Description

• The iliopsoas compartment is an anatomic space composed of the psoas major, psoas minor, and iliacus muscles, which mediate hip flexion and are innervated by L2–4.
• Commonly referred to as iliopsoas, this 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, and inserts on the lesser trochanter of the femur.
• Abscesses of the iliopsoas compartment are considered primary (hematogenous/lymphatic spread, idiopathic) or secondary (extension of nearby infection).
• Synonym(s): Iliopsoas abscess; Ilio-psoas abscess

Epidemiology

• In developed countries, >75% of psoas abscesses are secondary, usually originating from bone or inflammatory bowel disease (1,2,3).
• Primary psoas abscess is most common in developing and tropical countries.
• Mycobacterium tuberculosis (TB) with secondary vertebral osteomyelitis is a common cause of psoas abscess in countries where TB is prevalent, with about 5% of patients with vertebral TB developing psoas abscess.
• About 83% of primary psoas abscesses occur in patients <30 years old.
• In neonates and pediatric patients, psoas abscess is more likely to be primary, predominantly due to Staphylococcus aureus.
• Older patients are more predisposed to secondary psoas abscess due to increased incidence of age-related diseases, such as diverticulitis.
• About 57% of psoas abscesses occur on the right side, 40% on the left side, and 3% bilaterally.
• A male preponderance for psoas abscess has been reported.

• 3.9 cases per year before 1985 to about 12 cases per year in the 1990s
• 0.5 cases per 10,000 hospital admissions 1993–2004 to 6.5 cases per 10,000 hospital admissions 2005–2007 (urban, tertiary care center) (4)
• Rising incidence from increased prevalence of risk factors (immunosuppressant therapy, diabetes, etc.)
• Increased detection from improvement in quality and interpretation of cross-sectional imaging

Risk Factors

• Primary psoas abscess:
  • IV drug use
  • Diabetes
  • HIV/AIDS
  • Active malignancy
  • Other immunodeficiency states
• Other factors include trauma to the lumbar spinous processes incurring psoas muscle damage, and hematoma formation, as in the setting of hematologic disorders.
• Secondary psoas abscess: Skeletal infections (vertebral and pelvic osteomyelitis, spondylodiscitis), gastrointestinal infections (Crohn disease, ulcerative colitis, appendicitis, diverticulitis), genitourinary and gynecologic infections, and infections of aortic aneurysms (2,4):
  • Complications of hip arthroplasty, spinal surgery, aortic surgery, and kidney transplantation (2)
  • Paraspinal abscess, anastomotic leak, and hematoma set the stage for postoperative infection and abscess formation.
  • Cases have been reported of psoas abscess following acupuncture, secondary to spinal infection or bowel perforation.

Pathophysiology

• Primary psoas abscess results from hematogenous or lymphatic spread of infection from a distant source:
  • Primary abscesses are often monomicrobial, with 80% of infections due to S. aureus bacteremia or sepsis.
  • The rich blood supply of the iliopsoas muscles are thought to predispose these muscles to infection by hematogenous spread.
  • M. tuberculosis and S. aureus are the most common organisms causing iliopsoas abscess in HIV/AIDS patients, with primary abscesses more common than secondary.
  • Pseudomonas aeruginosa, Haemophilus aphrophilus, and Proteus mirabilis infections have also been reported.
• Secondary psoas abscess results from contiguous spread of infection from nearby structures of the musculoskeletal system, GI tract, genitourinary tract, and vasculature:
  • Secondary psoas abscesses are often polymicrobial, most commonly involving S. aureus from skeletal origins and Escherichia coli and other enteric organisms, such as Bacteroides spp. and Enterococcus faecalis, from GI and urinary sources (1).
• In recent years, MRSA has become the predominant pathogen in psoas abscesses with definitive microbiologic diagnosis, accounting for up to 25% of cases (4).
• The majority of psoas abscesses are unilateral and single, although multiple abscesses may be found in up to 25% of cases (2).

Commonly Associated Conditions

• Primary psoas abscess: IV drug use, diabetes, HIV/AIDS and other immunocompromised states, renal failure
• Secondary psoas abscess: Common: Inflammatory bowel disease, appendicitis, diverticulitis, and osteomyelitis. Infrequently, secondary psoas abscess has been associated with septic arthritis, colorectal carcinoma, trauma, endocarditis, hepatocellular carcinoma, pancreatitis, and Henoch-Schönlein purpura.

History

• Reported symptoms are often nonspecific indicators of generalized infection:
  • Fever, flank or abdominal pain, with or without radiation to the anterior hip and thigh
• Additional symptoms:
  • Limp, nausea, anorexia, malaise, and weight loss
• The classic symptom triad described by Myntner in 1881 of fever, abdominal or flank pain, and limp is present in <8% of cases.
• Most patients will present with pain, whereas persistent low-grade fever is found in 25–75% of cases (1,2).

