogenous Spread: Bacteria travel through the blood to the muscle.
  • Contiguous Spread: Infection spreads from adjacent structures:
    • Vertebral Osteomyelitis / Pott Disease (Tuberculous spondylitis).
    • Renal: Pyelonephritis or perinephric abscess.
    • Gastrointestinal: Crohn disease (due to the proximity of the psoas to the ileum/appendix).

2. Clinical Presentation

Diagnosis is often delayed because symptoms can be vague or mimic other abdominal/hip pathologies.

  • The “Classic” Triad: Fever, flank/back pain, and a palpable inguinal mass.
  • Psoas Sign: Abdominal or hip pain exacerbated by passive extension of the hip. This stretches the inflamed psoas muscle.
  • Microbiology:
    • Most Common: Staphylococcus aureus.
    • Secondary: Mycobacterium tuberculosis (especially in endemic areas or immunocompromised patients).

3. Diagnosis and Management

Modality Key Finding
Imaging (CT/MRI) Hypodense lesion (fluid collection) within the muscle plane. CT is the gold standard.
Laboratory Leukocytosis, elevated ESR/CRP.
Treatment Percutaneous or surgical abscess drainage + targeted antibiotic therapy.

Clinical Notes & Step 1 Pearls:

  • Crohn Disease Link: If a patient with a history of Crohn’s presents with new-onset hip pain and fever, suspect a psoas abscess.
  • Pott Disease: In a patient with TB, the psoas abscess is often called a “cold abscess” because it lacks the classic warmth and redness of an acute pyogenic infection.
  • Differential: Must be distinguished from appendicitis (which can also have a positive psoas sign if the appendix is retrocecal).

Activity: Psoas Abscess Diagnostic Logic

Quick Mnemonics:

Psoas Sign: Passive Extension = Pain.

S. aureus: Usually the “Gold” standard for skin/muscle abscesses.

Activity