Medical School · Year 3 · General Surgery · includes a quiz and discussion video

Seminar 10: Small Bowel Surgery

Year 3: General Surgery Clerkship

Learning Objectives

By the end of this seminar, students will be able to:

1. Describe small bowel anatomy and physiology
2. Evaluate and manage small bowel obstruction
3. Recognize Crohn's disease and surgical indications
4. Identify small bowel tumors and their management
5. Describe mesenteric ischemia and its treatment
6. Understand short bowel syndrome

I. Small Bowel Anatomy

The small intestine extends from the pylorus to the ileocecal valve and measures approximately 6 meters in length in the living patient, divided into three segments with distinct characteristics. The duodenum is the shortest segment at approximately 25 to 30 centimeters, primarily retroperitoneal except for the first portion, and divided into four parts defined by their relationship to the head of the pancreas. The jejunum comprises approximately 40 percent of the small bowel and is characterized by a thicker wall, larger vascular arcades, and longer vasa recta compared to the ileum. The ileum makes up the remaining 60 percent and is distinguished by smaller diameter, multiple vascular arcades, shorter vasa recta, and prominent Peyer's patches in the antimesenteric border.

The blood supply to the small intestine derives from the superior mesenteric artery, which arises from the aorta behind the neck of the pancreas and supplies all of the small bowel distal to the second portion of the duodenum. The jejunal and ileal branches arise from the left side of the SMA and form a series of arterial arcades within the mesentery that provide collateral circulation. Vasa recta are the terminal straight vessels that extend from the arcades to the mesenteric border of the bowel. The vasa recta do not anastomose along the bowel wall, making the antimesenteric border relatively watershed territory susceptible to ischemia. The duodenum has a dual blood supply from both the celiac trunk (via the gastroduodenal artery and superior pancreaticoduodenal branches) and the SMA (via the inferior pancreaticoduodenal branches), with the junction at the ampulla of Vater.

The wall of the small intestine consists of four layers that are essential for its digestive and absorptive functions. The mucosa contains villi, finger-like projections that vastly increase the surface area for absorption, with crypts of Lieberkuhn at their bases containing stem cells for epithelial renewal. The submucosa is a dense connective tissue layer containing the submucosal plexus of Meissner, blood vessels, and lymphatics. The muscularis propria consists of an inner circular and outer longitudinal layer, with the myenteric plexus of Auerbach between them coordinating peristalsis. The serosa is a thin mesothelial covering that is present only on the intraperitoneal portions of the small bowel, with the retroperitoneal duodenum covered by adventitia instead.

The small intestine has specialized regional features that reflect its varying functions along its length. The duodenum contains Brunner's glands in the submucosa that secrete alkaline mucus to neutralize gastric acid and protect the duodenal mucosa. The jejunum has the largest mucosal folds (plicae circulares) and longest villi, reflecting its role as the primary site of nutrient absorption. The ileum contains Peyer's patches, aggregates of lymphoid tissue in the submucosa that serve immune surveillance functions, and is the specific site of vitamin B12 and bile acid absorption in the terminal portion. The ileocecal valve marks the junction with the colon and provides a relative barrier to reflux of colonic contents, though its competence varies considerably among individuals.

<image>Panel A: Overview of small bowel segments showing duodenum with four parts, jejunum with thick folds, and ileum with thinner walls and Peyer's patches. Panel B: Arterial blood supply from superior mesenteric artery with jejunal and ileal branches forming arcades and vasa recta. Panel C: Cross-sectional histology showing four wall layers with detailed mucosal architecture including villi and crypts. Panel D: Comparison of jejunum versus ileum characteristics including wall thickness, vascular patterns, and specialized features.</image>

II. Small Bowel Physiology

The small intestine is responsible for the majority of nutrient digestion and absorption, with its massive surface area amplified approximately 600-fold by circular folds, villi, and microvilli. Carbohydrate digestion begins with salivary amylase and continues with pancreatic amylase in the small bowel lumen, with final digestion by brush border disaccharidases producing absorbable monosaccharides. Protein digestion involves gastric pepsin and pancreatic proteases (trypsin, chymotrypsin, carboxypeptidases) with brush border peptidases completing the process. Fat digestion requires bile salts for emulsification and micelle formation, with pancreatic lipase hydrolyzing triglycerides to fatty acids and monoglycerides that can be absorbed.

