Small Bowel & Malabsorption
Steatorrhea decoded: celiac, sprue, Whipple, SIBO, and carcinoid
Approach to malabsorption
Presentation. Malabsorption produces steatorrhea (bulky, greasy, floating, foul-smelling stools), weight loss, and deficiencies of the fat-soluble vitamins A, D, E, and K, vitamin B12, and iron. Deficiency clues include night blindness (A), osteomalacia and hypocalcemia (D), neuropathy and hemolysis (E), and easy bruising with a prolonged PT (K).
Localize the defect. The key question is whether the mucosa itself is damaged or the mucosa is intact but digestion fails. D-xylose, a monosaccharide absorbed by the proximal small bowel without any pancreatic digestion, answers it: absorption is LOW in mucosal disease (celiac) and NORMAL in pancreatic insufficiency.
| Test | Principle | Abnormal in |
|---|---|---|
| D-xylose absorption | Needs intact mucosa, not pancreatic enzymes | Low in mucosal disease (celiac); normal in pancreatic insufficiency |
| Fecal elastase | Pancreatic enzyme that survives intestinal transit | Low in pancreatic exocrine insufficiency |
| Hydrogen breath test | Bacteria ferment unabsorbed carbohydrate, releasing H2 | Positive in lactose intolerance and SIBO |
| Stool osmotic gap and pH | Unabsorbed sugar holds water; fermentation acidifies stool | ↑ Gap and pH <6 in lactose intolerance |
Celiac disease
Pathogenesis. An immune reaction to gliadin, a gluten protein found in wheat, rye, and barley, in carriers of HLA-DQ2 or DQ8. Tissue transglutaminase deamidates gliadin, which then binds DQ2/DQ8 more avidly and activates CD4+ T cells that damage the mucosa.
Serology. The best screen is anti-tissue transglutaminase (tTG) IgA, supported by anti-endomysial and anti-deamidated gliadin antibodies. Check total IgA first: selective IgA deficiency is associated with celiac and makes IgA-based tests falsely negative, so IgG-based tests are used instead. Confirm with duodenal biopsy.
Histology. Disease is most severe in the duodenum and proximal jejunum: villous atrophy, crypt hyperplasia, and intraepithelial lymphocytosis. The proximal pattern explains why iron deficiency anemia is a common presenting sign.
Dermatitis herpetiformis. The skin sign of celiac: intensely pruritic vesicles on extensor surfaces (elbows, knees) with IgA deposits at the tips of the dermal papillae. It responds to a gluten-free diet and dapsone.
Long-standing celiac disease raises the risk of enteropathy-associated T-cell lymphoma (EATL) and small-bowel adenocarcinoma. A lifelong gluten-free diet reverses the mucosal injury and lowers these risks.
Other causes of malabsorption
Tropical sprue. Histology resembles celiac, but the whole small bowel is involved, including the distal ileum, so both B12 and folate fall. It follows residence in or travel to tropical regions, is probably infectious, and responds to antibiotics (tetracycline) plus folate, not to a gluten-free diet.
Whipple disease. Infection with Tropheryma whipplei. Foamy PAS-positive macrophages stuff the lamina propria and block lymphatic drainage, causing fat malabsorption. The classic patient is an older white man with diarrhea and weight loss plus Arthralgias, Cardiac, and Neurologic involvement. Treat with ceftriaxone followed by long-term TMP-SMX.
Lactose intolerance. Brush-border lactase deficiency leaves lactose unabsorbed, causing osmotic diarrhea, bloating, and flatus after dairy. Villi are normal in the primary form; secondary deficiency follows mucosal injury such as gastroenteritis. The stool osmotic gap is increased, stool pH is low (<6) from bacterial fermentation, and the hydrogen breath test is positive.
Pancreatic exocrine insufficiency. Chronic pancreatitis, cystic fibrosis, or duct obstruction removes the enzymes needed for digestion, causing steatorrhea and fat-soluble vitamin (ADEK) malabsorption. Fecal elastase is low, while D-xylose absorption is normal because the mucosa is intact. B12 can also fall, because pancreatic proteases are needed to free B12 from its salivary binder. Treat with pancreatic enzyme replacement.
Small intestinal bacterial overgrowth (SIBO). Stasis from blind loops, dysmotility, diabetes, or scleroderma lets colonic-type bacteria flourish. They deconjugate bile acids (fat malabsorption) and consume B12 while synthesizing folate, so B12 falls and folate rises. The hydrogen breath test is positive; treat with rifaximin.
| Disorder | Key clue | B12 / folate | Treatment |
|---|---|---|---|
| Celiac disease | Anti-tTG IgA, duodenal villous atrophy, dermatitis herpetiformis | Iron and folate most affected (proximal disease) | Gluten-free diet |
| Tropical sprue | Tropical exposure; entire small bowel | Both low | Tetracycline + folate |
| Whipple disease | PAS-positive foamy macrophages; joints, heart, CNS | Variable | Ceftriaxone → TMP-SMX |
| SIBO | Stasis; positive breath test | B12 low, folate high | Rifaximin |
| Pancreatic insufficiency | Low fecal elastase, normal D-xylose | B12 may be low | Enzyme replacement |
| Lactose intolerance | Symptoms after dairy; low stool pH | Normal | Lactose avoidance |
Carcinoid tumors
The tumor. Carcinoids are well-differentiated neuroendocrine tumors, most often in the small bowel (ileum) and appendix. They form nests of uniform cells that stain for chromogranin A and synaptophysin, and they secrete serotonin.
- Bowel carcinoidSecretes serotonin into portal blood
- Liver MAO clears itFirst-pass metabolism: no syndrome
- Liver metastasesSerotonin drains directly into hepatic veins
- Systemic serotoninFlushing, diarrhea, wheezing
Carcinoid syndrome. Flushing, diarrhea, wheezing, and right-sided valve fibrosis: TIPS, for Tricuspid Insufficiency and Pulmonic Stenosis. The left heart is spared because lung MAO inactivates serotonin first. The syndrome appears only when tumor escapes first-pass hepatic metabolism, through liver metastases or a primary outside portal drainage (e.g., in the lung).
Diagnosis and treatment. Elevated urinary 5-HIAA, the serotonin metabolite, confirms the diagnosis (VMA and metanephrines are pheochromocytoma tests). Tumor diverts dietary tryptophan into serotonin, depleting niacin (B3) and causing pellagra: dermatitis, diarrhea, dementia. Octreotide controls symptoms, and localized tumors are resected.
Intussusception
Telescoping bowel. A proximal segment telescopes into the distal lumen, compressing its own mesenteric vessels. In toddlers it is usually ileocolic, with hypertrophied Peyer patches after a viral infection acting as the lead point. The picture is colicky pain with legs drawn up, currant-jelly stool (mucosal ischemia), and a sausage-shaped abdominal mass; ultrasound shows a target sign. An air or contrast enema is both diagnostic and therapeutic.
Intussusception in an adult implies a pathologic lead point, usually a tumor. Other associations are Meckel diverticulum and Henoch-Schönlein purpura (IgA vasculitis).