Perforated Sigmoid Colon With a Large Pelvic Abscess: CT Findings, Hinchey Stage II, and Diagnostic Pearls
Executive Answer
Perforated sigmoid colon with a large pelvic abscess is a serious complication of colonic disease, most commonly associated with diverticulitis when severe sigmoid diverticulosis is present. Contrast-enhanced computed tomography (CT) is essential for identifying the perforation, defining the extent of the abscess, evaluating adjacent organs, and assessing for additional signs of peritonitis. In this case, CT demonstrated a large, peripherally enhancing gas-and-fluid collection in the central pelvis, marked distal sigmoid and rectal wall thickening, and a small volume of intraperitoneal free fluid. The absence of upper abdominal free air does not exclude a localized colonic perforation. The combination of a large pelvic abscess and suspected sigmoid perforation is consistent with complicated diverticulitis and a Hinchey stage II pattern.
Why This Case Matters
Colonic perforation can present without the classic imaging pattern of diffuse pneumoperitoneum. When perforation is contained by adjacent tissues and inflammatory adhesions, the resulting infection may remain localized as an abscess rather than producing widespread contamination of the peritoneal cavity.
This distinction is clinically important. A radiologist who focuses exclusively on free intraperitoneal air may overlook a contained perforation, particularly when the dominant abnormality is a large pelvic collection.
This case illustrates three essential principles:
A large gas-and-fluid collection with peripheral enhancement should prompt careful evaluation for an underlying bowel perforation.
Severe sigmoid diverticulosis, adjacent bowel wall thickening, and a pelvic abscess support complicated diverticular disease as the likely cause.
The absence of upper abdominal free air does not rule out perforation, and the absence of generalized peritonitis on CT cannot be assumed from the imaging findings alone.
For radiologists and emergency physicians, recognizing the combination of a pelvic abscess and an abnormal adjacent bowel segment can accelerate appropriate surgical consultation and source-control planning.
Clinical Scenario
A man in his 60s presented with persistent abdominal pain and diarrhea. His C-reactive protein (CRP) level was elevated, indicating an inflammatory response. Abdominal CT was performed to investigate the symptoms and identify the underlying cause.
The available clinical information does not provide the exact CRP value, temperature, leukocyte count, hemodynamic status, duration of symptoms, or previous history of diverticulitis. Treatment details and clinical outcomes were also not reported.
The principal diagnostic challenge was to identify the source of the large pelvic collection and determine whether the findings represented complicated diverticulitis with a contained sigmoid perforation.
CT Imaging Findings
1. Large Gas-and-Fluid Collection in the Central Pelvis
The axial CT images demonstrate a large central pelvic collection containing gas and fluid, with peripheral contrast enhancement. In the clinical context of severe sigmoid diverticulosis and adjacent bowel wall inflammation, this appearance is highly suspicious for a large abscess associated with sigmoid colon perforation.
The combination of fluid, gas, and an enhancing peripheral wall supports the diagnosis of an organized infected collection rather than uncomplicated free fluid.
The collection's relationship to the sigmoid colon is particularly important. Identifying the likely site of bowel disruption helps establish the source of infection and provides information needed for treatment planning.
Clinical significance: A large pelvic abscess indicates complicated intra-abdominal infection and warrants prompt communication with the treating team. Management depends on the patient's physiological condition, the accessibility of the collection, and the feasibility of drainage or surgery.
2. Marked Distal Sigmoid and Rectal Wall Thickening
The axial images show marked wall thickening and edema involving the distal sigmoid colon and rectum against a background of severe sigmoid diverticulosis. No definite colonic stenotic lesion was identified.
These findings support an inflammatory process involving the distal colon. In the presence of a large adjacent pelvic abscess, complicated diverticulitis is the leading explanation.
However, bowel wall thickening is not specific to diverticulitis. The radiologist should evaluate the distribution of diverticula, the degree and symmetry of wall thickening, the surrounding inflammatory changes, and the relationship between the affected bowel segment and the abscess.
The absence of a definite stenotic lesion is useful, but it does not independently exclude an underlying colonic neoplasm.
3. Reactive Thickening of Adjacent Small-Bowel Loops
The coronal CT images demonstrate thickening of pelvic small-bowel loops, interpreted as likely reactive changes related to the adjacent inflammatory process.
