The third part of the duodenum is compressed between the aorta behind and the superior mesenteric artery in front
Expansion
The third (horizontal) part of the duodenum passes through the narrow angle between:
- The aorta posteriorly
- The superior mesenteric artery anteriorly
That angle is normally held open by a pad of retroperitoneal and mesenteric fat. Rapid weight loss, from anorexia nervosa, malignancy, burns, or after scoliosis correction or prolonged bed rest, removes the pad, narrowing the angle and compressing the duodenum.
Symptoms are postprandial epigastric pain, early satiety, and bilious vomiting relieved by lying prone or on the left side, which lets the small bowel fall away.
Note the companion condition sharing the same anatomy: the left renal vein also passes through the aortomesenteric angle, and its compression produces nutcracker syndrome with left flank pain and haematuria. The two can occur together.