Third Part of the Duodenum: Anatomical Definition
The third part of the duodenum is the horizontal (transverse) portion that crosses the midline from right to left at approximately the level of the third lumbar vertebra.
Anatomical Characteristics
The third part of the duodenum runs horizontally across the abdomen, positioned anterior to the aorta and inferior vena cava, and is crossed anteriorly by the superior mesenteric artery and vein. This anatomical relationship is clinically significant because compression of this segment by the superior mesenteric artery against the aorta can result in superior mesenteric artery syndrome, causing duodenal obstruction 1.
Key Anatomical Features:
- Location: Extends from the inferior duodenal flexure to the point where it ascends to become the fourth part
- Course: Runs horizontally from right to left at the L3 vertebral level
- Relationship to vessels: Passes between the superior mesenteric artery anteriorly and the aorta posteriorly
- Peritoneal coverage: Retroperitoneal structure
Clinical Significance
Superior Mesenteric Artery Syndrome
The third part's position makes it vulnerable to compression when the aortomesenteric angle narrows, presenting with chronic abdominal pain, nausea, postprandial early satiety, vomiting, and weight loss 1.
Anatomical Variants
While rare, the papilla of Vater can occasionally be located in the third portion of the duodenum rather than its typical position in the second part 2.
Imaging Considerations
The retroperitoneal location and proximity to multiple viscera means the third part can be involved in various primary and secondary pathologic processes that should be recognized on CT imaging, including inflammatory conditions, neoplasms, and developmental abnormalities 3.