Inflammation in the pelvis can extend to neighboring bowel loops, producing secondary wall thickening and edema. These changes may reflect local inflammatory involvement rather than a separate primary small-bowel disorder.
The key interpretive task is to distinguish reactive changes from an independent small-bowel abnormality. The distribution of inflammation and its spatial relationship to the abscess are useful in making this distinction.
4. Sagittal Assessment of the Pelvic Abscess
The sagittal CT images provide an additional perspective on the extent and location of the pelvic collection. Multiplanar assessment helps demonstrate the collection's relationship to the distal bowel and adjacent pelvic structures.
A small volume of intraperitoneal free fluid was present. No free intraperitoneal gas was identified in the upper abdomen. The urinary bladder wall was thickened, but no intravesical gas was present. No lymphadenopathy was identified. The cecum was not visualized in the reported examination findings.
The patient also had a history of posterior lumbar interbody fusion at L4–L5 and L5–S1, an incidental postsurgical finding rather than an established explanation for the pelvic inflammatory process.
Bladder wall thickening may be reactive in the setting of adjacent pelvic inflammation. The absence of intravesical gas means that a gas-containing enterovesical fistula was not demonstrated on this examination. Nevertheless, bladder wall thickening alone is nonspecific and must be interpreted in the clinical context.
Radiologist's Reasoning: From CT Findings to Diagnosis
The diagnosis rests on the integration of several findings rather than any single imaging feature.
First, identify the collection. A large gas-and-fluid collection with peripheral enhancement in the central pelvis is characteristic of an abscess in the appropriate clinical setting.
Second, identify the likely source. Marked distal sigmoid and rectal wall thickening, together with severe sigmoid diverticulosis, supports a colonic inflammatory source. The spatial relationship between the abnormal sigmoid segment and the abscess strengthens the diagnosis of a diverticular complication.
Third, evaluate the extent of perforation. A small volume of free intraperitoneal fluid was present, but upper abdominal free air was absent. This pattern is compatible with a localized or contained perforation. Pneumoperitoneum may be limited, absent on the imaged upper abdominal sections, or confined near the site of perforation.
Fourth, assess adjacent structures. Reactive pelvic small-bowel thickening and bladder wall thickening suggest local inflammatory effects. These findings should not automatically be interpreted as independent bowel or urinary tract disease.
Fifth, evaluate for additional complications. The radiologist should assess for diffuse peritoneal inflammation, distant collections, bowel obstruction, fistula formation, and suspicious focal colonic abnormalities. Not all of these complications were reported in this case, so their presence or absence should not be assumed beyond the documented findings.
Final Imaging Diagnosis
Perforated sigmoid colon with a large pelvic abscess, most likely related to complicated sigmoid diverticulitis.
The pelvic abscess pattern is consistent with Hinchey stage II. This classification describes a pelvic or more distant intra-abdominal abscess associated with diverticulitis; it should not be interpreted as proof of the patient's physiological stability or as a stand-alone treatment recommendation.
Understanding Hinchey Stage II Diverticulitis
The Hinchey classification is widely used to describe the severity of acute diverticulitis complicated by abscess or peritonitis.
In the original classification, stage I refers to a pericolic or mesenteric abscess, stage II to a pelvic, intra-abdominal, or retroperitoneal abscess, stage III to generalized purulent peritonitis, and stage IV to generalized fecal peritonitis.
CT-based modifications provide more detailed radiological staging, and terminology can differ among classification systems. Therefore, the classification system being used should be stated clearly.
In this case, the large pelvic abscess supports a Hinchey stage II pattern. The lack of upper abdominal free air does not downgrade the significance of the abscess or exclude a perforation.
Differential Diagnosis
Although complicated sigmoid diverticulitis is the leading diagnosis, several conditions can produce a pelvic abscess or inflammatory bowel wall thickening.
1. Perforated Colonic Diverticulitis
This is the most likely diagnosis because severe sigmoid diverticulosis, distal colonic wall thickening, and a large adjacent pelvic abscess occur together.
The distribution of diverticula and the relationship between the affected bowel segment and the collection are important discriminators.
