Retroperitoneal abscess is an infected fluid collection developing in the retroperitoneal space. Most common etiologies include bowel perforation, appendicitis, diverticulitis, pancreatitis, renal infection, vertebral osteomyelitis, and postoperative complications. It frequently involves the psoas muscle due to the direct anatomical connections of the psoas fascia with adjacent retroperitoneal organs. Rim-enhancing collection with low internal density and gas presence on CT are key diagnostic clues. The gas component is a product of anaerobic bacterial metabolism and represents the most specific finding of an infected collection. Urgent percutaneous or surgical drainage is required; without treatment, sepsis, fistula formation, or multiorgan failure may develop. Mortality rate is approximately 5-10% with appropriate treatment but approaches near 100% in untreated cases.
Age Range
20-70
Peak Age
45
Gender
Equal
Prevalence
Uncommon
Retroperitoneal abscess forms from bacterial infection spreading to the retroperitoneal space. The psoas muscle is the most common site because the psoas fascia has direct anatomical connections with adjacent retroperitoneal organs (vertebra, kidney, bowel), allowing free dissemination of infection along these fascial planes. Infection typically reaches the psoas via two pathways: hematogenous spread (especially in tuberculous abscess) or direct extension from adjacent organ infection (diverticulitis, Crohn disease, vertebral osteomyelitis). Inflammatory granulation tissue and neovascularization develop in the abscess wall; these newly formed, highly permeable vessels accumulate contrast agent and create the characteristic rim enhancement on CT. Purulent material in the abscess cavity contains degraded neutrophils, fibrin, bacterial debris, and proteolytic enzymes; this composition shows higher density than simple low-protein fluid (10-30 HU) but lower than solid tissue. Gas formation results from anaerobic bacterial metabolism (especially Bacteroides and Clostridium species), producing carbon dioxide, hydrogen, and hydrogen sulfide gases that appear as very low density (-1000 HU) on CT. If untreated, the inflammatory process progresses with invasion of surrounding tissues, formation of fistula tracts, erosion of the vena cava or aorta, and hematogenous dissemination with distant septic emboli.
Gas bubbles or air-fluid level within a rim-enhancing collection is pathognomonic for retroperitoneal abscess. Rim enhancement reflects granulation tissue neovascularization while gas reflects anaerobic bacterial metabolism, and the combination of these two findings is not seen in this configuration in any other retroperitoneal pathology.
On contrast-enhanced CT, the abscess wall shows intense rim enhancement reflecting the rich vascular structure of granulation tissue. Internal portion shows low density (10-30 HU) representing purulent material. Gas presence creates very low density foci or air-fluid levels within the collection and is the most reliable finding of anaerobic infection. Commonly seen in the psoas muscle with asymmetric size increase compared to the contralateral side. Surrounding fat infiltration (dirty fat) is typical and indicates spread of the inflammatory process into retroperitoneal fat planes. Abscess wall thickness is variable, with thick irregular wall suggesting chronic abscess.
Report Sentence
A rim-enhancing, low-density collection is seen in the retroperitoneal space/psoas muscle with gas components and surrounding inflammatory fat infiltration; findings are consistent with abscess.
On DWI, the abscess cavity shows marked diffusion restriction — high DWI signal, low ADC. This finding is very valuable for differentiating abscess from necrotic tumor and simple fluid, possessing the highest sensitivity and specificity among MRI findings for abscess diagnosis. High viscosity and cellular debris in purulent material restrict diffusion. ADC values are typically in the 0.4-0.8 x 10⁻³ mm²/s range, which is significantly lower than necrotic tumor (typically 1.0-1.5 x 10⁻³ mm²/s) and simple fluid (3.0+ x 10⁻³ mm²/s).
Report Sentence
The collection shows marked diffusion restriction on DWI (ADC 0.4-0.8 x 10⁻³ mm²/s) consistent with purulent content and abscess diagnosis.
On T2, the abscess cavity shows high signal (fluid-like) but may be slightly lower than pure fluid due to protein and cellular debris in purulent material. The abscess wall may show low-to-intermediate T2 signal reflecting the collagen content of fibrous granulation tissue. Surrounding tissue edema appears as high T2 signal indicating the extent of inflammatory spread. Psoas muscle enlargement with increased signal is seen; the normal homogeneous low-to-intermediate T2 signal of psoas muscle is disrupted with high T2 signal developing due to intramuscular edema.
Report Sentence
A high signal collection on T2 is seen with extensive surrounding tissue edema and increased psoas muscle signal; consistent with inflammatory collection.
On non-contrast CT, asymmetric enlargement and heterogeneous density of the psoas muscle is seen. Low-density areas (purulent fluid, 10-30 HU) and gas bubbles (air density, -1000 HU) within the muscle may be visible. Ipsilateral ureter or kidney displacement may accompany, indicating mass effect of the collection. The normal symmetric, smooth contoured, and homogeneous density appearance of the psoas muscle is lost. The abscess may extend caudally along the psoas muscle to the inguinal region because the psoas fascia continues to the iliac fossa and inguinal ligament.
Report Sentence
Asymmetric psoas muscle enlargement with heterogeneous density and intramuscular gas foci is seen; psoas abscess should be considered with extension toward the inguinal region.