Physical Exam

• Maneuvers that stretch or contract the inflamed psoas compartment result in considerable pain on the affected side.
• Pain with extension and internal rotation of the hip (the “psoas sign”) is the most common physical exam finding:
  • A positive psoas sign may be elicited by the examiner by placing a hand proximal to the patient’s knee on the affected side and asking the patient to raise the ipsilateral thigh against resistance.
  • A 2nd psoas test is performed by asking the patient to lie on the unaffected side and hyperextend the contralateral hip, eliciting pain with stretching of affected psoas muscle.
• These maneuvers may also be positive in the setting of other conditions causing iliopsoas inflammation, including retrocecal appendicitis. Sensitivity and specificity of the psoas sign have been reported at 16% and 95%, respectively (5).
• Rectal exam may aid in differentiating psoas abscess from retrocecal appendicitis, with pain on palpation of the retrovesical pouch more consistent with appendicitis.
• Whereas patients with psoas abscess may find relief from pain with full hip flexion, patients with hip pathology experience persistent or heightened pain.
• Distal extension of a psoas abscess may yield a painful or painless mass palpable below the inguinal ligament.
• ~50% of patients report abdominal tenderness, but guarding and rebound are uncommon.
• Most common findings in neonates include leg or groin swelling, limitation of leg motion, and pain, with fever occurring in a minority of cases.

Diagnostic Tests and Interpretation

• WBC >10,000/mL
• Anemia (hemoglobin <11 g/L)
• Thrombocytosis is less common.
• Elevations in BUN, sedimentation rate, and C-reactive protein (CRP) have been reported (2). CRP level has been shown to correspond with the severity of infection.

• CT is the gold standard imaging modality:
  • Sensitivity rate between 88–100%
  • Typically reveals a focal hypodense lesion consistent with an abscess
  • May demonstrate:
    • Enlargement of the iliopsoas muscle
    • Gas or air fluid levels within the muscle and fat stranding
  • Contrast CT may show rim enhancement of the abscess wall.
• Although not integral to the diagnosis of psoas abscess, MRI and technetium 99m scintigraphy may elucidate local infectious sources, such as vertebral osteomyelitis.
• Plain abdominal radiographs occasionally reveal the outline of an inflammatory mass.
• Chest plain films may identify scant pleural effusions or raised hemidiaphragm.
• US may disclose evidence of an inflammatory mass and is often diagnostic in cases of pediatric psoas abscess.
• US should be used as the initial imaging modality in neonates, followed by CT or MRI to further classify the abscess (6).
• An IV pyelogram may show deviation of the kidney and ureter, and barium studies may reveal bowel loop displacement and associated GI disease.

• Gram stain and culture of blood and aspirated abscess fluid confirm the diagnosis and guide antimicrobial treatment.
• AFB stain and mycobacterial culture if TB infection suspected

Differential Diagnosis

Retrocecal appendicitis, diverticulitis, bacterial infection or avascular necrosis of the hip, renal colic and pyelonephritis, arthritis, S1 disc herniation, inflammatory bowel disease, epidural abscess, vertebral osteomyelitis, endometriosis, and pelvic inflammatory disease; hematoma, tumor, and noninfectious inflammation of the iliopsoas compartment should also be considered.

Medication (Drugs)

• Broad-spectrum empiric antibiotics targeting staphylococcal (primary and secondary due to skeletal infections) and enteric organisms (secondary to intra-abdominal infection) are indicated in most cases.
• IV monotherapy with Unasyn, Zosyn, or a carbapenem is effective as initial treatment.
• Dual therapy with a 3rd-generation cephalosporin, such as ceftriaxone with metronidazole, is also adequate.
• Suspected MRSA infection is best treated with vancomycin, and, alternatively, linezolid or daptomycin.
• Coverage should be determined by culture sensitivity results from aspirated fluid.
• Although most psoas abscesses require drainage, abscesses ≤3 cm have been successfully managed by antibiotics alone.

Follow-Up Recommendations

Duration of antibiotic treatment should be tailored to the patient and may be continued for 2–6 weeks.

Patient Monitoring
Follow-up imaging may be warranted to verify adequate resolution of the fluid collection.

Diet

Patients should be kept NPO for percutaneous and surgical drainage procedures.

Prognosis

• A high index of suspicion for abscess in the iliopsoas compartment is crucial, as early intervention is associated with favorable outcomes.
• Delays in diagnosis can lead to serious complications, including septic shock, reported in up to 20% of cases.
• Mortality is reported at 2.4–19% for treated patients with primary and secondary abscesses, respectively. Untreated cases face a near 100% mortality rate (2).
• Mortality is significantly increased with age >65, bacteremia, and growth of E. coli in cultures (1).

Complications

• Risk factors for unfavorable outcomes include delays in treatment, bacteremia, and inadequate treatment (percutaneous drainage or antibiotics only) in patients with advanced age. About 40% of patients require >1 drainage procedure for full recovery. Incomplete drainage or suboptimal antibiotic treatment may lead to relapses of psoas abscess up to a year after initial presentation in 15–36% of cases.
• Most cases of relapse occur within 6 months of follow-up, but may occur after >1 year (1).

ICD-9

567.31 Psoas muscle abscess

ICD-10

• K68.12 Psoas muscle abscess

SNOMED

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