Psoas muscle abscess

Case contributed by Dr Amir Mahmud


Sudden-onset right-sided lower abdominal and back pain with associated fever. Recent history of appendectomy.

Patient Data

Age: 10 years
Gender: Female

There is a large 6.8 cm (AP) x 6.4 cm (Trans) x 10.8 cm (CC) irregular, heterogeneous hypodensity centered in the right psoas muscle, which extends inferiorly to involve the ipsilateral iliacus muscle as well. This is representative of a right psoas muscle abscess.

Associated mass effect is also seen, as evidenced by proximal right hydroureter. There is also a 12 mm x 10 mm obstructive staghorn calculus in the inferior major calyx of
the right kidney. No significant hydronephrosis/ peri-renal fat stranding.
The left kidney and ureter are normal in size and appearance.

The appendix is not visualized in this study in keeping with known history of appendectomy; however, the right iliac fossa has secondary signs of inflammation including fat stranding and hyperenhancement, which may represent a peri-appendiceal abscess and possible explanation for the etiology of the psoas muscle abscess.

Case Discussion

A psoas or iliopsoas abscess is an iliopsoas compartment fluid collection which is located within the retrofascial rather than retroperitoneal compartment.

These lesions may be broadly divided into two categories based on etiology and the presence of any underlying disease. primary and secondary:

Primary: (ilio)psoas abscesses usually occur through haematogenous seeding from a source of infection elsewhere in the body. Also, primary psoas abscesses are more commonly seen in patients with chronic immunosuppressive states, HIV, diabetes mellitus as well as in intravenous drug abusers. Commonly implicated organisms include Staphylococcus aureus and mixed gram-negative organisms.

Secondary: abscesses occur more frequently with spread from underlying Crohn disease being the commonest cause. Complications from appendicitis, urinary tract infections, vertebral osteomyelitis, infected abdominal aortic aneurysms, seeding from endocarditis and IUCDs are other documented causes.

CT scans will readily detect a psoas abscess. On CT, it will classically appear as a heterogenous low-attenuation lesion centered within, and causing expansion of, the psoas muscle belly. It will typically display rim enhancement post IV contrast administration. Extension from the psoas muscle into the iliacus muscle is considered a common sequela.

In the above case there was a definite history of recent appendectomy, with no other underlying co-morbidities and with fat stranding and a fluid locule seen in the right iliac fossa, it is most likely that complications due to the appendicitis may be the cause of this psoas abscess which extends to involve the iliacus muscle as well.

Common differentials include chronic or acute hematomas, metastases and lymphoma.

Percutaneous aspiration and drainage under image (CT/ultrasound) guidance may be required to confirm the diagnosis.


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Case information

rID: 75537
Published: 1st Apr 2020
Last edited: 2nd Apr 2020
Inclusion in quiz mode: Included
Institution: Aga Khan University Hospital Nairobi

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