2. Perforated Colorectal Carcinoma
Colorectal carcinoma can cause localized perforation, abscess formation, and adjacent inflammatory changes. A discrete irregular mass, asymmetric focal wall thickening, obstruction, or suspicious regional lymph nodes may increase concern for malignancy.
No definite stenotic lesion or lymphadenopathy was reported in this case. These findings are reassuring but do not definitively exclude an underlying neoplasm. Appropriate follow-up colonic evaluation should be considered after the acute inflammatory process has resolved, according to clinical circumstances and current guidance.
3. Other Causes of Pelvic Abscess
A pelvic abscess may arise from other intra-abdominal infections, postoperative complications, or inflammatory bowel disease. The differential depends on surgical history, symptoms, the distribution of inflammation, and the presence of a demonstrable bowel source.
The reported severe sigmoid diverticulosis and associated distal colonic inflammation make a diverticular origin more plausible than an unrelated pelvic infection.
4. Enterovesical Fistula
An enterovesical fistula is an abnormal communication between the bowel and urinary bladder and may complicate diverticular disease. Intravesical gas in a patient without recent urinary instrumentation can raise suspicion.
In this case, the bladder wall was thickened, but no gas was present within the bladder. The reported CT findings therefore do not establish an enterovesical fistula.
Diagnostic Algorithm for Suspected Colonic Perforation
A practical imaging-based approach is:
Clinical suspicion: Persistent abdominal pain, diarrhea, elevated inflammatory markers, or clinical deterioration should prompt assessment for complicated intra-abdominal disease.
Initial imaging: Contrast-enhanced CT of the abdomen and pelvis is generally the preferred examination when complicated diverticulitis or bowel perforation is suspected, unless contraindicated.
Identify the key findings: Evaluate bowel wall thickening, diverticulosis, extraluminal gas, gas-containing collections, peripheral enhancement, and free fluid.
Locate the source: Determine which bowel segment is most closely associated with the abscess and look for direct or indirect evidence of perforation.
Assess disease extent: Examine the entire abdomen and pelvis for additional collections, diffuse peritoneal inflammation, obstruction, fistulas, and other complications.
Communicate urgency: A large abscess associated with suspected bowel perforation warrants prompt communication with the treating emergency, surgical, and interventional radiology teams as appropriate.
Correlate with clinical status: Management decisions require examination findings, laboratory results, hemodynamic status, comorbidities, and assessment of whether drainage or operative source control is feasible.
This sequence supports timely diagnosis while avoiding the mistaken assumption that the absence of generalized pneumoperitoneum excludes a clinically important perforation.
Clinical Significance and Management Considerations
A large pelvic abscess caused by suspected sigmoid perforation represents complicated diverticulitis rather than uncomplicated bowel inflammation.
Management generally requires antibiotics and assessment for source control. Depending on the collection's size and accessibility, the patient's clinical condition, and local expertise, image-guided percutaneous drainage or operative intervention may be necessary. Antibiotic treatment alone may be considered in selected circumstances, but a large abscess requires careful clinical assessment and close monitoring.
The CT report should clearly communicate:
The suspected site and cause of perforation.
The presence and extent of the pelvic abscess.
Any additional collections or free fluid.
Evidence of diffuse peritoneal contamination, if present.
Relevant adjacent-organ findings.
Any suspicious colonic abnormality that could require follow-up evaluation.
The imaging findings alone do not establish which treatment was selected in this case. No treatment or outcome was reported in the available clinical information.
Why the Absence of Upper Abdominal Free Air Matters
Free intraperitoneal gas is an important sign of gastrointestinal perforation, but its distribution depends on the site, size, and containment of the perforation.
A localized perforation may produce an abscess in the pelvis without substantial gas extending into the upper abdomen. The inflammatory process can become confined by adjacent tissues, limiting the spread of contamination.
Consequently, a radiologist should not use the absence of upper abdominal free air as the sole criterion for excluding bowel perforation. The combination of a large gas-containing abscess, adjacent colonic inflammation, and severe diverticulosis may provide stronger evidence of a localized perforation than a search for diffuse pneumoperitoneum alone.
Diagnostic Risk: How This Finding Could Be Missed
Several interpretation challenges are relevant to this case.
