Horizontal part of duodenum

The horizontal (third) part of the duodenum (D3) is approximately 10 cm long and extends from the inferior duodenal flexure on the right side of the vertebral column at the level of L3-L4 vertebrae. It passes horizontally from right to left across the posterior abdominal wall, crossing the midline to reach the left side of the vertebral column, where it ascends as the fourth part of the duodenum. Like most of the duodenum, the horizontal part is retroperitoneal and is fixed in position.

Anatomical relations of the horizontal part of duodenum

Anteriorly, the horizontal part of duodenum is crossed by the superior mesenteric artery and vein at the root of the mesentery, and is also related to coils of jejunum and the mesentery. Posteriorly, it rests upon the third lumbar vertebra, the inferior vena cava, and the abdominal aorta, with the right psoas major muscle, right ureter, and right gonadal vessels also lying behind it. Superiorly, it is closely related to the head and uncinate process of the pancreas, while inferiorly it is related to loops of the jejunum.

Blood supply and lymphatics

The horizontal part receives its blood supply from the superior and inferior pancreaticoduodenal arteries, which form an important anastomosis between the coeliac trunk and superior mesenteric artery territories. Venous drainage follows corresponding veins into the portal venous system, while lymph drains to the pancreaticoduodenal and superior mesenteric lymph nodes.

Clinical correlate

The horizontal part of duodenum is important because it lies between the abdominal aorta posteriorly and the superior mesenteric vessels anteriorly. Sometimes, due to the loss of the fatty cushion separating these vessels can lead to compression of the D2 segment in between the superior mesenteric artery and abdominal aorta, that can result in superior mesenteric artery (SMA) syndrome. It is a rare condition causing duodenal obstruction, abdominal pain, vomiting, and weight loss.