What is the duodenum, including its anatomy, function, and common clinical issues?

The Duodenum: Anatomy, Function, and Clinical Significance

Anatomical Structure

The duodenum is the first and shortest segment of the small intestine, divided into four distinct parts: the superior (D1 or duodenal bulb), descending (D2), horizontal (D3), and ascending (D4) segments 1. The duodenal bulb is notably thicker than the descending duodenum, with tissue characteristics analogous to the gastric antrum 2.

Key Anatomical Relationships

• The D2 segment holds particular clinical significance due to its anatomical proximity to the gallbladder, right kidney, colon, and pancreas 1
• The duodenum's unique position—derived from both foregut and midgut embryologically—places it in close relationship with critical vascular and biliary structures 3
• The major and minor papillae are critical anatomical landmarks that must be identified during endoscopic evaluation 2
• The portal vein groove on the posterior-medial surface of the pancreatic head rests over the portal vein, serving as an important surgical landmark 4

Physiological Functions

The duodenum is far from having a secondary role in digestive performance and carries out fundamental activities essential for digestion and absorption 5.

Primary Functions

• The duodenum serves as the organ of choice for digestion and absorption of important nutritive substances 5
• Its motor and endocrine activity directs the functions of the stomach, intestine, and attached glands (pancreas and bile ducts) 5
• The proximal small intestine is dedicated to nutrient digestion and absorption while serving as a highly regulated immune site exposed to environmental factors 6
• Host-protective responses against pathogens and tolerance to food antigens are essential immunological functions 6

Common Clinical Pathologies

Congenital Conditions

Congenital anomalies like duplication cysts and diverticula are usually asymptomatic, while annular pancreas and malrotation may manifest in the first decade of life 3.

• Annular pancreas can cause obstruction and requires surgical intervention in symptomatic cases 3, 7
• Superior mesenteric artery syndrome and midgut volvulus represent potentially life-threatening developmental abnormalities 3, 7

Inflammatory and Infectious Diseases

Primary Crohn's disease of the duodenum is uncommon, occurring in 0.5-5% of patients based on clinical symptoms and radiographic findings, though endoscopic inflammation is found in 60-70% of cases 2.

• Common symptoms include upper abdominal pain, nausea and vomiting, weight loss, diarrhea, hematemesis, and anemia 2
• Initial management involves proton pump inhibitors and anti-inflammatory drugs, with steroids, immunomodulators, and anti-TNF agents reported as effective 2
• The main indication for surgery in primary gastroduodenal Crohn's disease is stricturing complications, with the majority of symptomatic patients eventually requiring surgery 2

Surgical Management of Duodenal Crohn's Disease

• Strictures in the antrum and duodenal bulb (50-60% of cases) are most commonly treated by distal gastrectomy with Roux-en-Y reconstruction 2
• For strictures of the second and third duodenal portions (25-30% of cases), stricturoplasty is the preferred treatment when technically possible, as there is no risk of blind loop, dumping, or anastomotic ulcerations 2
• Duodenal resection or pancreatoduodenectomy should be considered as last resort options 2
• Routine vagotomy is contraindicated in Crohn's patients 2
• Five-year local surgical recurrence is 5-8% for gastroduodenal lesions, though overall Crohn's disease recurrence is considerably higher due to concomitant jejunoileal and/or colonic disease 2

Neoplastic Lesions

Duodenal adenomas are encountered in up to 5% of routine upper endoscopies, with sporadic adenomas seen in up to 0.5% of patients, while upwards of 90% of patients with familial adenomatous polyposis will eventually develop duodenal adenomas 2.

Risk of Malignant Transformation

• Most duodenal adenomas contain only low-grade dysplasia; however, up to 20% may progress to high-grade dysplasia and approximately 5% to cancer 2
• Given their risk of malignant transformation, endoscopic resection of duodenal adenomas is generally recommended 2

Management Approach

• Non-neoplastic duodenal lesions (metaplastic foveolar epithelium, gastric heterotopia) may mimic neoplastic adenomatous pathology but do not require endoscopic resection unless symptomatic or bleeding 2
• Brunner gland hamartomas are benign lesions that generally do not require resection unless symptomatic or large (>2 cm) causing obstruction, pain, or bleeding 2
• Duodenal hamartomas associated with familial syndromes (Peutz-Jeghers, Cowden, juvenile polyposis) generally warrant endoscopic resection 2

Endoscopic Resection Considerations

• Confirmation that the lesion is not adjacent to or involving the major papilla is critical before polypectomy 2
• For lesions on the medial wall within 5 cm of the papilla, a duodenoscope should be used 2
• For lesions on the posterior/lateral wall and distal duodenum, a pediatric colonoscope may be useful 2
• Caution is advised with duodenal biopsies if future resection is likely, as this can result in underlying fibrosis leading to increased difficulties and risks at resection 2

Surveillance

• Initial endoscopic surveillance for a completely resected duodenal adenoma should be undertaken at 6-month intervals 2
• Given the high frequency of concomitant colonic adenomas in patients with duodenal adenomas, colonoscopy should be performed if a high-quality examination has not been performed in the past 3 years 2
• In familial adenomatous polyposis, duodenal adenomas should be considered for endoscopic resection based on size (≥1 cm), morphologic characteristics, advanced histology (high-grade dysplasia), and/or Spiegelman criteria 2

Traumatic Injuries

CT plays a vital role in diagnosing traumatic duodenal injury by differentiating between mural hematoma and duodenal perforation, as the latter requires immediate surgical intervention 3.

• Duodenal wall hematoma and traumatic perforation causing pneumoretroperitoneum can occur after blunt abdominal trauma 3
• In trauma settings, damage control surgery should be strongly considered rather than definitive reconstruction if the patient is physiologically deranged 8

Diagnostic Imaging

Cross-sectional imaging modalities, particularly CT, play a pivotal role in evaluating pathologies of the duodenum 1.

• CT provides superb anatomic detail and offers high diagnostic specificity by allowing direct imaging of the intestinal wall, secondary signs of bowel disease within the surrounding mesentery, and abnormal findings in adjacent structures 3
• Primary duodenal malignancies and local extension from adjacent malignancies can be diagnosed by CT reliably 3, 7
• Upper gastrointestinal series, ultrasound, and MR imaging also have roles in specific clinical scenarios 7

Critical Clinical Pitfalls

• The duodenum is often overlooked on imaging and in clinical practice despite its involvement by a multitude of primary and secondary processes 3, 7
• Inadequate dissection of the uncinate process during surgical resection can lead to positive margins and decreased survival in oncologic resections 8
• Unrecognized duodenal perforation after endoscopic resection may be life-threatening and often mandates surgery 2
• Extended Kocherisation is particularly useful for pancreatic head tumors requiring assessment of superior mesenteric artery involvement 8