Medical School · Year 2 · Gastrointestinal · includes a quiz and discussion video

Lecture 7: Small Intestine Disorders

Unit 2.2: Gastrointestinal System

Learning Objectives

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

1. Describe celiac disease pathophysiology, diagnosis, and management
2. Explain small intestinal bacterial overgrowth (SIBO)
3. Describe small bowel obstruction causes and management
4. Explain carcinoid tumors and carcinoid syndrome
5. Describe Meckel's diverticulum and other congenital anomalies
6. Explain the approach to chronic diarrhea

Lecture Content

Celiac Disease Overview

Celiac disease is an immune-mediated enteropathy triggered by ingestion of gluten in genetically susceptible individuals. Gluten refers to the storage proteins found in wheat (gliadin), barley (hordein), and rye (secalin). This condition affects approximately 1% of the population worldwide but remains significantly underdiagnosed because many patients have subtle or atypical presentations.

Genetic susceptibility is essential—virtually all patients with celiac disease carry HLA-DQ2 (approximately 95%) or HLA-DQ8 (approximately 5%). However, these alleles are common in the general population (30-40% of Caucasians), so their presence is necessary but not sufficient for disease development. Environmental factors, including the timing of gluten introduction in infancy, intestinal infections, and microbiome composition, influence whether genetically susceptible individuals develop clinical disease.

The pathophysiology begins when gliadin peptides cross the intestinal epithelium, either through transcellular transport or through disrupted tight junctions. In the lamina propria, tissue transglutaminase (tTG) deamidates specific glutamine residues in gliadin, creating negatively charged glutamic acid residues. These deamidated gliadin peptides bind with high affinity to HLA-DQ2 or HLA-DQ8 molecules on antigen-presenting cells. Presentation to CD4+ T cells triggers an adaptive immune response with cytokine release (interferon-gamma, IL-15, IL-21) that damages the intestinal epithelium. The result is villous atrophy, crypt hyperplasia, and infiltration of the lamina propria and epithelium with lymphocytes.

Clinical manifestations are highly variable. Classic GI symptoms include chronic diarrhea, steatorrhea, bloating, flatulence, and weight loss from malabsorption. However, many patients present with extraintestinal manifestations. Iron deficiency anemia (from duodenal iron malabsorption) is the most common presentation in adults. Dermatitis herpetiformis is a pruritic, blistering skin eruption on extensor surfaces that is considered pathognomonic for celiac disease. Bone disease (osteopenia, osteoporosis, osteomalacia) results from calcium and vitamin D malabsorption. Neurologic manifestations include peripheral neuropathy and cerebellar ataxia. Reproductive problems include infertility, recurrent miscarriage, and delayed puberty.

<image>Panel A: Genetic basis showing HLA-DQ2 and HLA-DQ8 molecules on chromosome 6, noted as necessary but not sufficient for disease development, present in 30-40% of Caucasians. Panel B: Immunologic pathway showing gliadin crossing the epithelium, tissue transglutaminase deamidating gliadin, deamidated gliadin binding HLA-DQ2 on antigen-presenting cells, presentation to CD4+ T cells, and cytokine release of interferon-gamma, IL-15, and IL-21 causing epithelial damage. Panel C: Histologic changes comparing normal villi to celiac villi with villous atrophy, crypt hyperplasia with elongated crypts, and increased intraepithelial lymphocytes exceeding 25 per 100 enterocytes. Panel D: Clinical manifestations organized by system including GI symptoms of diarrhea, steatorrhea, and bloating, hematologic presentation with iron deficiency anemia as the most common adult finding, dermatitis herpetiformis with blisters on extensor surfaces, bone disease with osteoporosis, neurologic manifestations of neuropathy and ataxia, and reproductive problems including infertility.</image>

Celiac Disease Diagnosis and Management

Diagnosis of celiac disease requires serologic testing followed by histologic confirmation, with the patient maintaining a gluten-containing diet throughout evaluation.

Serologic testing begins with IgA tissue transglutaminase (tTG-IgA), which is highly sensitive (>95%) and specific (>95%) for celiac disease. Total serum IgA should be measured simultaneously because IgA deficiency (present in 2-3% of celiac patients, ten times the general population rate) causes false-negative IgA-based tests. IgA endomysial antibodies (EMA) are highly specific and can be used to confirm positive tTG results. In IgA-deficient patients, IgG-based tests (deamidated gliadin peptide IgG, tTG-IgG) should be used.

