A 2-week-old female infant presents with acute-onset bilious vomiting and severe abdominal pain. On examination, the abdomen is distended and tender. Plain abdominal X-ray shows multiple dilated small bowel loops with air-fluid levels. Upper GI contrast study demonstrates the duodenum and jejunum positioned on the right side of the abdomen, with the cecum in the midline. The radiologist notes that the mesentery appears to be attached to a single narrow pedicle. Emergency laparotomy reveals the small bowel twisted around the superior mesenteric artery. What is the embryological abnormality responsible for this presentation?

Explanation

## Midgut Malrotation with Volvulus: Embryological Basis and Clinical Implications **Key Point:** The clinical and radiological findings in this case — duodenum and jejunum on the right side, cecum in the midline, narrow mesenteric pedicle, and volvulus around the SMA — are classic for **incomplete rotation of the midgut with failure of normal mesenteric fixation** (malrotation), NOT complete non-rotation. ### Distinguishing Malrotation from Non-Rotation | Feature | Normal Rotation | Malrotation (Incomplete) | Non-Rotation (Complete) | |---------|-----------------|--------------------------|-------------------------| | **Duodenum position** | Posterior, retroperitoneal (crosses midline) | Right of midline / anterior to SMA | Entirely right side | | **Jejunum/ileum position** | Left side | Right side | Right side | | **Cecum position** | Right lower quadrant | **Midline or subhepatic** | Right side | | **Mesenteric attachment** | Broad, fused posteriorly | **Narrow pedicle** | Single narrow pedicle | | **Volvulus risk** | Low | **High** | Extremely high | | **Typical age of presentation** | — | **2–4 weeks** | 1–2 weeks | ### Embryological Defect in Malrotation During weeks 6–10 of embryogenesis, the midgut herniates into the umbilical cord and normally undergoes **270° counterclockwise rotation** around the superior mesenteric artery (SMA) before returning to the abdominal cavity. In malrotation: 1. **Incomplete rotation:** The midgut rotates only **90–180°** instead of the full 270°, leaving the duodenojejunal junction to the right of the midline and the cecum in the midline or subhepatic position. 2. **Failure of mesenteric fixation:** Normally, the mesentery fuses broadly to the posterior peritoneum from the ligament of Treitz (left upper quadrant) to the right iliac fossa. In malrotation, this fusion fails, leaving the mesentery attached as a **narrow pedicle** around the SMA. 3. **Ladd's bands:** Peritoneal bands (Ladd's bands) form between the malpositioned cecum and the right lateral abdominal wall, crossing and potentially obstructing the duodenum. 4. **Volvulus predisposition:** The narrow mesenteric pedicle allows the entire midgut to twist around the SMA, causing acute obstruction and ischemia. **Why this is NOT non-rotation (Option C):** In complete non-rotation, the cecum remains on the **right side** (not the midline), and the entire intestine is on the right. The stem explicitly states the cecum is in the **midline** and the duodenum/jejunum are on the right — this pattern is characteristic of malrotation (incomplete rotation), not complete non-rotation. ### Clinical Pearl The imaging findings in this case are pathognomonic for malrotation with volvulus: - Duodenum and jejunum on the right side (failed to cross midline). - **Cecum in the midline** (hallmark of malrotation, not non-rotation). - Narrow mesenteric pedicle → volvulus around SMA. - Presentation at 2 weeks of age (consistent with malrotation; non-rotation typically presents at 1–2 weeks but with cecum on the right). **High-Yield:** The surgical treatment is the **Ladd's procedure** — detorsion of the volvulus, division of Ladd's bands, broadening of the mesenteric base, and appendectomy. **Mnemonic:** **Malrotation = Midline cecum + Narrow mesentery + Volvulus risk** [cite: Moore's The Developing Human, 11e, Ch 11; Langman's Medical Embryology, 15e, Ch 11; Sadler TW — Langman's Embryology; Nelson Textbook of Pediatrics, 21e]