Absorption of nutrients occurs through specific mechanisms along the small intestinal mucosa. Monosaccharides (glucose and galactose) are absorbed via sodium-dependent active transport (SGLT1), while fructose enters via facilitated diffusion (GLUT5). Amino acids and small peptides are absorbed through multiple transport systems, some sodium-dependent and others not. Fatty acids and monoglycerides diffuse into enterocytes, are re-esterified into triglycerides, packaged with cholesterol and proteins into chylomicrons, and exported via lymphatics rather than portal blood. Water-soluble vitamins are absorbed in the proximal small bowel, while vitamin B12 binds to intrinsic factor and is absorbed specifically in the terminal ileum, as are bile acids for enterohepatic circulation.

The small intestine absorbs approximately 8 to 9 liters of fluid daily, including both ingested fluids and gastrointestinal secretions. This fluid absorption is driven by sodium transport, with water following osmotically. Sodium absorption occurs through multiple mechanisms including sodium-nutrient cotransport (using glucose or amino acids), sodium-hydrogen exchange, and electrogenic sodium channels. Chloride follows sodium paracellularly or is absorbed via chloride-bicarbonate exchange. The jejunum absorbs the majority of water and electrolytes under normal conditions, but the ileum has tremendous reserve capacity. Diarrhea results when fluid delivery exceeds absorptive capacity or when secretion exceeds absorption, as occurs with secretory toxins or inflammatory conditions.

Small intestinal motility serves to mix luminal contents with digestive secretions and to propel chyme aborally while allowing sufficient time for digestion and absorption. During feeding, segmental contractions predominate, mixing chyme and exposing it to the mucosal surface without significant net propulsion. Peristalsis consists of a wave of contraction preceded by relaxation, propelling contents distally. The migrating motor complex (MMC) occurs during fasting, with periodic waves of contraction sweeping from the stomach through the small bowel approximately every 90 to 120 minutes, clearing residual material and bacteria. The enteric nervous system coordinates these patterns through the submucosal and myenteric plexuses, with modulation by extrinsic parasympathetic (stimulatory) and sympathetic (inhibitory) innervation.

<image>Panel A: Nutrient absorption mechanisms showing carbohydrate transport (SGLT1, GLUT5), amino acid transporters, and lipid absorption with chylomicron formation. Panel B: Regional specialization of absorption showing jejunal predominance for most nutrients and terminal ileum specificity for B12 and bile acids. Panel C: Fluid balance diagram showing input from diet and secretions versus absorption capacity in jejunum and ileum. Panel D: Motility patterns comparing fed-state segmentation with fasting migrating motor complex sweeping through the small bowel.</image>

III. Small Bowel Obstruction

Small bowel obstruction is one of the most common surgical emergencies, accounting for approximately 15 percent of emergency general surgery admissions. Adhesions from prior surgery are the leading cause in developed countries, responsible for approximately 60 percent of cases, followed by hernias at 15 to 20 percent and malignancy at 10 to 15 percent. Other causes include Crohn's disease strictures, intussusception, gallstone ileus, radiation enteritis, and foreign bodies. The pathophysiology involves proximal bowel dilation with fluid and gas accumulation, third-spacing of fluid into the bowel lumen and wall, and bacterial overgrowth, with potential progression to bowel wall compromise, ischemia, and perforation if obstruction is complete and prolonged.

Clinical presentation of small bowel obstruction includes the classic tetrad of abdominal pain, nausea and vomiting, abdominal distension, and obstipation. Pain is typically crampy and intermittent, coinciding with peristaltic waves against the obstruction, and is initially periumbilical reflecting the midgut origin. Vomiting is often bilious with proximal obstruction and may become feculent with distal or prolonged obstruction. Distension is more prominent with distal obstruction. Physical examination reveals a distended abdomen with high-pitched bowel sounds during early obstruction, progressing to absent bowel sounds with prolonged obstruction. Fever, tachycardia, localized tenderness, and peritoneal signs suggest strangulation, which requires urgent surgical intervention.

Diagnosis of small bowel obstruction relies on clinical assessment supported by laboratory and imaging findings. Laboratory studies reveal hemoconcentration from dehydration, electrolyte abnormalities (particularly hypokalemia and hypochloremia from vomiting), and elevated lactate or white blood cell count with strangulation. Plain abdominal radiographs show dilated small bowel loops with air-fluid levels, though they cannot reliably distinguish complete from partial obstruction or identify the transition point. CT scan with oral and intravenous contrast is the imaging modality of choice, providing information about the presence of obstruction, level and cause of obstruction, identification of a transition point, presence of closed-loop obstruction, and signs of bowel compromise including wall thickening, poor enhancement, pneumatosis, and mesenteric haziness.