Intestinal biopsy remains the gold standard for diagnosis. Multiple biopsies (at least 4-6) should be taken from the duodenal bulb and descending duodenum because villous atrophy can be patchy. The Marsh classification grades histologic severity: Marsh 0 is normal; Marsh 1 shows increased intraepithelial lymphocytes (>25 per 100 enterocytes); Marsh 2 adds crypt hyperplasia; Marsh 3 includes villous atrophy (3a partial, 3b subtotal, 3c total). Marsh 2-3 findings support the diagnosis in conjunction with positive serology.

HLA testing has a limited but specific role. Negative HLA-DQ2 and HLA-DQ8 testing essentially excludes celiac disease and is useful when the diagnosis is uncertain (equivocal serology, patient already on a gluten-free diet). However, positive HLA testing does not confirm celiac disease because these alleles are common.

Treatment is a strict, lifelong gluten-free diet (GFD). Patients must avoid all wheat, barley, and rye products. Oats are controversial but appear safe for most patients if uncontaminated. The threshold for safe gluten intake is less than 20 parts per million, the limit for "gluten-free" labeling. Safe alternatives include rice, corn, potato, quinoa, buckwheat, and certified gluten-free products. A dietitian referral is essential for education, as gluten is ubiquitous in processed foods.

Monitoring includes periodic serology (tTG-IgA should decline and normalize on GFD), symptom assessment, and consideration of repeat biopsy to confirm mucosal healing. Persistent symptoms despite dietary compliance should prompt evaluation for refractory celiac disease, inadvertent gluten exposure, or alternative diagnoses (SIBO, microscopic colitis, pancreatic insufficiency).

Complications of uncontrolled or refractory disease include enteropathy-associated T-cell lymphoma (EATL), small bowel adenocarcinoma, ulcerative jejunitis, and refractory celiac disease (Type I with normal intraepithelial lymphocyte phenotype has good prognosis; Type II with aberrant lymphocytes has poor prognosis and may progress to lymphoma).

<image>Panel A: Diagnostic algorithm starting with symptoms or risk factors leading to tTG-IgA plus total serum IgA testing, with IgG-based tests used if IgA deficient, and positive results prompting upper endoscopy with multiple duodenal biopsies graded by the Marsh classification from normal through 3c, with diagnosis confirmed at Marsh 2-3 with positive serology. Panel B: HLA testing showing that negative HLA-DQ2 and DQ8 essentially excludes celiac disease, useful when diagnosis is uncertain or the patient is already on a gluten-free diet. Panel C: Treatment with a strict lifelong gluten-free diet avoiding wheat, barley, and rye with safe alternatives including rice, corn, and potato, a threshold below 20 parts per million, and essential dietitian referral for education. Panel D: Monitoring with declining tTG-IgA levels on the gluten-free diet shown as a graph, repeat biopsy consideration to confirm mucosal healing, evaluation of persistent symptoms for SIBO, microscopic colitis, or inadvertent gluten exposure, and complications including enteropathy-associated T-cell lymphoma, adenocarcinoma, and refractory celiac disease Types I and II.</image>

Small Intestinal Bacterial Overgrowth

Small intestinal bacterial overgrowth (SIBO) occurs when excessive bacteria colonize the normally near-sterile small intestine. The traditional definition uses a bacterial count exceeding 10⁵ colony-forming units per milliliter on jejunal aspirate culture, though more recent definitions suggest thresholds as low as 10³ CFU/mL may be clinically relevant.

Several mechanisms normally prevent bacterial overgrowth. Gastric acid kills ingested bacteria. The migrating motor complex (MMC) sweeps bacteria distally during fasting. The ileocecal valve prevents colonic bacterial reflux. Intestinal immunoglobulins (particularly secretory IgA) suppress bacterial growth. Bile has bacteriostatic properties.

Risk factors for SIBO disrupt these protective mechanisms. Anatomic abnormalities include surgical blind loops (after Billroth II or Roux-en-Y procedures), strictures, diverticula, and small bowel fistulas. Motility disorders such as diabetic enteropathy, scleroderma, and chronic intestinal pseudo-obstruction allow bacterial stasis. Hypochlorhydria from chronic PPI use, atrophic gastritis, or gastric surgery permits bacterial survival. Immune deficiency (IgA deficiency, HIV) impairs local defense. Other conditions include cirrhosis, chronic pancreatitis, and advanced age.

