The duodenal bulb is the triangular 1st portion of the duodenum. It is suspended by the hepatoduodenal ligament, which also contains the bile duct, portal vein, and hepatic artery. The bulb is the only intraperitoneal portion of the duodenum.

The descending duodenum is the 2nd portion and is the site of the major pancreaticobiliary papilla (of Vater), the entry of the common bile and pancreatic ducts.

The transverse duodenum is its 3rd portion, and it crosses between the aorta and the superior mesenteric vessels.

The ascending duodenum is the 4th portion, and it ends at the duodenojejunal junction, which is fixed in place by the suspensory ligament of the duodenum (ligament of Treitz). The duodenojejunal junction usually lies at about the same level as the pylorus and the T12 vertebra.

The duodenal wall consists of 4 layers: The mucosa, submucosa, circular, and longitudinal smooth muscle.

Brunner glands secrete mucus and alkaline fluid with proteolytic enzymes. These are most prominent within the proximal duodenum and may enlarge to simulate multiple polyps (Brunner gland hypertrophy) or may develop into a benign neoplastic mass (Brunner gland adenoma).

The 2nd and 3rd portions of the duodenum are closely attached to the pancreatic head, and resection of either organ generally requires resection of both, known as a pancreaticoduodenectomy (Whipple procedure).

The duodenum occupies the anterior pararenal space of the retroperitoneum along with the pancreas and vertical colon segments. Inflammatory, or less commonly malignant processes, affecting 1 of these organs often spread to affect the others.

Duodenal ulcers and erosions are common, with a multifactorial etiology, including Helicobacter pylori infection. Erosion of the duodenal mucosa makes it vulnerable to the caustic effects of acid and digestive enzymes produced by the stomach.

The 2nd portion of duodenum lies just anterior to the right renal hilum. Inflammation originating in the duodenum (perforated ulcer) or pancreatic head (pancreatitis) may extend into the right perirenal space, potentially simulating primary renal inflammation.

Duodenal ulcers may perforate, often resulting in collections of gas and fluid that are both intra- and retroperitoneal, reflecting the dual compartment location of the duodenum.

Congenital diverticula commonly arise from the 2nd and 3rd portions of the duodenum and are usually of no clinical concern. A fluid-filled diverticulum could be mistaken for a cystic pancreatic mass. Periampullary diverticula may be associated with biliary disease and are prone to iatrogenic perforation if endoscopic papillotomy is performed. Diverticula may also perforate spontaneously or as the result of feeding tube placement.

Duodenal tumors are uncommon relative to the rest of the GI tract and are often associated with various syndromes. Patients with Gardner syndrome, for example, have an increased prevalence of duodenal adenomas and carcinomas, as well as ampullary carcinomas. Multiple endocrine neoplasia type 1 (MEN1) is associated with duodenal carcinoid tumors, in addition to tumors of the parathyroid, pancreas, and pituitary.

The 3rd portion of the duodenum is adjacent to the aorta and often lies at the proximal end of abdominal aortic aneurysms. Spontaneously, or more commonly following surgical or endovascular stent graft repair of an aneurysm, a fistula between the aorta and duodenum may form (aorto-enteric fistula), often with fatal consequences.

Air-contrast upper GI series remains the most accurate imaging test for detecting mucosal erosions, ulcers, or polypoid lesions, although its use for these indications has decreased precipitously with increased use of endoscopy.

Fluoroscopic studies may also be employed to evaluate functional abnormalities of the duodenum, such as the SMA syndrome. A “megaduodenum” may also result from scleroderma, and the duodenum is often involved in patients with celiac-sprue.