Initial management of small bowel obstruction involves resuscitation, decompression, and determination of the need for operative intervention. Fluid resuscitation corrects the significant hypovolemia from third-spacing and vomiting, typically requiring several liters of crystalloid. Nasogastric decompression relieves proximal dilation and reduces the risk of aspiration. Electrolyte abnormalities are corrected. Nonoperative management with "drip and suck" is appropriate for partial adhesive small bowel obstruction in patients without peritonitis, with resolution occurring in 60 to 80 percent within 48 to 72 hours. Indications for surgery include complete obstruction, closed-loop obstruction, strangulation (fever, tachycardia, peritonitis, elevated lactate), hernia as the cause, failure of nonoperative management, or first presentation of SBO without prior surgery suggesting a mechanical cause requiring diagnosis.

<image>Panel A: Causes of small bowel obstruction comparing adhesive bands, incarcerated hernia, and malignant stricture with respective radiographic appearances. Panel B: CT scan demonstrating small bowel obstruction with dilated proximal loops, decompressed distal bowel, and transition point identified by arrow. Panel C: Signs of strangulation on CT including bowel wall thickening, poor enhancement, mesenteric haziness, and pneumatosis. Panel D: Management algorithm for adhesive small bowel obstruction showing pathway from initial resuscitation through nonoperative trial versus operative indications.</image>

IV. Operative Management of Small Bowel Obstruction

The decision to operate for small bowel obstruction must weigh the risks of continued nonoperative management against operative morbidity, with several factors guiding this decision. Absolute indications for surgery include peritonitis from perforation or strangulation, hemodynamic instability, closed-loop obstruction on imaging, and obstruction caused by incarcerated hernia. Relative indications include failure to improve after 48 to 72 hours of nonoperative management, complete obstruction (no air in colon), and virgin abdomen without prior surgery suggesting a cause requiring surgical diagnosis. Water-soluble contrast challenge (Gastrografin) can help predict which patients with partial obstruction will resolve nonoperatively; appearance of contrast in the colon within 24 hours indicates high likelihood of resolution without surgery.

Operative approach to adhesive small bowel obstruction involves systematic exploration and adhesiolysis with the goal of relieving obstruction while minimizing bowel injury. The abdomen is entered through or near the previous incision, with careful dissection to enter the peritoneal cavity safely. The entire small bowel is run from the ligament of Treitz to the terminal ileum, identifying and lysing all adhesions. The transition point where dilated proximal bowel meets decompressed distal bowel is identified and the obstructing adhesive band divided. Bowel viability is assessed after lysis; questionable segments should be observed for return of color and peristalsis, with Doppler assessment of mesenteric flow if needed. Non-viable bowel requires resection with primary anastomosis in most cases.

Laparoscopic management of small bowel obstruction has gained acceptance for selected cases, offering the potential benefits of reduced wound complications, faster recovery, and fewer subsequent adhesions. Ideal candidates have limited prior surgery (one or two previous operations), a clear transition point on imaging suggesting a single adhesive band, partial rather than complete obstruction, and absence of peritonitis. Technical considerations include safe entry technique using open (Hasson) approach or optical trocar in a location away from previous incisions, gentle manipulation of dilated fragile bowel, adequate visualization of the transition point, and low threshold for conversion if visualization is poor or multiple dense adhesions are encountered.

Bowel resection is required when non-viable bowel is identified or when the cause of obstruction requires resection, such as tumor or stricture. The mesentery is divided close to the bowel wall for benign disease or at the root for malignancy. The bowel is divided with a stapler or sharply between clamps. Primary anastomosis is performed in most cases, either hand-sewn (single or double layer) or stapled (functional end-to-end). Factors favoring diversion with delayed anastomosis or stoma creation include peritonitis with gross contamination, hemodynamic instability, malnutrition, and immunosuppression. The anastomosis should be tension-free with adequate blood supply, and the mesenteric defect should be closed to prevent internal herniation.