Search limited to free air: A reader may focus on pneumoperitoneum and overlook a contained perforation with a large abscess.
Failure to identify the bowel source: A pelvic collection may be described without adequately evaluating the adjacent sigmoid colon and its diverticula.
Satisfaction of search: Once the abscess is recognized, secondary findings such as reactive small-bowel thickening, bladder wall thickening, or additional collections may receive insufficient attention.
Overinterpretation of reactive changes: Adjacent bowel wall thickening may be incorrectly labeled as a separate primary bowel disorder without considering the distribution of pelvic inflammation.
Premature exclusion of malignancy: The absence of a definite stenotic lesion should not be treated as definitive exclusion of colorectal cancer when appropriate follow-up evaluation is otherwise indicated.
These are general diagnostic risks, not evidence that an error occurred in this patient's care.
AI in Radiology: Potential Value and Limitations
Artificial intelligence may support the interpretation of complicated abdominal CT examinations by helping identify abnormal gas collections, segment fluid collections, flag bowel wall abnormalities, or prioritize examinations with suspected acute abdominal complications.
In a hypothetical clinical workflow, a detection or triage system could flag a large pelvic gas-and-fluid collection for radiologist review. Image segmentation tools might help quantify the collection's dimensions or volume when such measurements are clinically useful.
However, these are potential applications, not documented interventions in this case. No AI system, diagnostic performance data, or AI-assisted outcome was reported.
Where AI Could Help
Detection support: Flagging a suspicious gas-containing collection for review.
Quantitative analysis: Assisting with reproducible measurement of a collection across serial examinations.
Workflow prioritization: Supporting escalation of potentially urgent findings within a validated clinical workflow.
Structured reporting: Prompting documentation of the collection, suspected source, free fluid, extraluminal gas, and possible complications.
Where AI Could Fail
AI may miss a small or anatomically complex perforation, misclassify postoperative or intraluminal gas, or fail to distinguish a primary bowel lesion from reactive changes. Performance may also vary with image quality, scanner protocols, patient anatomy, and the prevalence of relevant abnormalities in the training data.
A model that detects an abscess does not necessarily identify its cause, determine whether the bowel is perforated, or establish the appropriate treatment.
Radiologist review remains essential. AI should function as decision support rather than an autonomous replacement for clinical interpretation.
Healthcare Workflow and System-Level Implications
The clinical value of CT extends beyond identifying an abscess. A useful report establishes the likely source of infection, describes the extent of disease, and communicates findings that may influence urgent management.
When the report clearly identifies a large pelvic abscess and suspected sigmoid perforation, it can support timely coordination among emergency physicians, surgeons, radiologists, and interventional radiologists.
Conversely, an incomplete description of the collection or failure to communicate suspected perforation may contribute to delays in further assessment. Such delays can complicate clinical decision-making and increase the burden of care, although no delay or adverse outcome was documented in this case.
If AI is introduced into this workflow, hospitals should evaluate whether it improves detection, prioritization, communication, and reporting completeness. Evaluation should include false-positive alerts, false negatives, integration with the picture archiving and communication system (PACS), alert fatigue, and the need for human verification. Benefits should be demonstrated locally rather than assumed.
Practical Radiology Pearls
A large gas-and-fluid collection with peripheral enhancement in the pelvis should raise strong suspicion for an abscess.
Severe sigmoid diverticulosis and adjacent colonic wall thickening support a diverticular source when these findings accompany a pelvic abscess.
Absence of upper abdominal pneumoperitoneum does not exclude localized colonic perforation.
Small-volume free fluid does not, by itself, establish generalized peritonitis.
Reactive thickening of adjacent small-bowel loops may reflect inflammation spreading from a neighboring pelvic process.
Bladder wall thickening may be reactive; it does not independently establish a urinary tract infection or enterovesical fistula.
The absence of a definite stenotic lesion or lymphadenopathy does not definitively exclude colorectal malignancy.
Multiplanar CT review helps define the collection's extent and its relationship to the bowel and pelvic organs.
A large abscess warrants prompt clinical assessment for antibiotics and appropriate source control.
AI may assist with detection and workflow prioritization, but the diagnosis and management plan require clinical correlation and human oversight.
