midgut rotation

**Normal development** - The midgut extends from the distal duodenum (distal to the ampulla of Vater/bile duct opening) to the proximal 2/3 of the transverse colon (junction of proximal 2/3 and distal 1/3). It is supplied by the SMA, which forms the axis of rotation. - Week 6: Rapid elongation of the midgut forms the primary intestinal loop, which has a cranial (proximal) limb and a caudal (distal) limb. The cranial limb becomes the distal duodenum, jejunum, and proximal ileum. The caudal limb becomes the distal ileum, cecum, appendix, ascending colon, and proximal 2/3 of the transverse colon. The vitelline (omphalomesenteric) duct connects the loop to the yolk sac at the apex. Because the abdominal cavity is small (large liver and kidneys), the loop undergoes physiologic herniation into the umbilical cord. - Rotation: While herniated (weeks 6-10), the loop rotates 90° counterclockwise (viewed from the front). After it returns to the abdomen (week 10), it rotates a further 180° counterclockwise. The total is 270° counterclockwise around the SMA. Result: the duodenum ends up posterior to the SMA, the transverse colon ends up anterior to the SMA, the cecum ends up in the right lower quadrant, and the colon ends up in a "frame" configuration around the small bowel. - Week 10: The loop returns to the abdomen (the jejunum returns first, the cecum last). The cecum initially lies in the right upper quadrant and descends to the right iliac fossa, and the appendix descends with it. The ascending and descending colon become secondarily retroperitoneal, and the duodenum (except the first part) becomes retroperitoneal.

**Clinical correlates** 1. **Malrotation** (incomplete or abnormal rotation) - The cecum is often located in the upper abdomen or left side. Peritoneal bands (Ladd bands) run from the cecum to the right abdominal wall and cross the duodenum, causing duodenal obstruction. - The mesenteric base is narrow, which predisposes to **midgut volvulus**: the bowel twists around the SMA, causing vascular compromise, ischemia, and infarction. - Presentation: Usually in the first month of life, with **bilious vomiting**, abdominal distension, and, with ischemia, bloody stools and shock. Can present later with chronic intermittent symptoms. - Diagnosis: The upper GI contrast series is the gold standard. Findings: abnormal position of the duodenojejunal junction (does not cross the midline to the left of the vertebral body or lie at the level of the pylorus), a "corkscrew" appearance of the duodenum/jejunum, and a "bird's beak" sign with volvulus. Ultrasound may show the "whirlpool sign" (SMV wrapping around the SMA) and reversal of the normal SMA/SMV relationship. Abdominal x-ray may be normal or show a "double bubble" if obstructed. - Treatment: Emergent surgery (**Ladd procedure**: detorsion of the volvulus, division of Ladd bands, broadening the mesentery, and appendectomy, with the bowel placed in a nonrotated position, small bowel on the right and colon on the left). - Associations: Heterotaxy syndromes, congenital diaphragmatic hernia, gastroschisis, omphalocele, and duodenal atresia.

2. **Reverse rotation**: The midgut rotates clockwise, so the transverse colon lies posterior to the SMA (and the duodenum anterior), which can cause colonic obstruction.

3. **Nonrotation**: The small bowel is on the right and the colon is on the left, with a high risk of volvulus.

4. **Failure of the gut to return to the abdomen**: **Omphalocele** is a persistent herniation with a membranous sac covering the bowel (the sac is made of amnion and peritoneum). It is associated with other anomalies and chromosomal abnormalities (trisomy 13, 18, Beckwith-Wiedemann). **Gastroschisis** is a paraumbilical abdominal wall defect (usually right of the umbilicus) with bowel extruding without a sac, and it has few associated anomalies. It is associated with young maternal age.

5. **Meckel diverticulum**: Persistent vitelline duct (rule of 2s: 2% of the population, 2 feet from the ileocecal valve, 2 inches long, 2 types of ectopic tissue (gastric and pancreatic), presents by age 2). Presents with painless lower GI bleeding.

6. **Intestinal atresia/stenosis** and **apple-peel (Christmas tree) atresia**: Due to vascular accidents (SMA branch occlusion) in utero.

**High-yield board points** - Rotation is 270° counterclockwise around the SMA. - Physiologic herniation occurs at weeks 6-10, and the gut returns at week 10. - Neonate with bilious vomiting = malrotation with volvulus until proven otherwise (a surgical emergency). - Ladd bands cause duodenal obstruction in malrotation. - Upper GI series shows a "corkscrew" or "bird's beak" appearance. - Foregut = celiac artery; midgut = SMA; hindgut = IMA.