A 55-year-old woman with a history of chronic constipation presents with acute onset severe left lower abdominal pain and abdominal distension. On examination, she is afebrile but has a markedly distended, tympanic abdomen. Plain abdominal radiograph shows a massively dilated sigmoid colon with a characteristic 'coffee bean' or 'omega loop' appearance. The proximal colon and small bowel are also dilated. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Sigmoid Volvulus **Key Point:** The pathognomonic 'coffee bean' or 'omega loop' sign on plain radiography is diagnostic of sigmoid volvulus, representing the twisted, dilated sigmoid colon. ### Radiographic Hallmarks of Sigmoid Volvulus | Feature | Description | Diagnostic Value | |---------|-------------|-------------------| | **Coffee Bean Sign** | Twisted sigmoid loop with narrowing at the site of torsion | Pathognomonic on frontal view | | **Omega Loop** | U-shaped or omega-shaped configuration | Seen on lateral view | | **Transition Point** | Abrupt narrowing at the level of torsion | Marks the site of obstruction | | **Proximal Dilation** | Dilated colon and small bowel proximal to volvulus | Secondary to obstruction | | **Whirl Sign** (CT) | Spiral arrangement of mesentery and vessels | Confirms torsion | **High-Yield:** Sigmoid volvulus accounts for 5–10% of large bowel obstructions in developed countries but up to 40% in developing countries (including India). It is the second most common cause of large bowel obstruction after carcinoma. **Clinical Pearl:** Sigmoid volvulus typically occurs in elderly patients with chronic constipation or neuropsychiatric disorders. The classic presentation is acute onset colicky pain with abdominal distension. ### Mechanism and Pathophysiology Sigmoid volvulus occurs when the sigmoid colon twists around its mesentery, causing: 1. Mechanical obstruction to fecal flow 2. Venous congestion and ischemia if not promptly relieved 3. Risk of perforation if decompression is delayed **Mnemonic — VOLVULUS:** **V**olvulus (twisted bowel), **O**mega loop appearance, **L**arge bowel obstruction, **V**enous congestion risk, **U**rgent decompression needed, **L**ong mesentery predisposes, **U**sually sigmoid, **S**evere pain and distension. ### Differential Diagnosis at a Glance | Condition | Radiographic Sign | Key Differentiator | |-----------|-------------------|-------------------| | **Sigmoid Volvulus** | Coffee bean / omega loop | Twisted appearance, transition at torsion site | | **Toxic Megacolon** | Diffuse colonic dilation > 6 cm | Associated with IBD/infection, systemic toxicity | | **Cecal Carcinoma** | Focal narrowing (apple core) | Gradual onset, weight loss, anemia | | **Diverticulitis** | Localized colonic wall thickening | Fever, left lower quadrant tenderness, no obstruction | [cite:Robbins 10e Ch 17]