Psoas muscle abscess and sacroiliac joint osteomyelitis

Case contributed by Dr Rajalakshmi Ramesh


Six-week history of progressively worsening, non-radiating, lumbar back pain, on a background history of concomitant intravenous drug use. On admission, normal WCC and elevated CRP. Diffuse bilateral lower limb weakness on clinical examination.

Patient Data

Age: 45 years
Gender: Male

A large loculated fluid collection lies within the distal right psoas muscle, and measures 4.2 x 7.7 x 11.5 cm in dimension. The proximal aspect of this fluid collection lies at the L5 vertebral level, and its distal most aspect lies approximately at the level of the superior pubic rami and is above the psoas' distal insertion. The affected portion of the right psoas muscle is expanded. Erosion of the right sacroiliac joint consistent with a septic arthritis. Subtle fat stranding is seen around the posterior aspect of the cecum, which lies directly anterior to the fluid collection. Large right distal psoas fluid collection is consistent with a right psoas abscess secondary to an infected right SI joint.

Large, complex cystic mass within the right psoas and iliacus muscles measuring 7.1 by 7.4 cm. The right sacro-iliac joint is abnormal, with subtle cortical erosion and fluid within the joint and marrow edema evident in the adjacent sacrum and iliac bones. The marrow signal is of diffusely low to intermediate signal in both T1 and T2 weighted images. These findings are highly concerning for an infective sacroiliitis and right psoas abscess. The differential is psoas hematoma or less likely tumor. Diffuse low to intermediate signal marrow may represent red marrow hyperplasia or diffuse infiltration. There is also a focal central-right paracentral disc protrusion at L5-S1, contacting the right S1 nerve root as it buds off the thecal sac.

The patient underwent a CT-guided percutaneous drainage of the psoas collection, with insertion of pigtail catheter. 

Using sterile technique, a 10 French Navare catheter was inserted into the right psoas collection using a Seldinger technique, without immediate complication. A sample has been sent for a microbiological culture.

Cultures of the aspirated psoas collection grew methicillin-resistant Staphylococcus aureus. Blood cultures and transesophageal echocardiogram were negative for culture growth and valvular pathology respectively. The patient was treated with a four week course of intravenous following by an eight week course of flucloxacillan. Repeat scanning upon completion of this antibiotic course demonstrated complete resolution of the right psoas abscess. 

High precontrast T1 and T2 marrow signal on the sacral and iliac sides of the right SI joint not consistent with acute bone marrow edema and probably representing the sequelae of previous infection. Low signal sclerosis and possible erosion of the (partially imaged) right SI joint. No evidence on this study that the complex right psoas and iliac is muscle collection has recurred.

Minor L4/L5 disc bulge with minor bilateral lateral recess stenosis without nerve root impingement. L5/S1 disc space height loss and desiccation. Circumferential L5/S1 disc bulge. The focal central-right paracentral disc protrusion at L5/S1 identified on the previous MRI is significantly reduced in size. It contacts but does not impinge the descending S1 nerve roots bilaterally.

Case Discussion

This case illustrates the occurrence of psoas muscle abscess that was managed with percutaneous drainage and antibiotic therapy.  



Case courtesy of Associated Professor Pramit Phal

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Case information

rID: 34252
Published: 12th Feb 2015
Last edited: 2nd Apr 2020
Inclusion in quiz mode: Included