What are the anatomy, functions, blood supply, innervation, common diseases, and management of the duodenum?

Duodenum: Comprehensive Medical Overview

Anatomical Structure and Divisions

The duodenum is the first 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, 2

• The D1 segment (duodenal bulb) has a thicker wall compared to the descending duodenum, which provides some protective effect against perforation during procedures 3
• The D2 segment holds particular clinical significance due to its anatomical proximity to the gallbladder, right kidney, colon, and pancreas 1
• The duodenum develops from both foregut and midgut embryologically, with the ventral and dorsal pancreatic buds fusing during development 4, 5
• The portal vein groove is a smooth-surfaced landmark on the posterior-medial surface of the pancreatic head that rests over the portal vein, critical for surgical orientation 4

Blood Supply and Innervation

The duodenum receives dual arterial supply through anterior and posterior pancreaticoduodenal arcades formed by branches from the celiac axis and superior mesenteric artery. 4

• Complete mobilization of the portal and superior mesenteric veins from the uncinate process is essential for pancreatic head resections 4, 6
• The superior mesenteric artery (SMA) margin represents the most critical surgical margin, involving soft tissue directly adjacent to the proximal 3-4 cm of the SMA 4
• Pancreatic lymphatic drainage follows the arterial supply, with specific nodal stations including perigastric, suprapancreatic, celiac, porta hepatis, and pancreaticoduodenal lymph nodes for pancreatic head tumors 4

Physiological Functions

The duodenum serves as the organ of choice for digestion and absorption of important nutritive substances, while its motor and endocrine activity directs the functions of the stomach, intestine, pancreas, and bile ducts. 7

• The duodenum controls gastric emptying through complex motor mechanisms 8
• Its endocrine activity fundamentally regulates digestive performance throughout the gastrointestinal tract 7
• The duodenum's relationship to bile and pancreatic ducts makes it central to digestive enzyme delivery 8

Common Pathological Conditions

Congenital Anomalies

• Annular pancreas may manifest in the first decade of life and can cause duodenal obstruction 5, 2
• Duplication cysts are usually asymptomatic incidental findings 5, 2
• Diverticula typically have no pathologic significance and are asymptomatic 5, 2
• Superior mesenteric artery syndrome and midgut malrotation require recognition as they can present with obstruction 5, 2

Inflammatory Conditions

• Crohn's disease of the duodenum is uncommon (0.5-5% based on clinical/radiographic findings, but up to 60-70% show endoscopic inflammation), typically affecting the antrum and pylorus, with stricturing complications being the main surgical indication 9
• Peptic ulcer disease and Cameron's erosions in large hiatal hernias are commonly overlooked lesions 9
• Groove pancreatitis can secondarily involve the duodenum 5

Neoplastic Conditions

• Adenocarcinoma is the most common primary duodenal malignancy and can be reliably diagnosed with CT 5, 2
• Lymphoma can involve the duodenum primarily or secondarily 5, 2
• Gastrointestinal stromal tumors (GISTs) require tissue diagnosis with immunohistochemical staining 3, 5
• Neuroendocrine tumors require chromogranin A, synaptophysin, and Ki-67 staining for diagnosis 3
• Younger patients are more likely to have Dieulafoy's lesion, while older patients (>40 years) are prone to vascular lesions comprising up to 40% of obscure bleeding causes 9

Traumatic Injuries

• Duodenal wall hematoma can occur after blunt abdominal trauma and is managed conservatively 5
• Traumatic duodenal perforation causing pneumoretroperitoneum requires immediate surgical intervention 5
• In trauma settings, damage control surgery should be strongly considered rather than definitive reconstruction if the patient is physiologically deranged 6

Diagnostic Approach

Imaging Modalities

Cross-sectional imaging, particularly CT, plays a pivotal role in evaluating duodenal pathologies because it allows direct imaging of the intestinal wall, secondary signs of bowel disease within the surrounding mesentery, and abnormal findings in adjacent structures. 1, 5, 2

• CT provides superb anatomic detail and offers high diagnostic specificity for detection of duodenal pathologies 5
• Upper gastrointestinal series (UGIS) remains useful for evaluating certain conditions 2
• Ultrasound and MRI have limited but specific roles in duodenal evaluation 2

Endoscopic Evaluation

For D1 lesions, a standard forward-viewing gastroscope with a clear distal cap attachment should be used for lesions on the anterior, posterior, or lateral wall, while a side-viewing duodenoscope should be used for D1 lesions on the medial wall within 5 cm of the ampulla. 3

• Careful endoscopic evaluation using high-definition white light and chromoendoscopy can distinguish neoplastic from non-neoplastic lesions 3
• Document lesion size, Paris morphology, location, wall orientation, and relationship to the papilla 3

