Description

• The iliopsoas compartment is a retroperitoneal anatomic space composed of the psoas major, psoas minor, and iliacus muscles, which mediate hip flexion and are innervated by L2–L4.
• The muscle group originates from the lateral borders of the 12th thoracic to 5th lumbar vertebrae, passes posterior to the inguinal ligament and anterior to the hip joint, inserting on the lesser trochanter.
• Abscesses are primary (hematogenous or lymphatic spread) or secondary (extension of nearby infection).
• Synonym(s): iliopsoas abscess

Epidemiology

Incidence

• Historically quite rare
• Incidence increased with HIV, IV drug use, and immunosuppressant therapy (1).
• Improved imaging techniques have led to more frequent diagnosis (1).

Prevalence

• Primary abscesses account for ~ 1/3 of cases worldwide; most are in developing countries (1,2).
• Primary abscesses more common in infants and are associated with Staphylococcus aureus (1,2)
• Secondary psoas abscesses are more common (particularly in developed countries) (1,2).
• Secondary abscesses in the elderly commonly associated with age-related diseases (e.g., diverticulitis, urinary tract infections)

Etiology and Pathophysiology

• Primary psoas abscess results from hematogenous or lymphatic spread of infection from a distant source.
  • Iliopsoas has a rich blood supply, predisposing to primary infection.
  • Primary abscesses are often monomicrobial, with S. aureus as the predominant organism (1,3).
  • Mycobacterium tuberculosis and S. aureus are the most common causative organisms in HIV patients (2).
• Secondary psoas abscess results from contiguous spread from nearby musculoskeletal, gastrointestinal (GI), genitourinary (GU), and vascular structures.
  • S. aureus most common from a skeletal source (1,2,3)
  • Escherichia coli most common from GI and GU source (1,2)
  • Other causes are Bacteroides sp., Enterococcus faecalis, Streptococcus viridans from GI source, and Klebsiella pneumoniae from GI and GU sources (2).
  • Secondary psoas abscesses from GI and GU sources may be polymicrobial (1,2).
  • M. tuberculosis can cause secondary psoas abscess from vertebral, GI, and GU spread.
• Methicillin-resistant S. aureus (MRSA) accounted for up to 25% of cases in one study (3).
• Most psoas abscesses are unilateral and solitary (1).

Risk Factors

• Primary psoas abscess:
  • IV drug use
  • Diabetes
  • HIV/AIDS
  • Active malignancy
  • Renal failure
  • Prior psoas muscle trauma (1,2,3)
• Secondary psoas abscess:
  • Skeletal infections
  • Intra-abdominal infection
  • GU infections
  • Recent surgery (1,2,3)

History

• Symptoms often nonspecific
• The most common symptoms are fever, flank pain, abdominal pain, and limp; >50% present with pain and 25–75% low-grade fever (1,2).
• Pain may radiate to anterior hip and thigh.
• Guarding, rebound, or other signs of peritoneal inflammation are less common (1,2,3).
• Other symptoms may include nausea, anorexia, malaise, weight loss, and groin swelling.

Physical Exam

• Maneuvers manipulating the inflamed psoas compartment result in pain on the affected side.
• Pain with extension and internal rotation of the hip (psoas sign) is a common exam finding (2,3).
  • Place a hand proximal to the knee on the affected side and have patient lift thigh against resistance.
  • Alternative: Lay on the unaffected side and hyperextend the contralateral hip to stretch the psoas muscle—positive test if painful.
• Psoas maneuvers may be positive in other conditions causing iliopsoas inflammation (e.g., retrocecal appendicitis).
• Patients often hold the hip flexed to relieve pain.
• Extension of a psoas abscess below the inguinal ligament may result in a palpable mass.
• Common findings in neonates include leg or groin swelling, limitation of leg motion, pain, and fever.

Differential Diagnosis

Retrocecal appendicitis, diverticulitis, septic arthritis, avascular necrosis of the hip, renal colic, pyelonephritis, arthritis, lumbar disc herniation, inflammatory bowel disease, epidural abscess, vertebral osteomyelitis, endometriosis, pelvic inflammatory disease, hematoma, tumor, and inflammation of the iliopsoas compartment

Diagnostic Tests & Interpretation

Initial Tests (lab, imaging)

• Labs: WBC (>10,000/mL), anemia (hemoglobin <11 g/L), elevated ESR and CRP, and may have elevated liver enzymes and creatinine kinase (1)
• Contrast enhanced CT is preferred imaging modality (4)[C].
  • Sensitivity of 88–100%
  • Typically reveals a focal, hypodense lesion
  • May also demonstrate enlargement of the iliopsoas muscle; air-fluid levels within the muscle; fat stranding and/or rim enhancement
  • CT helps guide PCD.
• MRI similar sensitivity to contrast CT but takes longer and cannot guide PCD (4)
• Plain abdominal radiographs may outline an inflammatory mass.
• Chest radiograph may identify small pleural effusions or raised hemidiaphragm.
• Ultrasound (US) may be used for initial imaging for suspected pediatric or neonatal psoas abscess (4).
• US and MRI preferred for pregnant women (4)[C]

Diagnostic Procedures/Other

• Culture of abscess fluid confirms diagnosis and guides antimicrobial treatment.
• AFB stain and mycobacterial culture if TB suspected

Medication

• Culture results should guide antibiotic coverage (drug resistance is becoming more common).
• Broad-spectrum empiric antibiotics to target Staphylococcus (due to skeletal infections) and enteric organisms (due to intra-abdominal infection)
• IV monotherapy with ticarcillin clavulanic acid 3.1 g IV q6h, piperacillin-tazobactam 3.375 g IV q6h, or meropenem 1 g q8h are effective as initial treatment (6)[C].
• Dual therapy with a 3rd-generation cephalosporin, such as ceftriaxone 1 g IV q12–24h with metronidazole 500 mg IV q8h, is also adequate.
• Treat MRSA with vancomycin 15 mg/kg IV q8–12h. Daptomycin 6 mg/kg IV QD or linezolid 600 mg IV/PO twice daily are alternatives (6)[C].
• Although most psoas abscesses require drainage, some (≤3 cm) can be managed by antibiotics alone.
• After surgical drainage, treat TB per current guidelines.

Follow-up Recommendations

Most patients require 3 to 6 weeks of antibiotic treatment. TB requires 9 to 12 months of treatment.

Patient Monitoring
Follow-up imaging to verify resolution if necessary

Diet

NPO for percutaneous and surgical drainage procedures

Prognosis

• A high index of suspicion is important for early intervention and improved outcomes.
• Delays in diagnosis contribute to complications, including septic shock and death.
• Mortality rate in treated adults is ~5% (1,5). Untreated cases have ~100% mortality.
• Mortality increases with advanced age, multiple comorbidities, delayed diagnosis, bacteremia/sepsis, and underlying secondary causes (1,2,5).
• Children have a favorable prognosis with prompt diagnosis and treatment.

Complications

• Inadequate treatment associated with unfavorable outcomes. About 40% of patients require >1 drainage procedure for full recovery (5).
• Incomplete drainage or suboptimal antibiotic treatment may lead to relapses up to a year after initial presentation in 16% of cases (1).
• Spinal cord compression has been reported via direct extension of psoas abscess.
• Most relapses occur within 6 months of treatment; can occur over a year after treatment

ICD-10

• K68.12 Psoas muscle abscess

ICD-9

• 567.31 Psoas muscle abscess

SNOMED

• 235997006 Abscess iliopsoas non-tuberculous (disorder)
• 266463007 Iliopsoas abscess (disorder)