<image>Panel A: Intraoperative photograph showing adhesive band causing small bowel obstruction with dilated proximal and decompressed distal bowel. Panel B: Technique for running the bowel from ligament of Treitz to identify transition point and perform systematic adhesiolysis. Panel C: Assessment of bowel viability after adhesiolysis showing viable pink bowel with peristalsis versus questionable dusky segment. Panel D: Stapled functional end-to-end anastomosis technique following small bowel resection.</image>

V. Crohn's Disease

Crohn's disease is a chronic inflammatory bowel disease characterized by transmural inflammation that can affect any portion of the gastrointestinal tract from mouth to anus, with the terminal ileum and colon being most commonly involved. The etiology involves complex interactions between genetic susceptibility, environmental triggers, intestinal microbiome alterations, and dysregulated immune responses. The disease characteristically demonstrates skip lesions with intervening normal bowel, transmural inflammation leading to fistulae and strictures, and a tendency for recurrence even after surgical resection. Unlike ulcerative colitis, which is limited to the mucosa and can be cured by proctocolectomy, Crohn's disease has no surgical cure, and the goal of surgery is to manage complications while preserving bowel length.

Clinical manifestations of Crohn's disease vary depending on the location and behavior of disease. Ileitis presents with right lower quadrant pain, diarrhea, and weight loss, and may mimic acute appendicitis on initial presentation. Colonic disease causes diarrhea (often bloody), abdominal cramping, and urgency. Perianal disease affects approximately one-third of patients and includes fissures (often lateral and multiple, unlike typical midline fissures), fistulae, and abscesses. Disease behavior is classified as inflammatory (mucosal disease), stricturing (fibrostenotic narrowing causing obstruction), or penetrating (fistulae and abscesses). Extraintestinal manifestations affect multiple organ systems including joints (arthritis), skin (erythema nodosum, pyoderma gangrenosum), eyes (uveitis), and liver (primary sclerosing cholangitis).

Diagnosis of Crohn's disease requires integration of clinical, endoscopic, radiologic, and histologic findings. Ileocolonoscopy with biopsies is the primary diagnostic modality, revealing patchy erythema, aphthous ulcers progressing to linear ulcers with a cobblestone appearance, and stenosis. Histology shows transmural inflammation, non-caseating granulomas (pathognomonic but present in only 30 percent of biopsies), and architectural distortion. Cross-sectional imaging with CT or MR enterography evaluates small bowel disease, identifies complications including abscess, fistula, and stricture, and provides information about disease activity through bowel wall enhancement and thickening. Video capsule endoscopy allows direct visualization of small bowel mucosa but should not be used in patients with strictures due to risk of capsule retention.

Medical management of Crohn's disease aims to induce and maintain remission, with treatment selection based on disease severity, location, and behavior. Corticosteroids are effective for inducing remission but not for maintenance due to toxicity with prolonged use. Immunomodulators including azathioprine, 6-mercaptopurine, and methotrexate are used for maintenance. Biologic agents have revolutionized treatment, with anti-TNF agents (infliximab, adalimumab) effective for both induction and maintenance, and newer agents including vedolizumab (anti-integrin) and ustekinumab (anti-IL-12/23). Nutritional support is important, as malnutrition is common due to decreased intake, malabsorption, and increased metabolic demands. Smoking cessation is strongly emphasized as smoking significantly worsens disease course and increases surgical recurrence.

<image>Panel A: Endoscopic appearance of Crohn's disease showing aphthous ulcers, deep linear ulcers, and cobblestone mucosa. Panel B: CT enterography demonstrating terminal ileal wall thickening, mesenteric fat stranding, and complications including abscess and fistula. Panel C: Gross pathology specimen showing transmural inflammation, fat wrapping, stricture formation, and skip lesions. Panel D: Disease behavior classification comparing inflammatory, stricturing, and penetrating phenotypes with clinical implications.</image>

VI. Surgical Management of Crohn's Disease

Approximately 70 to 80 percent of patients with Crohn's disease will require surgery at some point during their lifetime, though the threshold for surgery has evolved with effective medical therapies. Indications for surgery include failure of medical management with persistent symptoms despite adequate therapy, stricturing disease causing obstruction not amenable to endoscopic dilation, penetrating disease with abscess or fistula, fulminant colitis or toxic megacolon, cancer or dysplasia, and growth retardation in pediatric patients. Surgery should be viewed as part of multimodality management rather than a last resort, and earlier surgical intervention for localized disease may be appropriate in carefully selected patients.