On US, a heterogeneous, irregularly marginated collection with increased echogenicity is seen. Internal echoes (debris, septations, fibrin particles) favor abscess and differentiate from simple fluid. Gas creates hyperechoic artifact with possible posterior acoustic shadowing or reverberation artifact. Percutaneous drainage can be performed under US guidance, which is the preferred treatment modality especially for superficial retroperitoneal abscesses. On Doppler US, increased vascularity (inflammatory hyperemia) around the abscess may be observed.
Report Sentence
An irregularly marginated collection with increased echogenicity, internal echoes, debris and gas artifacts is seen in the retroperitoneal area on US; inflammatory hyperemia is present in surrounding tissues.
On contrast-enhanced MRI, the abscess wall shows distinct rim enhancement with variable wall thickness (2-10 mm). In acute abscess, the wall is thin and smooth, while in chronic abscess it may be thick and irregular. Internal cavity does not enhance. Increased enhancement in surrounding tissues indicates parenchymal extension of the inflammatory process. Gadolinium accumulation on T1 post-contrast sequence highlights the permeable neovascularization of the abscess wall.
Report Sentence
Distinct rim enhancement of the collection wall is seen on contrast-enhanced MRI with non-enhancing internal cavity; consistent with abscess.
On contrast-enhanced CT arterial and portal venous phases, adjacent organ pathology indicating abscess etiology is investigated. Vertebral body destruction and disc space loss suggest osteomyelitis/spondylodiscitis. Bowel wall thickening, pericolic/periileal inflammatory changes indicate diverticulitis or Crohn disease. Renal cortical abscess or pyelonephritis findings suggest renal-origin abscess. Periappendiceal inflamed fat and appendiceal wall thickening indicate ruptured appendicitis.
Report Sentence
Destructive vertebral changes/bowel wall thickening adjacent to the abscess are noted and should be evaluated as potential etiologic source.
Criteria
Infected collection localized to or predominantly involving the psoas muscle
Distinct Features
May be associated with vertebral osteomyelitis or Crohn disease; abscess may extend beyond the muscle to iliac fossa and inguinal region as the psoas fascia continues to these areas; calcified wall and psoas calcification may be seen in tuberculous abscess
Criteria
Infected collection localized to the perirenal space originating from renal infection
Distinct Features
Develops as a complication of pyelonephritis from renal cortical abscess rupture; may remain encapsulated within Gerota fascia or extend to pararenal space; accompanying renal cortical scar or obstructive uropathy provides etiologic clue
Criteria
Infected collection developing after surgery from anastomotic leak or contamination
Distinct Features
Near surgical site, usually developing 5-10 days postoperatively; may contain gas but distinction from normal postoperative gas is important (postoperative gas usually resorbs within 7 days); if localized near anastomotic line, leak should be considered
Criteria
Chronic abscess from Mycobacterium tuberculosis, usually associated with vertebral osteomyelitis
Distinct Features
Known as cold abscess with low inflammatory response; calcified wall, caseous necrosis, extension along psoas; vertebral destruction and disc space loss (Pott disease) accompanies; endemic area or immunosuppression history
Distinguishing Feature
Hemorrhage shows high density (50-80 HU) on non-contrast CT without rim enhancement because granulation tissue has not developed around it; no gas component. Abscess shows low density (10-30 HU) with rim enhancement and gas. Clinically, hemorrhage has sudden onset while abscess has insidious course.
Distinguishing Feature
Lymphoma appears as a solid homogeneous mass without cavitation, rim enhancement, or gas; shows low-to-moderate homogeneous enhancement. Abscess is a cavitary collection characterized by rim enhancement + gas + clinical infection signs. On DWI, lymphoma shows very low ADC (0.4-0.6) but is differentiated from abscess by absence of rim enhancement.
Distinguishing Feature
Müllerian cyst is a thin-walled, uniform low-density benign cystic lesion without enhancement; wall thickness is 1-2 mm with smooth uniform borders. Abscess is characterized by thick irregular wall enhancement, gas, and surrounding inflammatory changes (dirty fat). Clinically, Müllerian cyst is asymptomatic while abscess presents with fever and pain.
Distinguishing Feature
Tailgut cyst is a presacral multiloculated cystic lesion; DWI negative, thin septations and stained glass pattern; no solid enhancing component. Abscess is differentiated by DWI positivity (marked diffusion restriction), rim enhancement, and clinical infection findings.
Urgency
emergentManagement
interventionalBiopsy
Not NeededFollow-up
specialist-referralRetroperitoneal abscess requires emergency intervention and untreated mortality can approach 100%. Percutaneous drainage (CT or US guided) is the preferred primary treatment with success rates of 80-90% in unilocular abscesses. Multiple drainage catheters or surgical drainage may be needed for multiloculated, septated, or organized abscesses. Broad-spectrum antibiotics (gram-negative + anaerobic coverage) are initiated simultaneously with drainage and narrowed based on culture results. Underlying etiology must be investigated: bowel pathology (diverticulitis, Crohn disease, appendicitis perforation), vertebral infection (osteomyelitis, spondylodiscitis, tuberculosis), renal infection (obstructive pyelonephritis, renal cortical abscess), or postoperative complication (anastomotic leak). Treatment efficacy is monitored with serial CT; decrease in abscess size, gas resolution, and reduction of surrounding inflammatory changes indicate healing.
Retroperitoneal abscess requires urgent treatment. Percutaneous drainage (CT or US-guided) is the primary treatment. Broad-spectrum antibiotic therapy is initiated. Underlying cause (spinal infection, Crohn, diverticulitis) should be investigated. Tuberculous abscess should be considered especially in endemic regions.