• Clinical science
  • Clinician

Psoas abscess

Summary

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.

Etiology

  • Primary psoas abscess [1]
    • 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. [1]
    • More common in Asia and Africa [2]
  • Secondary psoas abscess
    • Caused by contiguous spread of a pathogen from an adjacent infectious focus
    • Primary focus may be: [1][3][4]
      • Gastrointestinal
      • Musculoskeletal
      • Genitourinary
      • Cardiovascular
      • Postprocedural
    • Different causative pathogens are associated with different primary infectious foci.
  • Risk factors [1]
  • Causative pathogens [3]
Causative pathogens of psoas abscess
Pathogen Properties
S. aureus
  • Most common causative pathogen [1]
  • Typically associated with a skeletal primary focus
  • Incidence of MRSA is not well established.
E. coli
  • Second most common causative pathogen [1]
  • Typically associated with gastrointestinal or urinary tract primary focus

Bacteroides spp., S. viridans

  • Frequently isolated, especially from a gastrointestinal focus [1][3]
M. tuberculosis

Clinical features

  • Classic triad: present in ∼ 30% of cases [5]
  • Nonspecific symptoms: lower abdominal pain, malaise, weight loss
  • Examination findings
    • 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). [1]

Diagnostics

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. [6]

Laboratory studies [1][5][7]

Obtain blood cultures (and pus/aspirate cultures, if possible) before initiating empiric antibiotic therapy.

Imaging

CT abdomen and pelvis with IV contrast (axial section) [1][5][8]

  • Indication: imaging modality of choice [1][5][8]
  • Characteristic findings
    • 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

MRI of the pelvis and abdomen with IV contrast [8]

  • Indications
    • Suspected psoas abscess in a patient with contraindications for CT scan
    • Suspected spinal pathology and evaluation of back pain [5]
    • Consider MR enterography if Crohn disease is suspected to be the underlying etiology. [1]
    • Pregnant women: MRI without contrast is preferred
  • Characteristic findings

Abdominal ultrasound [9]

  • Indications
    • 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
  • Supportive findings
    • 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.

If no primary focus of infection can be identified, primary psoas abscess is most likely. [6]

Differential diagnoses

See also differential diagnosis of low back pain.

The differential diagnoses listed here are not exhaustive.

Treatment

General principles [1][5][12]

Empiric antibiotic therapy for psoas abscess [1][5][13]

Abscess drainage

Image-guided percutaneous drainage (PCD)

  • Indications
    • Size of the abscess > 3.5 cm [12][11][1]
    • Abscess anatomically amenable to PCD
  • Procedure
    • Under image guidance, drain the abscess and place the drainage catheter in the abscess cavity to allow further drainage.
    • CT-guided drainage is preferred. [11]
    • Ultrasound-guided drainage may be attempted if experienced radiologists are available. [9]

Surgical drainage

  • Indications
  • Procedure: open surgery or laparoscopic drainage

Send abscess fluid for culture and tailor antibiotic therapy to the sensitivity reports.

Acute management checklist

  • 1. Shields D, Robinson P, Crowley TP. Iliopsoas abscess – A review and update on the literature. Int J Surg. 2012; 10(9): pp. 466–469. doi: 10.1016/j.ijsu.2012.08.016.
  • 2. Ricci MA, Rose FB, Meyer KK. Pyogenic psoas abscess: Worldwide variations in etiology. World J Surg. 1986; 10(5): pp. 834–842. doi: 10.1007/bf01655254.
  • 3. López VN, Ramos JM, Meseguer V, et al. Microbiology and Outcome of Iliopsoas Abscess in 124 Patients. Medicine. 2009; 88(2): pp. 120–130. doi: 10.1097/md.0b013e31819d2748.
  • 4. Huang J-J, Ruaan M-K, Lan R-R, Wang M-C. Acute Pyogenic Iliopsoas Abscess in Taiwan: Clinical Features, Diagnosis, Treatments and Outcome. J Infect. 2000; 40(3): pp. 248–255. doi: 10.1053/jinf.2000.0643.
  • 5. Tomich EB, Della-Giustina D. Bilateral psoas abscess in the emergency department. The western journal of emergency medicine. 2009; 10(4): pp. 288–91. pmid: 20046252.
  • 6. Yoo JH, Kim EH, Song HS, Cha JG. A case of primary psoas abscess presenting as buttock abscess. Journal of Orthopaedics and Traumatology. 2009; 10(4): pp. 207–210. doi: 10.1007/s10195-009-0074-2.
  • 7. Nakamura T, Morimoto T, Katsube K, Yamamori Y, Mashino J, Kikuchi K. Clinical characteristics of pyogenic spondylitis and psoas abscess at a tertiary care hospital: a retrospective cohort study. J Orthop Surg. 2018; 13(1). doi: 10.1186/s13018-018-1005-9.
  • 8. Muttarak M, Peh WCG. CT of Unusual Iliopsoas Compartment Lesions. RadioGraphics. 2000; 20(suppl_1): pp. S53–S66. doi: 10.1148/radiographics.20.suppl_1.g00oc07s53.
  • 9. Chern C-H, Hu S-C, Kao W-F, Tsai J, Yen D, Lee C-H. Psoas abscess: Making an early diagnosis in the ED. Am J Emerg Med. 1997; 15(1): pp. 83–88. doi: 10.1016/s0735-6757(97)90057-7.
  • 10. Signore A, Sconfienza LM, Borens O, et al. Consensus document for the diagnosis of prosthetic joint infections: a joint paper by the EANM, EBJIS, and ESR (with ESCMID endorsement). Eur J Nucl Med Mol Imaging. 2019; 46(4): pp. 971–988. doi: 10.1007/s00259-019-4263-9.
  • 11. Tabrizian P. Management and Treatment of Iliopsoas Abscess. Arch Surg. 2009; 144(10): p. 946. doi: 10.1001/archsurg.2009.144.
  • 12. Yacoub WN, Sohn HJ, Chan S, et al. Psoas abscess rarely requires surgical intervention. The American Journal of Surgery. 2008; 196(2): pp. 223–227. doi: 10.1016/j.amjsurg.2007.07.032.
  • 13. Gilbert DN, Chambers HF, Eliopoulos GM, et al. The Sanford Guide to Antimicrobial Therapy 2019. Sperryville, VA: Antimicrobial Therapy, Incorporated, 2019; 2019.
  • 14. Deng Y, Zhang Y, Song L, et al. Primary iliopsoas abscess combined with rapid development of septic shock. Medicine. 2018; 97(51): p. e13628. doi: 10.1097/md.0000000000013628.
  • Torres GM, Cernigliaro JG, Abbitt PL, et al. Iliopsoas compartment: normal anatomy and pathologic processes. RadioGraphics. 1995; 15(6): pp. 1285–1297. doi: 10.1148/radiographics.15.6.8577956.
last updated 06/03/2020
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