Preoperative optimization is crucial for achieving good surgical outcomes in Crohn's disease patients who often present malnourished and on immunosuppressive medications. Nutritional status should be optimized, with preoperative enteral nutrition for 2 to 4 weeks in severely malnourished patients improving anastomotic healing. Abscesses should be drained percutaneously if possible prior to elective surgery to convert a septic operation to a clean one. Steroids should be weaned when feasible, though this is not always possible. The timing of surgery relative to biologic therapy is debated, with some evidence suggesting increased infectious complications within 4 to 8 weeks of anti-TNF therapy, though recent data are conflicting. Cross-sectional imaging provides a roadmap of disease extent and complications to guide surgical planning.

Surgical principles in Crohn's disease emphasize bowel preservation and limited resection, as the disease will recur and reoperation may be necessary. Resection should be limited to grossly diseased bowel causing symptoms, with margins of only 2 centimeters required (microscopic disease at margins does not affect recurrence rates). Strictureplasty is an alternative to resection for short strictures, particularly in patients with extensive disease or prior resections, and includes Heineke-Mikulicz (for strictures less than 10 cm), Finney (10-20 cm), and Michelassi side-to-side isoperistaltic strictureplasty (greater than 20 cm) techniques. Primary anastomosis is appropriate in most cases, with a stapled side-to-side configuration preferred by many surgeons as it may have lower recurrence rates than hand-sewn end-to-end anastomosis.

Specific surgical scenarios in Crohn's disease require tailored approaches. Acute ileitis discovered during appendectomy should prompt careful inspection; if the cecum and appendiceal base are healthy, appendectomy is appropriate to prevent diagnostic confusion in the future. Inflammatory masses in the right lower quadrant may require ileocecal resection once optimized. Enterocutaneous fistulae are managed with nutritional support and sepsis control, with resection of the diseased bowel segment when fit for surgery. Enteroenteric fistulae typically require resection of the diseased segment with simple closure of the secondarily involved loop if it is otherwise healthy. Perianal disease is managed with drainage of abscesses, seton placement for complex fistulae, and medical therapy, with definitive fistula repair reserved for patients with controlled luminal disease.

<image>Panel A: Surgical decision-making algorithm for Crohn's disease showing indications for surgery and choice between resection versus strictureplasty. Panel B: Strictureplasty techniques comparing Heineke-Mikulicz for short strictures, Finney for intermediate, and Michelassi for long strictures. Panel C: Ileocolic resection specimen with gross pathology showing typical Crohn's features including fat wrapping, stricture, and fistula tract. Panel D: Management of enterocutaneous fistula showing staged approach from sepsis control through nutritional optimization to definitive surgery.</image>

VII. Small Bowel Tumors

Small bowel tumors are uncommon, representing only 3 to 6 percent of gastrointestinal tumors despite the small intestine comprising 75 percent of alimentary tract length. Neuroendocrine tumors (carcinoids) are the most common primary small bowel malignancy, followed by adenocarcinoma, lymphoma, and gastrointestinal stromal tumors. Benign tumors include adenomas, lipomas, hemangiomas, and hamartomas associated with Peutz-Jeghers syndrome. The relative rarity of small bowel tumors compared to colorectal cancer may relate to rapid transit time reducing carcinogen exposure, liquid contents causing less mechanical irritation, lower bacterial counts than the colon, and robust mucosal immunity including secretory IgA.

Small bowel carcinoid tumors are well-differentiated neuroendocrine tumors arising from enterochromaffin cells, most commonly located in the distal ileum and often presenting with multiple primaries. Symptoms include vague abdominal pain, intermittent obstruction from the desmoplastic reaction the tumor induces in the mesentery, and gastrointestinal bleeding. Carcinoid syndrome, characterized by flushing, diarrhea, wheezing, and right-sided valvular heart disease, occurs when serotonin and other vasoactive substances reach the systemic circulation, which requires hepatic metastases or extraintestinal primary since the liver normally metabolizes these substances. Treatment for localized disease is surgical resection with wide mesenteric excision to encompass involved lymph nodes. Metastatic disease is managed with somatostatin analogs, cytoreductive surgery when feasible, and liver-directed therapies.