Biopsy Considerations

Biopsy should be reserved for genuine uncertainty about neoplastic versus non-neoplastic lesions, subepithelial tumors requiring immunohistochemical staining, or when endoscopic resection is unlikely due to patient comorbidities. 3

• Critical pitfall: Biopsy can induce submucosal scarring that makes subsequent endoscopic resection more difficult, increases perforation risk, and may convert a resectable lesion into one requiring surgery 3
• The risk of bleeding with duodenal biopsy exists, though the duodenal bulb's thicker wall provides some protective effect 3
• For celiac disease diagnosis, four biopsies from the second part of the duodenum plus two from the bulb are recommended 3
• Avoid repeated or aggressive biopsy in patients with familial adenomatous polyposis who may require future endoscopic resection 3

Histological Assessment for Celiac Disease

When evaluating duodenal biopsies for celiac disease, proper orientation is essential for assessment of villous height to crypt depth ratio (normal >3:1), and intraepithelial lymphocytes (IELs) should be counted with <25 IELs/100 enterocytes considered normal. 9

• The report should state number of biopsies, orientation, architectural features (normal, partial, subtotal, or total villous atrophy), lamina propria content, presence of Brunner's glands, and crypt hyperplasia 9
• Lymphocytic duodenosis (≥25 IELs/100 enterocytes with normal villous architecture) occurs in 3.8% of the population negative for celiac serology and is associated with H. pylori infection, immune disorders, drugs, and neoplasia 9
• Other causes of villous atrophy include immune disorders, food hypersensitivity, infection, drugs, and neoplasia—70% of patients with non-celiac enteropathy were initially misdiagnosed as having celiac disease 9

Management Strategies

Medical Management of Crohn's Disease

• Initial management includes proton pump inhibitors and anti-inflammatory drugs 9
• Steroids, immunomodulators, and anti-TNF agents have been reported effective 9
• The best medical prophylaxis after surgery for gastroduodenal Crohn's disease is not known due to small case numbers 9

Surgical Management

For Crohn's disease of the antrum and duodenal bulb, stricturoplasty and Roux-en-Y bypass are considered effective surgical options, while strictures of the second and third duodenal portions are best treated with stricturoplasty when technically possible. 9

• Surgery must be as conservative as possible, preferably with wedge resection and primary closure 9
• Duodenal resection or pancreatoduodenectomy should be considered as last resort 9
• Routine vagotomy is contraindicated in Crohn's patients 9
• 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 9
• Extended Kocherisation is particularly useful for pancreatic head tumors requiring assessment of superior mesenteric artery involvement 6
• Inadequate dissection of the uncinate process can lead to positive margins and decreased survival in oncologic resections 6

Management of Obscure GI Bleeding

Patients with occult GI blood loss and iron deficiency anemia with negative EGD and colonoscopy need comprehensive evaluation, including capsule endoscopy to identify an intestinal bleeding lesion, as angiectasia accounts for up to 80% of obscure bleeding causes. 9

• Review capsule endoscopy in its entirety because it may provide clues to bleeding from stomach and colon overlooked by EGD and colonoscopy 9
• Be aggressive in investigating younger patients because small bowel tumors are the most common cause of obscure bleeding in this population 9
• Less common etiologies of bleeding originating in the C-loop of duodenum include hemobilia, hemosuccus pancreaticus, and aortoenteric fistula 9

Short Bowel Syndrome Considerations

Patients with short bowel and retained functional colon may need parenteral nutrition if less than 50 cm of small intestine remains, while jejunostomy patients with less than 100 cm of jejunum are likely to need parenteral saline long-term. 9

• Hypomagnesemia is common in jejunostomy patients and is treated by correcting sodium depletion, oral or intravenous magnesium supplements, and occasionally oral 1-alpha hydroxycholecalciferol 9
• Gall stones are common (45%) in jejunostomy patients, probably resulting from gall bladder stasis 9
• Jejunum-colon patients have a 25% chance of developing symptomatic calcium oxalate renal stones due to increased colonic absorption of dietary oxalate 9

Critical Clinical Pitfalls

• The duodenum is often overlooked on imaging despite its involvement by multiple primary and secondary processes 5, 2
• Bleeding lesions commonly overlooked in the upper GI tract include Cameron's erosions, fundic varices, peptic ulcer disease, angioectasias, Dieulafoy's lesion, and gastric antral vascular ectasia 9
• Confusion in short bowel patients may result from D-lactic acidosis (only in patients with preserved colon) or hyperammonaemia, both requiring specific treatment approaches 9
• Drug absorption is significantly impaired in short bowel patients, requiring much higher doses than usual (e.g., thyroxine, warfarin, digoxin) or intravenous administration 9