Symptoms result from bacterial competition for nutrients and production of toxic metabolites. Bloating and flatulence are common because bacterial fermentation of carbohydrates produces gas (hydrogen, methane, CO2). Diarrhea occurs because deconjugated bile acids irritate the colonic mucosa and cause secretory diarrhea. Steatorrhea develops because deconjugated bile acids cannot form micelles effectively, impairing fat absorption. Vitamin B12 deficiency is characteristic because bacteria avidly consume dietary B12. Paradoxically, folate may be elevated because bacteria synthesize folate. Weight loss results from malabsorption.

Diagnosis is challenging because the gold standard (jejunal aspirate and culture) is invasive and not widely available. Breath testing is the practical alternative. Patients ingest glucose or lactulose; bacteria metabolize these substrates, producing hydrogen or methane that is absorbed and exhaled. A rise in breath hydrogen (or methane) indicates bacterial fermentation. Glucose hydrogen breath testing is more specific but may miss distal SIBO; lactulose testing is more sensitive but less specific. Given test limitations, empiric antibiotic therapy is often appropriate when clinical suspicion is high.

Treatment uses antibiotics to reduce bacterial load. Rifaximin is preferred because it is non-absorbed, concentrates in the gut, has broad-spectrum activity, and has minimal resistance development. Alternatives include metronidazole, ciprofloxacin, amoxicillin-clavulanate, or doxycycline. A typical course lasts 7-14 days. Addressing underlying causes (prokinetics for dysmotility, surgical correction of blind loops) helps prevent recurrence. Recurrence is common, and some patients require cyclic antibiotic therapy. Dietary approaches including low-FODMAP diets may reduce symptoms.

<image>Panel A: Normal protective mechanisms preventing small intestinal bacterial overgrowth including gastric acid killing ingested bacteria, the migrating motor complex sweeping bacteria distally during fasting, the ileocecal valve preventing colonic bacterial reflux, and secretory IgA suppressing bacterial growth. Panel B: Risk factors disrupting protective mechanisms including anatomic abnormalities such as surgical blind loops, strictures, and diverticula, motility disorders from diabetic autonomic neuropathy and scleroderma, hypochlorhydria from chronic PPI use, and immune deficiency including IgA deficiency. Panel C: Symptoms with underlying mechanisms showing bloating and gas from bacterial fermentation of carbohydrates producing hydrogen and methane, diarrhea from deconjugated bile acids, steatorrhea from micelle formation failure, low vitamin B12 from bacterial consumption, and paradoxically elevated folate from bacterial synthesis. Panel D: Diagnosis with breath testing apparatus and graph showing a rise in hydrogen or methane after substrate ingestion, and treatment with rifaximin as the preferred antibiotic, addressing the underlying cause, and preventing recurrence with cyclic antibiotics or dietary approaches.</image>

Small Bowel Obstruction

Small bowel obstruction (SBO) is a common surgical emergency resulting from mechanical blockage of intestinal flow. Understanding the causes, presentation, and management principles is essential.

Etiology varies by patient history. In developed countries, adhesions from prior abdominal surgery are the dominant cause (60-70% of cases), typically developing months to years after the original procedure. Hernias (inguinal, femoral, incisional, internal) cause 10-15% and may present as the first episode without prior surgery. Malignancy (primary small bowel tumors or metastatic disease, particularly peritoneal carcinomatosis) accounts for 10-15%. Crohn's disease causes strictures that may obstruct. Other causes include volvulus, intussusception, gallstone ileus (a gallstone eroding through the gallbladder into the duodenum and impacting in the ileum), and foreign bodies.

Pathophysiology follows a predictable sequence. Obstruction prevents aboral passage of intestinal contents. Proximal to the obstruction, the bowel dilates as gas and fluid accumulate. Normally, the small intestine secretes and reabsorbs several liters of fluid daily; obstruction blocks reabsorption, causing fluid sequestration in the dilated loops (third-spacing). Increased intraluminal pressure eventually compromises mucosal blood flow. If the mesenteric vessels are compromised (strangulation), ischemia progresses to necrosis and perforation. Closed-loop obstruction (obstruction at two points, as in volvulus or an incarcerated hernia) is particularly dangerous because intraluminal pressure rises rapidly.

Clinical presentation includes the cardinal features. Colicky abdominal pain reflects peristaltic waves attempting to overcome the obstruction; it becomes constant if ischemia develops. Vomiting occurs early with proximal obstruction (often bilious) and later with distal obstruction (may be feculent). Abdominal distension develops as gas and fluid accumulate. Obstipation (absence of flatus and stool) is a late finding and indicates complete obstruction; continued passage suggests partial obstruction. Physical examination reveals a distended, tympanitic abdomen with high-pitched bowel sounds early that diminish as ileus sets in. Tenderness, peritoneal signs, fever, and tachycardia raise concern for strangulation.