Frequently Asked Questions
1. What is the key CT finding in perforated sigmoid diverticulitis with a pelvic abscess?
The key finding is a pelvic gas-and-fluid collection with peripheral enhancement associated with an inflamed sigmoid colon, particularly when severe diverticulosis is present. The combination supports an abscess caused by complicated diverticular disease.
2. Can sigmoid colon perforation occur without upper abdominal free air?
Yes. A contained perforation may produce a localized abscess without substantial pneumoperitoneum in the upper abdomen. The absence of distant free air does not exclude perforation.
3. What does Hinchey stage II mean?
Hinchey stage II traditionally describes diverticulitis complicated by a pelvic, intra-abdominal, or retroperitoneal abscess. CT-based modifications may use different stage definitions, so the classification system should be specified.
4. What is the role of contrast-enhanced CT?
Contrast-enhanced CT helps identify the inflamed bowel segment, detect abscesses and extraluminal gas, evaluate adjacent organs, and assess the extent of complications. It also helps guide decisions about drainage or surgery in conjunction with clinical assessment.
5. Does the absence of a colonic stenosis rule out colorectal cancer?
No. The absence of a definite stenotic lesion lowers concern for an obvious obstructing lesion but does not definitively exclude malignancy. Follow-up colonic evaluation may be appropriate after recovery, particularly when a diverticular abscess has occurred.
6. Why might the small bowel and bladder show wall thickening?
Inflammation from a pelvic abscess can affect adjacent structures, producing reactive wall thickening. The findings must be correlated with the distribution of inflammation and clinical information.
7. Does bladder wall thickening establish an enterovesical fistula?
No. Bladder wall thickening is nonspecific. Intravesical gas, in the appropriate context and without recent instrumentation, may raise suspicion for a fistula, but no intravesical gas was reported in this case.
8. How is a large diverticular abscess generally managed?
Management depends on the patient's clinical condition, abscess size and accessibility, and local expertise. Antibiotics are generally required, and image-guided drainage or surgery may be considered for source control. The optimal approach must be determined by the treating team.
9. Can AI diagnose a colonic perforation on CT?
AI may help flag suspicious collections or support image analysis and workflow prioritization. It can also produce false-positive or false-negative results. Clinical interpretation by a radiologist remains necessary.
Educational Quiz
Question 1. A patient with severe sigmoid diverticulosis has a large pelvic gas-and-fluid collection with peripheral enhancement. Which diagnosis is most likely?
A. Uncomplicated diverticulosis
B. Complicated sigmoid diverticulitis with abscess
C. Simple urinary bladder wall thickening
D. Uncomplicated small-bowel enteritis
Correct answer: B. The collection and adjacent sigmoid abnormalities strongly support a diverticular abscess, with perforation suspected in this clinical context.
Question 2. No free air is visible in the upper abdomen. Which interpretation is most appropriate?
A. Bowel perforation is excluded.
B. The collection cannot be an abscess.
C. A localized or contained perforation remains possible.
D. The findings establish uncomplicated diverticulitis.
Correct answer: C. A contained perforation may produce a localized abscess without distant pneumoperitoneum.
Question 3. What is the most appropriate interpretation of adjacent small-bowel wall thickening?
A. It always indicates primary small-bowel disease.
B. It may represent reactive inflammation from the pelvic process.
C. It proves an enteric fistula.
D. It excludes diverticulitis.
Correct answer: B. Adjacent bowel wall thickening may be reactive and should be interpreted in relation to the location and extent of inflammation.
Question 4. What is a reasonable role for AI in this case type?
A. Independently determine the need for surgery.
B. Replace radiologist review.
C. Support detection, measurement, or workflow prioritization, subject to validation and human oversight.
D. Exclude colorectal cancer based on the absence of a stenosis.
Correct answer: C. AI can support selected imaging tasks but cannot independently establish the full diagnosis or management plan.
Key Takeaways
A large pelvic abscess associated with severe sigmoid diverticulosis and distal colonic wall thickening strongly suggests complicated sigmoid diverticulitis.
The combination of a gas-and-fluid collection and adjacent bowel inflammation supports a suspected contained sigmoid perforation.
Absence of upper abdominal free air does not exclude localized bowel perforation.
The pelvic abscess pattern is consistent with Hinchey stage II, subject to the classification system used.
