## Distinguishing Mechanical Small Bowel Obstruction from Paralytic Ileus ### Key Radiographic Features **Key Point:** The hallmark discriminator is the **presence of valvulae conniventes (plicae circulares) crossing the entire width of the dilated small bowel loop** in mechanical obstruction, versus their absence or partial involvement in paralytic ileus. ### Comparative Table | Feature | Mechanical SBO | Paralytic Ileus | | --- | --- | --- | | **Valvulae conniventes** | Cross entire width of bowel | Sparse, do not cross full width | | **Air-fluid levels** | Present (both conditions) | Present (both conditions) | | **Bowel caliber** | Markedly dilated proximal to obstruction | Uniformly dilated | | **Colonic gas** | Absent or minimal | Often present | | **Transition zone** | Sharp cutoff visible | Gradual tapering | ### Why Valvulae Conniventes Matter 1. **Mechanical obstruction** creates a **pressure gradient** proximal to the block, causing the mucosa to fold maximally — valvulae become prominent and **span the entire bowel diameter**. 2. **Paralytic ileus** has no mechanical block; bowel is uniformly atonic and dilated, so valvulae are **flattened and do not cross the full width**. **High-Yield:** This is the **single most reliable plain film sign** to differentiate the two on initial radiography. **Clinical Pearl:** Air-fluid levels are present in both conditions and therefore **not discriminatory**. Free air suggests perforation (a complication), not the underlying obstruction type. ### Additional Context - **Transition zone** (sharp change from dilated to collapsed bowel) also favors mechanical obstruction but may not always be visible. - **Colonic gas** is often absent in mechanical SBO (due to the proximal block) but present in ileus (because the entire GI tract is atonic). [cite:Robbins & Cotran 10e Ch 17] 
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