Diagnosis relies on imaging. Abdominal X-ray shows dilated small bowel loops (>3 cm) with air-fluid levels on upright films and minimal colonic gas. CT with contrast is the gold standard, identifying the transition point (where dilated bowel meets decompressed bowel), the cause of obstruction, and signs of strangulation (bowel wall thickening, mesenteric haziness, decreased wall enhancement, free fluid). Laboratory findings may include leukocytosis, elevated lactate (concerning for ischemia), and electrolyte abnormalities.

<image>Panel A: Etiology pie chart showing adhesions from prior surgery at 60-70% as the dominant cause, hernias at 10-15%, malignancy at 10-15%, Crohn's disease causing strictures at 5%, and other causes including volvulus, intussusception, and gallstone ileus. Panel B: Pathophysiology sequence from obstruction to proximal bowel dilation, fluid sequestration from blocked reabsorption, increased intraluminal pressure, mucosal ischemia, and progression to strangulation and necrosis, with closed-loop obstruction from volvulus noted as particularly dangerous. Panel C: Clinical features including colicky abdominal pain from peristaltic waves attempting to overcome the obstruction, vomiting that is bilious early and feculent late, abdominal distension, and obstipation with absence of flatus and stool indicating complete obstruction. Panel D: Imaging showing abdominal X-ray with dilated small bowel loops and air-fluid levels, CT scan identifying the transition point where dilated bowel meets decompressed bowel, and findings concerning for strangulation including bowel wall thickening, free fluid, and decreased wall enhancement.</image>

Small Bowel Obstruction Management

Management of SBO requires determining whether operative or non-operative treatment is appropriate.

Initial management for all patients includes bowel rest (NPO), nasogastric tube placement for decompression (relieves vomiting and reduces aspiration risk), IV fluid resuscitation (significant third-spacing requires aggressive replacement), and electrolyte correction. Serial abdominal examinations assess for deterioration.

Indications for urgent surgical intervention include signs of strangulation or perforation (peritonitis, fever, tachycardia, elevated lactate, sepsis), complete obstruction that does not resolve, closed-loop obstruction, incarcerated or strangulated hernia, and CT findings concerning for ischemia.

Non-operative management is appropriate for partial SBO (passage of some flatus or stool), adhesive SBO without signs of strangulation, and patients who have responded to non-operative management in the past. Water-soluble contrast (Gastrografin) administered via NG tube can be both diagnostic and therapeutic: if contrast reaches the colon within 24 hours, resolution is likely and surgery is probably unnecessary; if contrast does not progress, surgery is typically needed. Gastrografin's hyperosmolar nature draws fluid into the lumen, which may help relieve partial obstruction.

The duration of non-operative management is debated but generally should not exceed 48-72 hours without evidence of improvement. Prolonged conservative management risks ischemia developing unrecognized. Close monitoring is essential.

Surgical options depend on the cause. Adhesiolysis (division of adhesions) is performed for adhesive SBO. Hernia repair addresses incarcerated hernias. Bowel resection is necessary for necrotic segments, tumors, or strictures not amenable to strictureplasty. Strictureplasty preserves bowel length in Crohn's disease by widening the strictured segment without resection. Intestinal bypass is occasionally performed for unresectable obstruction.

Prevention of recurrent adhesive SBO includes minimizing peritoneal trauma, using laparoscopic techniques when possible, and applying adhesion barriers (though evidence for these is mixed).

<image>Panel A: Initial management for all patients including bowel rest with NPO status, nasogastric tube placement for decompression, IV fluid resuscitation for third-space losses, electrolyte correction, and serial abdominal examinations to assess for deterioration. Panel B: Indications for urgent surgical intervention including signs of strangulation with peritonitis, fever, and elevated lactate, complete obstruction that does not resolve, closed-loop obstruction, and incarcerated or strangulated hernia, with surgical options of adhesiolysis, hernia repair, bowel resection for necrotic segments, and strictureplasty. Panel C: Non-operative management pathway for partial SBO and adhesive SBO without strangulation, including the Gastrografin trial where contrast reaching the colon within 24 hours predicts resolution while failure to progress indicates the need for surgery, with a maximum 48-72 hours without improvement. Panel D: Prevention of recurrent adhesive SBO through minimizing peritoneal trauma, using laparoscopic techniques when possible, and applying adhesion barriers.</image>

Carcinoid Tumors and Syndrome

Small bowel neuroendocrine tumors (NETs), traditionally called carcinoids, arise from enterochromaffin cells and are the most common primary malignancy of the small intestine. The ileum is the most frequent small bowel location. These tumors produce serotonin and other bioactive peptides.