Reactive small-bowel and bladder wall thickening should be interpreted in the context of adjacent pelvic inflammation.
CT findings should be communicated promptly because a large abscess may require antibiotics and assessment for image-guided drainage or operative source control.
Follow-up colonic evaluation may be appropriate after recovery to assess for an underlying lesion when clinically indicated.
AI may assist detection and workflow, but its output requires radiologist verification and clinical correlation.
References
[1] World Society of Emergency Surgery, “2020 update of the WSES guidelines for the management of acute colonic diverticulitis in the emergency setting,” World Journal of Emergency Surgery, vol. 15, article 32, 2020. DOI: https://doi.org/10.1186/s13017-020-00313-4.
[2] J. Hall et al., “The American Society of Colon and Rectal Surgeons Clinical Practice Guidelines for the Treatment of Left-Sided Colonic Diverticulitis,” Diseases of the Colon & Rectum, vol. 63, no. 6, pp. 728–747, 2020. DOI: https://doi.org/10.1097/DCR.0000000000001679.
[3] A. F. Peery, A. Shaukat, and L. S. Strate, “AGA Clinical Practice Update on Medical Management of Colonic Diverticulitis: Expert Review,” Gastroenterology, vol. 160, no. 3, pp. 906–911.e1, 2021. DOI: https://doi.org/10.1053/j.gastro.2020.09.059.
[4] A. F. Peery, “Management of colonic diverticulitis,” BMJ, vol. 372, article n72, 2021. DOI: https://doi.org/10.1136/bmj.n72.
[5] F. Tiralongo et al., “Acute Colonic Diverticulitis: CT Findings, Classifications, and a Proposal of a Structured Reporting Template,” Diagnostics, 2023. DOI: https://doi.org/10.3390/diagnostics13243680.
[6] A. T. Hawkins et al., “The American Society of Colon and Rectal Surgeons Clinical Practice Guidelines for the Management of Left-Sided Colonic Diverticulitis,” Diseases of the Colon & Rectum, vol. 69, no. 10, pp. 2431–2450, 2026. DOI: https://doi.org/10.1097/DCR.0000000000004382.
[7] A. F. Peery et al., “ACG Clinical Guideline: Colonic Diverticulitis,” The American Journal of Gastroenterology, vol. 121, no. 7, pp. 1549–1561, 2026. DOI: https://doi.org/10.14309/ajg.0000000000004047.
[8] Korean Association for the Study of Intestinal Diseases, “Evidence-based clinical practice guidelines for diagnosis and medical management of acute colonic diverticulitis developed by the Korean Association for the Study of Intestinal Diseases (KASID),” 2026.
[9] World Society of Emergency Surgery et al., “WSES/GAIS/SIS-E/WSIS/AAST global clinical pathways for patients with intra-abdominal infections,” World Journal of Emergency Surgery, 2021. DOI: https://doi.org/10.1186/s13017-021-00387-8.
[10] World Society of Emergency Surgery, “WSES Guidelines for the management of acute left sided colonic diverticulitis in the emergency setting,” World Journal of Emergency Surgery, 2016. DOI: https://doi.org/10.1186/s13017-016-0095-0.
Medical Disclaimer
This article is intended for medical education and professional discussion. It does not replace individualized clinical assessment, radiologist interpretation, consultation with a surgeon, or institution-specific protocols. Diagnosis and treatment decisions should be made by the responsible clinical team using the complete imaging examination, laboratory results, physical examination, and the patient's overall condition.
Continue Learning
Bowel Perforation on CT: Key Imaging Signs and Diagnostic Pitfalls — [INTERNAL LINK PLACEHOLDER]
Colonic Diverticulitis on CT: Uncomplicated Versus Complicated Disease — [INTERNAL LINK PLACEHOLDER]
Hinchey Classification of Acute Diverticulitis: Imaging and Clinical Significance — [INTERNAL LINK PLACEHOLDER]
Abdominal CT in the Emergency Department: Recognizing Time-Critical Findings — [INTERNAL LINK PLACEHOLDER]
AI in Emergency Radiology: Detection Support, Triage, and Diagnostic Limitations — [INTERNAL LINK PLACEHOLDER]
Comments
Post a Comment