A 52-year-old woman with a history of left-sided colonic cancer treated with partial colectomy 2 years ago now presents with acute onset of severe left lower quadrant pain, abdominal distension, and inability to pass stool or flatus for 48 hours. On examination, the abdomen is distended and tympanic. Plain abdominal radiograph shows a grossly dilated colon (>8 cm) with a transition zone at the sigmoid region. The small bowel loops are normal in caliber. What is the most likely diagnosis?

Explanation

## Diagnosis: Acute Colonic Obstruction ### Key Radiographic Features **Key Point:** A dilated colon (>8 cm) with a sharp transition zone at the sigmoid region indicates mechanical colonic obstruction. The normal caliber of small bowel loops excludes small bowel pathology and confirms the obstruction is distal to the ileocecal valve. ### Imaging Findings: Colonic vs. Small Bowel Obstruction | Feature | Colonic Obstruction | Small Bowel Obstruction | |---------|-------------------|------------------------| | Bowel caliber | >6 cm (dilated) | 3–4 cm (dilated) | | Mucosal pattern | Haustra (incomplete) | Valvulae conniventes (complete) | | Distribution | Peripheral (colon outline) | Central (clustered) | | Small bowel | Normal caliber | Dilated | | Transition zone | Sigmoid/rectum (left lower quadrant) | Variable location | ### Clinical Context **Clinical Pearl:** History of colonic cancer with surgical resection 2 years ago raises suspicion for **anastomotic stricture** or **recurrent malignancy** as the cause of obstruction. Sigmoid colon is the most common site of colonic obstruction due to its narrow caliber and fixed position. **High-Yield:** The "closed-loop obstruction" risk is high in colonic obstruction with a competent ileocecal valve — the colon cannot decompress proximally, leading to rapid perforation risk (caecal diameter >12 cm = imminent perforation). ### Differential Diagnosis of Colonic Obstruction Causes **Mnemonic: MAST** — **M**alignancy (most common in developed countries), **A**nastomotic stricture, **S**igmoid volvulus, **T**umor (diverticular stricture). ### Pathophysiology 1. Mechanical obstruction at sigmoid region (stricture, tumor, or volvulus). 2. Proximal colon dilates due to increased intraluminal pressure. 3. If ileocecal valve is competent → closed-loop obstruction → risk of caecal perforation. 4. If ileocecal valve is incompetent → small bowel can decompress → lower perforation risk. ### Management Algorithm ```mermaid flowchart TD A[Acute colonic obstruction]:::outcome --> B{Caecal diameter}:::decision B -->|< 12 cm| C[CT abdomen for staging]:::action B -->|> 12 cm| D[Imminent perforation risk]:::urgent C --> E{Cause identified?}:::decision E -->|Malignancy| F[Surgical resection]:::action E -->|Stricture| G[Endoscopic dilation ± stent]:::action E -->|Volvulus| H[Endoscopic decompression]:::action D --> I[Urgent surgical intervention]:::urgent ``` **Next Steps:** - **CT abdomen with IV contrast** is the gold standard to identify the cause (stricture vs. tumor vs. volvulus), assess for perforation, and stage malignancy if present. - **Colonoscopy** may be therapeutic (decompression, stent placement) if safe. - **Surgical consultation** if perforation suspected or endoscopy contraindicated. [cite:Robbins 10e Ch 17]