Most small bowel carcinoids are slow-growing but have malignant potential. At diagnosis, about 80% have already metastasized to regional lymph nodes or liver because they are often asymptomatic until advanced. Local symptoms include vague abdominal pain, intermittent obstruction (the tumor and its desmoplastic reaction can kink the bowel), and GI bleeding.

Carcinoid syndrome develops only when tumor products reach the systemic circulation, which typically requires hepatic metastases. Normally, serotonin and other peptides released into the portal circulation are metabolized during first-pass hepatic metabolism. With liver metastases (or with tumors draining outside the portal system, such as ovarian carcinoids), vasoactive substances bypass hepatic degradation and cause systemic effects.

The syndrome's manifestations reflect serotonin and other peptide actions. Flushing is the most common symptom—episodic reddening of the face and upper body lasting minutes to hours, often triggered by alcohol, stress, or certain foods. Secretory diarrhea results from serotonin's effects on intestinal motility and secretion. Wheezing and bronchospasm occur in some patients. Carcinoid heart disease affects the right heart: serotonin causes fibrotic thickening of the tricuspid and pulmonic valves, leading to regurgitation and stenosis. The left heart is typically spared because the lungs metabolize serotonin. Carcinoid crisis—life-threatening hypotension, bronchospasm, and flushing—can be precipitated by anesthesia, surgery, or tumor manipulation.

Diagnosis relies on demonstrating elevated serotonin metabolites and localizing the tumor. 24-hour urine 5-hydroxyindoleacetic acid (5-HIAA), the major serotonin metabolite, is more than 95% specific when elevated. Chromogranin A is a useful serum tumor marker. Imaging includes CT or MRI for anatomic assessment, somatostatin receptor scintigraphy (Octreoscan), and Gallium-68 DOTATATE PET-CT, which is highly sensitive for somatostatin receptor-positive tumors.

Treatment is multimodal. Surgical resection is curative for localized disease and may be palliative for hepatic metastases (debulking, ablation, embolization). Somatostatin analogs (octreotide, lanreotide) are the mainstay of symptom control—they inhibit hormone release and also have antiproliferative effects. Peptide receptor radionuclide therapy (PRRT) using Lutetium-177 DOTATATE delivers targeted radiation to somatostatin receptor-positive tumor cells. Telotristat inhibits tryptophan hydroxylase (the rate-limiting enzyme in serotonin synthesis) and is used for diarrhea refractory to somatostatin analogs. Everolimus (an mTOR inhibitor) is approved for progressive disease.

<image>Panel A: Tumor location with the ileum highlighted as the most common small bowel site and enterochromaffin cell of origin illustrated, alongside the requirement for carcinoid syndrome showing that normally serotonin is metabolized by hepatic first-pass but with liver metastases it reaches the systemic circulation. Panel B: Carcinoid syndrome manifestations including episodic flushing of the face and upper body, secretory diarrhea, wheezing and bronchospasm, and carcinoid heart disease with right-sided valve thickening causing tricuspid regurgitation and pulmonic stenosis while the left heart is spared because the lungs metabolize serotonin, with carcinoid crisis from anesthesia or surgery as a warning. Panel C: Diagnostic workup showing 24-hour urine 5-HIAA as greater than 95% specific, chromogranin A as a serum tumor marker, and Gallium-68 DOTATATE PET-CT imaging showing liver metastases as highly sensitive for somatostatin receptor-positive tumors. Panel D: Treatment pyramid with surgical resection curative for localized disease, somatostatin analogs octreotide and lanreotide for symptom control and antiproliferative effect, peptide receptor radionuclide therapy with Lutetium-177 DOTATATE, telotristat for diarrhea refractory to somatostatin analogs, and everolimus for progressive disease.</image>

Other Small Bowel Tumors

Small bowel malignancies are rare, accounting for only 3-5% of GI tract cancers, despite the small intestine representing 75% of GI tract length and 90% of mucosal surface area.

Small bowel adenocarcinoma is the most common primary malignancy of the small bowel but is still rare. The duodenum is the most frequent site, with decreasing incidence distally. Risk factors include celiac disease (threefold increased risk even with a gluten-free diet), Crohn's disease (particularly in bypassed loops), familial adenomatous polyposis (duodenal adenomas progress to cancer), Lynch syndrome, and Peutz-Jeghers syndrome. Presentation includes obstruction, bleeding, pain, or incidental discovery. Treatment is surgical resection; chemotherapy (typically FOLFOX) is used for advanced disease. Prognosis is generally poor, with 5-year survival around 30%.