Small bowel adenocarcinoma has similar risk factors to colorectal cancer and is associated with Crohn's disease, celiac disease, familial adenomatous polyposis, and Lynch syndrome. The duodenum is the most common site, followed by the jejunum and ileum. Presentation is often late with obstruction, bleeding, or perforation. Staging and prognosis are similar to colorectal cancer, with node-positive disease having significantly worse outcomes. Surgical treatment involves segmental resection with regional lymphadenectomy for jejunal and ileal tumors, while duodenal tumors may require pancreaticoduodenectomy if involving the ampulla or periampullary region. Adjuvant chemotherapy with FOLFOX is recommended for node-positive disease based on extrapolation from colorectal cancer data.

Gastrointestinal stromal tumors of the small bowel are the second most common site after the stomach, arising from the interstitial cells of Cajal. Small bowel GISTs have higher malignant potential than gastric GISTs at equivalent size and mitotic rate. Presentation includes bleeding, obstruction, or incidental discovery. Treatment is surgical resection with negative margins; unlike gastric cancer, wide margins and lymphadenectomy are not required. Adjuvant imatinib is recommended for high-risk tumors based on size, mitotic rate, and location. Unresectable or metastatic disease is treated with imatinib as first-line therapy, with second-line options including sunitinib and regorafenib for resistant disease.

<image>Panel A: CT appearance comparing small bowel carcinoid with desmoplastic mesenteric mass, adenocarcinoma with apple-core stricture, and GIST with smooth submucosal mass. Panel B: Carcinoid tumor specimen showing multiple small bowel primaries with mesenteric nodal involvement and retractile mesentery. Panel C: Capsule endoscopy images of small bowel tumors showing submucosal lesion, ulcerated adenocarcinoma, and bleeding angiodysplasia. Panel D: Risk stratification for small bowel GIST showing higher malignant potential compared to gastric GIST at equivalent size.</image>

VIII. Mesenteric Ischemia

Mesenteric ischemia results from inadequate blood flow to meet intestinal metabolic demands and can be classified as acute or chronic, arterial or venous. Acute mesenteric ischemia is a surgical emergency with mortality rates of 60 to 80 percent despite modern treatment. Causes include superior mesenteric artery embolism (accounting for approximately 50 percent, typically from cardiac source), SMA thrombosis (20 to 25 percent, occurring on pre-existing atherosclerotic stenosis), non-occlusive mesenteric ischemia (NOMI, 20 percent, from low-flow states and vasoconstriction), and mesenteric venous thrombosis (5 to 10 percent). The jejunum and ileum are most vulnerable, while the duodenum is relatively protected by its dual blood supply.

Clinical presentation of acute mesenteric ischemia classically features pain out of proportion to physical examination findings, though this early presentation is often not recognized. Patients experience severe periumbilical pain with a relatively benign abdomen initially, as ischemia affects the visceral afferent fibers before peritoneal signs develop from transmural necrosis. Risk factors for arterial embolism include atrial fibrillation, recent myocardial infarction, and cardiac valve disease. Risk factors for thrombosis include known peripheral vascular disease and prior mesenteric symptoms suggesting chronic mesenteric ischemia. As ischemia progresses to infarction, patients develop bloody diarrhea, peritoneal signs, and systemic inflammatory response with acidosis and multiorgan failure.

Diagnosis of acute mesenteric ischemia requires a high index of suspicion, as laboratory findings are nonspecific until late in the course. Lactate elevation suggests tissue hypoperfusion but is a late finding. Leukocytosis, hemoconcentration, and metabolic acidosis develop with progression. CT angiography is the diagnostic study of choice, demonstrating arterial occlusion (embolus as meniscus sign, thrombosis with calcified atherosclerotic disease), bowel wall changes (thickening, pneumatosis, absence of enhancement), portal venous gas, and mesenteric haziness. Plain radiographs may show thumb-printing (edematous bowel wall) or pneumatosis intestinalis late in the course. In patients with peritonitis, immediate surgery is indicated without delay for imaging.

Treatment of acute mesenteric ischemia depends on the etiology and extent of bowel involvement. Fluid resuscitation, broad-spectrum antibiotics, and anticoagulation (to prevent thrombus propagation) are initiated immediately. For SMA embolism without peritonitis, interventional approaches including catheter-directed thrombolysis and aspiration embolectomy may be attempted. For SMA thrombosis, endovascular stenting may restore flow if the patient is stable. Patients with peritonitis require exploratory laparotomy for bowel assessment, resection of frankly necrotic bowel, and arterial revascularization (open embolectomy, thrombectomy, or bypass) when feasible. Second-look laparotomy at 24 to 48 hours is often planned to reassess bowel viability after revascularization, as the demarcation between viable and nonviable bowel becomes clearer over time.

