Psoas abscess (also called iliopsoas abscess) is a rare condition characterized by a localized collection of pus in the iliopsoas muscle compartment. It is categorized into primary psoas abscess (caused by hematogenous or lymphatic spread of a pathogen) and secondary psoas abscess (resulting from contiguous spread from an adjacent infectious focus). The most common causes of secondary psoas abscess are infections of gastrointestinal or musculoskeletal origin. Infections are often monomicrobial, with S. aureus most frequently isolated, followed by E. coli. Polymicrobial abscesses are most commonly of gastrointestinal origin. Computed tomography is the diagnostic modality of choice. Empiric antibiotic therapy should be administered as soon as blood cultures and, if possible, abscess cultures are obtained. A small abscess (< 3.5 cm) in a nonseptic patient can be managed with antibiotic therapy alone. Larger or multiloculated abscesses should be drained under image guidance or with surgery. In a psoas abscess of any kind, the underlying cause should be evaluated and treated.
Primary psoas abscess 
- Caused by hematogenous or lymphatic spread of a pathogen from a distant source
- Most often caused by a single pathogen
- Psoas muscle injury (e.g., trauma, ischemia) increases the likelihood of infection. 
- More common in Asia and Africa 
Secondary psoas abscess
- Caused by contiguous spread of a pathogen from an adjacent infectious focus
Primary focus may be: ; 
- Gastrointestinal: e.g., IBD (especially Crohn disease), colitis, appendicitis, diverticulitis, pancreatitis
- Musculoskeletal: e.g., osteomyelitis (esp. of the vertebrae), Pott disease, paraspinal abscess, septic arthritis (commonly of the hip in children), prosthetic joint infection
- Genitourinary: e.g., pyelonephritis, cystitis
- Cardiovascular: e.g., infective endocarditis, endocarditis, infected aortic endograft
- Postprocedural: e.g., after vascular surgery, arterial catheterization, spinal surgery, instrumentation of the urinary tract
- Different causative pathogens are associated with different primary infectious foci.
Risk factors 
- Immunodeficiency (e.g., diabetes mellitus, HIV)
- Intravenous drug use
Causative pathogens 
- Both primary and secondary psoas abscesses are most often monomicrobial.
- Secondary abscesses are more likely to be polymicrobial than primary abscesses.
|Causative pathogens of psoas abscess|
|E. coli|| |
Bacteroides spp., S. viridans
|M. tuberculosis|| |
Classic triad: present in ∼ 30% of cases 
- Antalgic gait
- Lower back pain (flank pain)
- Nonspecific symptoms: lower abdominal pain, malaise, weight loss
- Affected hip lies in a flexed and externally rotated position at rest.
- Passive extension and/or internal rotation of the affected hip elicits pain.
- A tender/nontender mass may be palpable in the ipsilateral iliac or inguinal region.
- Additional features: symptoms related to underlying conditions may be present (e.g., colitis, UTI). 
Imaging is required to confirm the clinical diagnosis of psoas abscess. Laboratory studies provide supportive evidence of an acute infectious process and may be used to monitor response to therapy. Tests to evaluate for a suspected primary focus should be guided by the pretest probability of the underlying etiology, as determined by a thorough history and physical examination. If no obvious primary focus of infection can be identified, the psoas abscess is presumed to be a primary psoas abscess. 
Laboratory studies 
Routine tests for suspected psoas abscess
- CBC: ↑ WBC; anemia may be present
- Inflammatory markers: ↑ ESR, ↑ CRP
- ↑ Creatine kinase
- Procalcitonin: not routinely indicated; usually elevated in patients with sepsis
Gram stain and culture of blood and abscess aspirate
- A definitive microbiological diagnosis can be made in ∼ 75% of patients. 
- The isolated organism may indicate the primary source of infection (see causative pathogens in ''Etiology'').
- Consider an acid-fast stain (Ziehl-Neelsen stain) and special culture medium if tuberculosis is suspected.
Tests to evaluate for an underlying etiology
- Urinalysis and leukocyte esterase in suspected UTI
- HIV serologies/PCR in suspected HIV infection. 
- Diagnostic workup for tuberculosis in endemic regions
- Serology for suspected inflammatory bowel disease (see ''Diagnostics'' of ulcerative colitis and Crohn disease)
Obtain blood cultures (and pus/aspirate cultures, if possible) before initiating empiric antibiotic therapy.