Lymphoma in the small intestine includes several types. Diffuse large B-cell lymphoma is the most common primary intestinal lymphoma. MALT lymphoma can occur in the small bowel. Enteropathy-associated T-cell lymphoma (EATL) specifically associates with celiac disease—particularly refractory celiac disease Type II—and carries a dismal prognosis. Mantle cell lymphoma causes lymphomatous polyposis with multiple polyps. Treatment is typically chemotherapy; surgery is reserved for complications.

GISTs (gastrointestinal stromal tumors) occur in the small intestine (second most common site after stomach). They arise from interstitial cells of Cajal and are characterized by KIT or PDGFRA mutations. Treatment is surgical resection for localized disease and imatinib for unresectable or metastatic tumors.

Metastatic disease to the small bowel is more common than primary malignancy. Melanoma has a particular predilection for small bowel metastases—any patient with melanoma presenting with GI bleeding or obstruction should be evaluated for small bowel involvement. Lung and breast carcinomas also metastasize to the small bowel.

<image>Panel A: Small bowel adenocarcinoma with the duodenum as the most frequent site, risk factors including celiac disease, Crohn's disease, familial adenomatous polyposis, Lynch syndrome, and Peutz-Jeghers syndrome, presentation with obstruction and bleeding, treatment with surgical resection and FOLFOX chemotherapy, and poor prognosis with 5-year survival around 30%. Panel B: Small bowel lymphoma types including diffuse large B-cell lymphoma as the most common, MALT lymphoma, enteropathy-associated T-cell lymphoma with celiac disease association and poor prognosis, and mantle cell lymphoma causing lymphomatous polyposis, with chemotherapy as the treatment mainstay. Panel C: Small intestinal gastrointestinal stromal tumors as the second most common GIST site after the stomach, characterized by KIT or PDGFRA mutations, treated with surgical resection for localized disease and imatinib for unresectable or metastatic tumors. Panel D: Metastatic disease to the small bowel as more common than primary tumors, with melanoma having particular predilection for small bowel metastases shown as multiple lesions, along with lung and breast carcinoma metastases.</image>

Meckel's Diverticulum

Meckel's diverticulum is the most common congenital anomaly of the GI tract, resulting from incomplete obliteration of the vitelline (omphalomesenteric) duct. It is classically described by the "rule of 2s": present in 2% of the population, located within 2 feet (60 cm) of the ileocecal valve, approximately 2 inches (5 cm) long, twice as common in males symptomatically, most commonly presents before age 2, and contains 2 types of ectopic tissue.

As a true diverticulum, Meckel's contains all layers of the intestinal wall (unlike false or pulsion diverticula). The clinical significance stems from the presence of ectopic tissue in approximately 50% of cases—most commonly gastric mucosa, which secretes acid, and less commonly pancreatic tissue. This ectopic gastric mucosa can cause acid-mediated ulceration of adjacent ileal mucosa.

Most Meckel's diverticula are asymptomatic and discovered incidentally. Complications occur in approximately 4% of cases. Hemorrhage is the most common presentation in children: ectopic gastric mucosa produces acid that ulcerates the adjacent ileal mucosa, causing painless lower GI bleeding. The blood is typically brick-red or maroon because it originates in the distal small bowel. Adults are more likely to present with obstruction (from intussusception with the diverticulum as a lead point, volvulus around a persistent fibrous band to the umbilicus, or adhesions) or diverticulitis (inflammation mimicking appendicitis but located in the midabdomen or left lower quadrant).

Diagnosis is challenging. The Meckel's scan (Technetium-99m pertechnetate scintigraphy) detects ectopic gastric mucosa because pertechnetate is taken up by gastric parietal cells. It is approximately 85% sensitive in children but less reliable in adults. CT may identify the diverticulum but can be falsely negative. Video capsule endoscopy and deep enteroscopy may visualize the lesion. Many cases are diagnosed only at surgery for unexplained GI bleeding or obstruction.

Treatment is surgical resection of the diverticulum and adjacent ileal segment for symptomatic cases. Management of incidentally discovered Meckel's is controversial: some surgeons recommend prophylactic resection in young patients, especially if the diverticulum appears to contain ectopic tissue or has a narrow base, while others favor observation given the low complication rate.