<image>Panel A: CT angiography demonstrating SMA embolus with abrupt cutoff and meniscus sign versus SMA thrombosis with calcified atherosclerotic disease at the origin. Panel B: CT findings of mesenteric ischemia showing bowel wall thickening, pneumatosis intestinalis, portal venous gas, and mesenteric haziness. Panel C: Intraoperative appearance of ischemic bowel showing spectrum from viable (pink) through questionable (dusky) to necrotic (black/green). Panel D: Treatment algorithm for acute mesenteric ischemia based on etiology and presence of peritonitis.</image>

IX. Chronic Mesenteric Ischemia and Venous Thrombosis

Chronic mesenteric ischemia, also known as intestinal angina, results from atherosclerotic stenosis of the mesenteric vessels, typically requiring involvement of at least two of the three main vessels (celiac, SMA, IMA) due to extensive collateralization. The classic presentation is postprandial abdominal pain occurring 15 to 30 minutes after eating and lasting 1 to 2 hours, leading to "food fear" and weight loss as patients reduce oral intake to avoid pain. Physical examination is often unremarkable except for evidence of malnutrition and potentially an abdominal bruit. Risk factors are those for atherosclerotic disease including smoking, hyperlipidemia, hypertension, and diabetes. The diagnosis requires a high index of suspicion in patients with unexplained weight loss and postprandial pain, particularly those with known vascular disease.

Diagnostic evaluation of chronic mesenteric ischemia demonstrates vascular stenosis or occlusion in the appropriate clinical context. CT angiography or MR angiography reveals stenosis or occlusion of at least two mesenteric vessels, typically at their origins where atherosclerosis predominates. Duplex ultrasonography can measure flow velocities in the celiac and SMA, with elevated velocities indicating stenosis, though this is operator-dependent and limited by bowel gas. Catheter angiography is reserved for cases where intervention is planned. Importantly, asymptomatic mesenteric stenosis is common and does not require treatment; intervention is indicated only for symptomatic disease.

Treatment of chronic mesenteric ischemia aims to restore adequate intestinal blood flow to relieve symptoms and prevent progression to acute ischemia. Endovascular therapy with angioplasty and stenting has become first-line treatment due to lower morbidity compared to open surgery, with technical success rates exceeding 90 percent. The SMA is the most important vessel to revascularize. Limitations include restenosis rates of 20 to 40 percent requiring reintervention. Open surgical revascularization options include antegrade or retrograde bypass to the SMA and celiac artery, transaortic endarterectomy, and reimplantation. Open surgery has superior patency but higher perioperative morbidity, making it appropriate for endovascular failures or patients with anatomy unsuitable for stenting.

Mesenteric venous thrombosis is a less common cause of intestinal ischemia that may present acutely, subacutely, or chronically. Risk factors include hypercoagulable states (both inherited such as Factor V Leiden and acquired such as malignancy), portal hypertension, abdominal infection, and recent abdominal surgery. Symptoms are often more indolent than arterial ischemia, with abdominal pain present for days to weeks before presentation. CT demonstrates thrombus in the superior mesenteric vein or portal vein, often with bowel wall thickening. Treatment is primarily anticoagulation with heparin followed by long-term warfarin, achieving resolution in most cases. Surgery is reserved for peritonitis indicating bowel infarction, with resection of necrotic bowel and continued anticoagulation postoperatively.

<image>Panel A: CT angiography showing severe stenosis at the origins of the celiac trunk and superior mesenteric artery with post-stenotic dilation. Panel B: Comparison of endovascular stenting technique for SMA stenosis versus open surgical bypass from the aorta. Panel C: CT demonstrating mesenteric venous thrombosis with filling defect in SMV and portal vein with associated bowel wall thickening. Panel D: Treatment algorithm for mesenteric venous thrombosis showing anticoagulation as primary therapy with surgery reserved for peritonitis.</image>

X. Short Bowel Syndrome

Short bowel syndrome is a malabsorptive condition resulting from extensive small bowel resection or disease, defined functionally by the inability to maintain fluid, electrolyte, and nutritional balance on a normal diet. Anatomic definitions vary, but generally less than 200 centimeters of residual small bowel is considered short bowel, with less than 100 centimeters indicating severe short bowel requiring parenteral nutrition. Causes include massive resection for mesenteric ischemia, multiple resections for Crohn's disease, trauma, radiation enteritis, and necrotizing enterocolitis in neonates. The clinical impact depends on the length of remaining bowel, which segments are preserved (jejunum versus ileum), presence or absence of the ileocecal valve and colon, and remaining bowel health.