CT abdomen and pelvis with IV contrast (axial section) 
- Indication: imaging modality of choice 
- Enlargement of the ipsilateral psoas muscle
- Well-defined abscess: encapsulated hypodense lesion with peripheral enhancement, with or without gas shadows, within the psoas muscle
- Signs of an intraabdominal or pelvic primary focus on infection, if present (e.g. peritoneal fat stranding adjacent to inflamed bowel, destruction of the vertebral body in vertebral osteomyelitis)
MRI of the pelvis and abdomen with IV contrast 
- Suspected psoas abscess in a patient with contraindications for CT scan
- Suspected spinal pathology and evaluation of back pain 
- Consider MR enterography if Crohn disease is suspected to be the underlying etiology. 
- Pregnant women: MRI without contrast is preferred
- Similar to those in CT
- Abscess: hypointense in T1, hyperintense in T2
Abdominal ultrasound 
- An alternative to MRI in patients with contraindications for CT scan
- May be used as a first-line imaging modality to evaluation for intraabdominal infection
- Asymmetrical swelling of the psoas muscle
- The abscess appears as a hypoechoic collection within the psoas muscle.
- Signs of intra-abdominal or pelvic primary focus
Imaging to evaluate for a suspected primary focus
A primary focus within the abdomen or spine may be discovered incidentally during the workup for psoas abscess.
- Consider the following:
- MRI of the spine in suspected vertebral osteomyelitis
- Imaging of the hip or knee in suspected prosthetic joint infection 
- TTE/TEE in suspected infective endocarditis
- Chest x-ray in suspected pulmonary focus (e.g., tuberculosis)
If no primary focus of infection can be identified, primary psoas abscess is most likely. 
See also differential diagnosis of low back pain.
- Inflammatory bowel disease (especially Crohn disease) 
- Pancreatitis 
- Colorectal neoplasia
- Osteomyelitis (most commonly vertebral osteomyelitis)
- Paraspinal abscess
- Septic arthritis of the hip (especially in children) 
- Muscle strain
- Iliopsoas bursitis
- Metastatic deposits within the iliopsoas muscle
- Osteoarthritis of the hip
- Prosthetic joint infection of the hip 
- Aortic aneurysm
- Infected iliac artery endograft
The differential diagnoses listed here are not exhaustive.
General principles 
- Antibiotic therapy and drainage of the abscess are the mainstays of therapy.
- Consider a trial of antibiotic therapy alone in small abscesses (< 3.5 cm). 
- Abscess drainage (either image-guided or surgical) is indicated in septic patients or if the abscess is large or multiloculated.
- Identify and treat the underlying condition (e.g., appendectomy in appendicitis; tuberculosis therapy if tuberculosis is confirmed)
Empiric antibiotic therapy for psoas abscess 
- Coverage: empiric therapy should provide coverage of S. aureus and E. coli.
- One of the following beta-lactams with beta-lactamase inhibitor:
- OR combination therapy with metronidazole PLUS one of the following:
- A third-generation cephalosporin (e.g., ceftriaxone )
- A fluoroquinolone (e.g., levofloxacin )
- PLUS consider one of the following for MRSA coverage :
- Tailor antibiotic therapy to culture sensitivity reports when they are available.
- Switch to oral antibiotics once the patient is afebrile.
- Duration of antibiotic therapy: usually 4–6 weeks 
Image-guided percutaneous drainage (PCD)
- Under image guidance, drain the abscess and place the drainage catheter in the abscess cavity to allow further drainage.
- CT-guided drainage is preferred. 
- Ultrasound-guided drainage may be attempted if experienced radiologists are available. 
- Multiloculated abscesses
- Catheters repeatedly blocked by thick pus
- Abscess inaccessible to percutaneous drainage
- Septic shock or sepsis not responding to antibiotic therapy
- Primary source of infection requires surgical management 
- Procedure: open surgery or laparoscopic drainage
Send abscess fluid for culture and tailor antibiotic therapy to the sensitivity reports.
Acute management checklist
- Identify and treat sepsis, if present.
- Obtain blood cultures and, if possible, abscess fluid cultures before administering antibiotics.
- Administer empiric antibiotic therapy for psoas abscess. 
- Consult surgery and, if available, interventional radiology for abscess drainage.
- Provide supportive care (e.g., analgesia, antiemetics, IV fluids)
- Consider ICU admission in patients with sepsis.