<image>Panel A: Embryology showing the vitelline duct that normally obliterates, with persistence leaving Meckel's diverticulum as a true diverticulum containing all intestinal wall layers on the antimesenteric border of the ileum within 2 feet of the ileocecal valve. Panel B: The rule of 2s with 2% prevalence, 2 feet from the ileocecal valve, 2 inches long, twice as commonly symptomatic in males, most commonly presenting before age 2, and containing 2 types of ectopic tissue, with gastric mucosa as the most common producing acid that ulcerates adjacent ileal mucosa causing painless GI bleeding with brick-red blood. Panel C: Complications by age showing children most commonly presenting with hemorrhage from ectopic gastric mucosa, and adults more likely presenting with obstruction from intussusception or volvulus, or diverticulitis mimicking appendicitis. Panel D: Diagnosis with Meckel's scan using Technetium-99m pertechnetate showing uptake at the diverticulum and stomach with approximately 85% sensitivity in children, CT that may miss the diagnosis, and surgery as often the definitive diagnostic modality, with treatment being surgical resection for symptomatic cases.</image>

Short Bowel Syndrome

Short bowel syndrome (SBS) is intestinal failure resulting from surgical resection, congenital defect, or disease-associated loss of absorption, leaving insufficient functional bowel to maintain nutritional and fluid homeostasis without supplementation. It is defined functionally rather than by a specific length, though fewer than 200 cm of remaining small bowel typically requires support.

Common causes include massive resection for mesenteric ischemia (arterial thrombosis or embolism), multiple resections for Crohn's disease, midgut volvulus (particularly in neonates), trauma, and radiation enteritis. Congenital short bowel occurs rarely.

Several factors determine the severity of SBS. The length of remaining bowel is the most important factor—more preserved intestine means better absorptive capacity. Which segment was resected matters: jejunal resection is better tolerated because the ileum can adapt, whereas ileal resection is more consequential because the ileum uniquely absorbs bile acids (leading to bile acid diarrhea if the colon is present or fat malabsorption if absent) and vitamin B12. The presence of the colon in continuity significantly improves outcomes—the colon absorbs water and electrolytes and can salvage energy from malabsorbed carbohydrates through bacterial fermentation to short-chain fatty acids. Preservation of the ileocecal valve slows transit and prevents reflux of colonic bacteria.

Three phases characterize the clinical course. The acute phase (weeks to months) features massive fluid and electrolyte losses, profound diarrhea, and metabolic instability; total parenteral nutrition (TPN) is typically required. The adaptation phase (months to 2 years) sees structural and functional changes that improve absorption: villous hyperplasia, increased crypt depth, intestinal lengthening, and upregulation of transporters. Enteral nutrition strongly stimulates adaptation and should be initiated early. The maintenance phase achieves stability; some patients become independent of parenteral support, while others require lifelong supplementation.

Management involves fluid and electrolyte replacement, TPN for those unable to maintain nutrition enterally, early and progressive enteral feeding (to stimulate adaptation), antidiarrheal agents (loperamide, diphenoxylate), PPI therapy (gastric hypersecretion occurs after resection and contributes to diarrhea), and bile acid sequestrants (cholestyramine) if the colon is present and bile acid diarrhea contributes. Teduglutide, a GLP-2 analog, promotes intestinal adaptation by increasing villous height, crypt depth, and mesenteric blood flow; it can reduce or eliminate parenteral support requirements in some patients. Intestinal transplantation is reserved for patients with intestinal failure-associated liver disease or inability to maintain vascular access for TPN.

<image>Panel A: Causes of short bowel syndrome including mesenteric ischemia with an occluded artery, Crohn's disease with multiple resections, midgut volvulus, and trauma, with fewer than 200 cm of remaining small bowel typically requiring nutritional support. Panel B: Factors affecting severity including length of remaining bowel, segment resected with ileal loss worse than jejunal because the ileum uniquely absorbs bile acids and vitamin B12, presence of the colon in continuity for water absorption and fermentation of carbohydrates to short-chain fatty acids, and preservation of the ileocecal valve to slow transit. Panel C: Three clinical phases showing the acute phase with massive fluid losses and TPN required, the adaptation phase with enteral feeding stimulating villous hypertrophy, increased crypt depth, and transporter upregulation, and the maintenance phase where some patients achieve enteral autonomy. Panel D: Treatment options including fluid and electrolyte replacement, TPN, early enteral nutrition to stimulate adaptation, antidiarrheal agents, PPI therapy, bile acid sequestrants, teduglutide as a GLP-2 analog promoting intestinal adaptation, and intestinal transplantation for liver failure or loss of vascular access.</image>

Approach to Chronic Diarrhea

Chronic diarrhea is defined as loose or watery stools lasting more than 4 weeks. A systematic approach based on pathophysiology guides efficient diagnosis.