The physiologic consequences of extensive small bowel resection reflect the loss of absorptive surface area and loss of specialized regional functions. Fluid and electrolyte losses are often massive in the early postoperative period, with high-output stomal losses potentially exceeding 3 to 4 liters daily. Loss of the terminal ileum eliminates the specific site of vitamin B12 absorption (requiring lifelong parenteral supplementation) and bile acid absorption (causing bile acid diarrhea and steatorrhea from depleted bile acid pool). Loss of the ileocecal valve allows bacterial reflux from the colon with small intestinal bacterial overgrowth. Loss of intestinal hormones disrupts the normal feedback mechanisms regulating gastric acid secretion and gastric emptying, with gastric hypersecretion common in the early period.

Management of short bowel syndrome proceeds through three phases: acute, adaptation, and maintenance. The acute phase lasts weeks to months after resection and requires parenteral nutrition and careful management of fluid and electrolyte losses. The adaptation phase extends over 1 to 2 years as remaining bowel undergoes structural and functional changes including villous hypertrophy, increased crypt depth, and upregulated transporter expression. Enteral nutrition is essential for stimulating adaptation and should be started as soon as feasible. Dietary modifications include small frequent meals, limiting simple sugars, separating fluids from solids, and oral rehydration solutions rather than hypotonic fluids. Pharmacologic therapies include anti-diarrheal agents, antisecretory medications (PPI, H2 blockers, octreotide), and bile acid sequestrants (cholestyramine).

Patients with severe short bowel syndrome who cannot achieve nutritional autonomy require long-term parenteral nutrition (PN), with associated complications including catheter-related infections, catheter thrombosis, and intestinal failure-associated liver disease. Teduglutide, a glucagon-like peptide-2 analog, promotes intestinal adaptation and can reduce parenteral nutrition requirements in some patients. Surgical options for short bowel syndrome include longitudinal intestinal lengthening and tailoring (Bianchi procedure), serial transverse enteroplasty (STEP) to increase functional bowel length, and intestinal transplantation for patients with life-threatening PN complications or PN failure. Intestinal transplantation outcomes have improved significantly, with 5-year survival approaching 70 percent at experienced centers, though lifelong immunosuppression is required.

<image>Panel A: Anatomic factors determining severity of short bowel syndrome comparing jejunum-colon anatomy versus end-jejunostomy and their clinical implications. Panel B: Timeline of intestinal adaptation showing structural changes including villous hypertrophy and crypt deepening. Panel C: Dietary and pharmacologic management strategies for different phases of short bowel syndrome. Panel D: Surgical options for short bowel syndrome including serial transverse enteroplasty (STEP) technique and intestinal transplantation.</image>

Summary

• Small bowel anatomy: duodenum, jejunum, ileum; blood supply from SMA with arcades and vasa recta
• Small bowel obstruction: adhesions most common cause; CT to evaluate; nonoperative management for partial SBO without signs of strangulation
• Indications for surgery: complete obstruction, closed-loop, strangulation, hernia, failure of nonoperative management
• Crohn's disease: transmural inflammation, skip lesions, strictures, fistulae; surgery for complications; bowel-preserving techniques
• Strictureplasty: bowel-sparing alternative to resection for Crohn's strictures
• Small bowel tumors: carcinoid most common malignancy; adenocarcinoma, GIST, lymphoma also occur
• Acute mesenteric ischemia: surgical emergency; pain out of proportion to exam; CT angiography to diagnose; revascularize and resect necrotic bowel
• Chronic mesenteric ischemia: postprandial pain with weight loss; endovascular stenting first-line treatment
• Short bowel syndrome: malabsorption after extensive resection; intestinal adaptation over 1-2 years; parenteral nutrition for severe cases
• Terminal ileum: specific site for B12 and bile acid absorption; loss requires B12 supplementation

Key Terms

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