Psoas abscess

A psoas abscess (more accurately iliopsoas as the iliacus muscle is frequently involved) is a collection of pus in the iliopsoas compartment.

Infection is normally unilateral and usually arises by hematogenous spread or by contiguous spread from local infection:

• primary abscess arising from hematogenous spread is commonly caused by occult S. aureus bacteraemia and usually is associated with intravenous drug use or immunocompromise
• other pathogens, including S. pneumoniae, Streptococcus milleri, group A streptococci, E. coli, Pseudomonas, Haemophilus, Proteus and Pasteurella have been reported.

Historically, secondary psoas abscess was most often due to TB with local spread from adjacent involved vertebral bodies or, less frequently, after blood-borne infection:

• in recent decades, with the decline in tuberculosis in the developed world, there has been a shift to psoas abscess arising from genitourinary or gastrointestinal infections especially in immunocompromised patients
• any age group may be affected, and there is no gender bias
• is a rare condition with vague clinical presentation. The classical triad of fever, limp, and back pain is present in <30% of patients.

Clinical features

• action of the psoas muscle is to flex and internally rotate the hip owing to its tendinous insertion into the lesser trochanter and so patients with irritation of the psoas muscle by an abscess may hold the hip in this fixed position
  • may also elicit a positive aggravating ‘psoas stretch test
• with a neglected psoas abscess pus may track along the psoas tendon to appear as a tender swelling below the inguinal ligament. Patients are usually apyrexial

Diagnosis:

• a high index of clinical suspicion is essential for the diagnosis of IPA
• investigations which can aid in diagnosis are imaging techniques while direct aspiration and microbiological cultures are very specific
  • other investigations such as erythrocyte sedimentation rate and leucocyte count are nonspecific although anaemia, leucocytosis and an elevated ESR are common findings
  • blood cultures may be positive, and cultures of abscess fluid should be performed
  • imaging is the gold standard for detection of psoas infection. CT and MRI scans are the preferred modalities and clearly demonstrate the abscess
  • ultrasonography is less effective

Differential diagnosis:

• the condition may be confused with a femoral hernia or enlarged inguinal lymph nodes

Treatment involves drainage and intravenous antibiotics. Adjacent or concomitant infection elsewhere in the body should be identified and treated appropriately with debridement, resection, or antibiotics.

References

1. Kradin R. Psoas Abscess. Diagnostic Pathology of Infectious Disease, Second Edition, 2018.
2. Chern C et al. Psoas abscess: making an early diagnosis in the ED. Am J Emerg Med. 1997 Jan;15(1):83-8
3. Taiwo B. Psoas abscess: a primer for the internist. South Med J. 2001 Jan;94(1):2-5.
4. Hsieh MS et al. Features and treatment modality of iliopsoas abscess and its outcome: A 6-year hospital-based study. BMC Infect Dis. 2013;13:578.