Classification by mechanism identifies four categories. Osmotic diarrhea results from non-absorbed solutes drawing water into the lumen. Secretory diarrhea results from active secretion of fluid exceeding absorption. Fatty (steatorrhea) diarrhea results from fat malabsorption. Inflammatory diarrhea results from mucosal damage with exudation of blood and inflammatory cells.

Key features distinguish these categories. Osmotic diarrhea stops with fasting because the osmotically active substrate is no longer ingested; stool has an increased osmotic gap (>125 mOsm/kg). Secretory diarrhea continues despite fasting; stool osmotic gap is normal (<50 mOsm/kg). Steatorrhea presents with bulky, oily, malodorous stools that float. Inflammatory diarrhea produces bloody stools with mucus and is associated with elevated fecal calprotectin or lactoferrin.

The stool osmotic gap is calculated as: 290 − 2 × (stool Na⁺ + stool K⁺). A gap exceeding 125 mOsm/kg suggests osmotic diarrhea; a gap under 50 mOsm/kg suggests secretory diarrhea.

Common causes vary by mechanism. Osmotic diarrhea: lactose intolerance (most common worldwide), sorbitol or fructose malabsorption, magnesium-containing antacids or laxatives. Secretory diarrhea: bile acid diarrhea (ileal resection or dysfunction), microscopic colitis, neuroendocrine tumors (carcinoid, VIPoma), certain infections. Malabsorptive/fatty diarrhea: celiac disease, chronic pancreatitis, SIBO, intestinal lymphangiectasia. Inflammatory diarrhea: inflammatory bowel disease (Crohn's, UC), infectious colitis, ischemic colitis.

Initial workup includes complete blood count (anemia suggests malabsorption or blood loss), comprehensive metabolic panel (electrolyte derangement, hypoalbuminemia), inflammatory markers (CRP, ESR), celiac serology (tTG-IgA), thyroid function (hyperthyroidism causes diarrhea), and stool studies (fecal calprotectin, fecal fat, ova and parasites, Clostridioides difficile). If inflammatory markers are elevated, colonoscopy with biopsies is indicated. If malabsorption is suspected, evaluate for celiac disease, pancreatic insufficiency, and SIBO. For persistent unexplained secretory diarrhea, consider microscopic colitis (requires colonoscopy with biopsies even if mucosa appears normal), bile acid malabsorption (SeHCAT test or empiric trial of bile acid sequestrant), and rare causes (hormone-secreting tumors).

<image>Panel A: Definition of chronic diarrhea as loose stools for more than 4 weeks, with the stool osmotic gap formula of 290 minus 2 times the sum of stool sodium and potassium, distinguishing osmotic diarrhea that stops with fasting and has a gap exceeding 125 mOsm/kg from secretory diarrhea that persists with fasting and has a gap under 50 mOsm/kg. Panel B: Four mechanism-based classifications showing osmotic diarrhea from lactose intolerance, sorbitol, and magnesium, secretory diarrhea from bile acid malabsorption, microscopic colitis, and carcinoid, fatty or malabsorptive diarrhea with oily floating stools from celiac disease, chronic pancreatitis, and SIBO, and inflammatory diarrhea with blood, mucus, and elevated calprotectin from IBD and infection. Panel C: Initial workup including CBC, comprehensive metabolic panel, CRP, celiac serology with tTG-IgA, thyroid function, and stool studies including fecal calprotectin, fecal fat, ova and parasites, and C. difficile testing. Panel D: Further evaluation with colonoscopy with biopsies if inflammatory markers are elevated or diarrhea is unexplained, breath testing for SIBO, SeHCAT test or empiric bile acid sequestrant trial for bile acid malabsorption, and evaluation for rare causes including hormone-secreting tumors.</image>

Summary

• Celiac disease: Gluten-triggered; tTG-IgA testing; villous atrophy; gluten-free diet
• SIBO: Excess small bowel bacteria; breath testing; rifaximin treatment
• SBO: Adhesions most common; NG decompression; surgery for complete/strangulated
• Carcinoid: Ileal NETs; syndrome with hepatic metastases; octreotide for symptoms
• Meckel's: Rule of 2s; gastric ectopia → bleeding; Meckel scan
• Short bowel: Malabsorption; adaptation occurs; teduglutide aids
• Chronic diarrhea: Classify by type (osmotic, secretory, fatty, inflammatory)

Key Terms

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