Small Bowel Disorders
Surgery · GI Surgery · lean revision notes
Small Bowel Disorders
A high-yield cluster for NEET PG GI surgery, knitting together embryology, vascular anatomy and operative judgement. The favourites are Meckel's diverticulum, paediatric intussusception, acute mesenteric ischaemia, short bowel syndrome and small-bowel carcinoid — each a reliable source of case-based MCQs.
Overview & why it is tested
The small intestine (duodenum, jejunum, ileum) spans roughly 6 metres and is the principal site of digestion and absorption. Surgically it is "forgiving" tissue with a rich blood supply and good healing, yet it harbours a handful of classic disorders that combine embryology, imaging and a clear drug/operation of choice — exactly the recipe examiners like. Master the eponyms, cut-offs and named criteria below and you will clear most stems.
Classification of small bowel disorders
| Category | Representative conditions | Core NEET PG hook |
|---|---|---|
| Congenital / embryological | Meckel's diverticulum, duplication cysts, malrotation | Vitellointestinal duct, rule of twos |
| Mechanical obstruction | Adhesions (commonest cause overall), intussusception, hernia, gallstone ileus | Air-fluid levels, target sign |
| Vascular | Acute mesenteric ischaemia (embolic/thrombotic/NOMI), mesenteric venous thrombosis | Pain out of proportion, lactate |
| Functional / metabolic | Short bowel syndrome | <200 cm residual length, TPN |
| Neoplastic | Carcinoid (NET), adenocarcinoma, GIST, lymphoma | Serotonin, 5-HIAA, octreotide |
| Inflammatory / infective | Crohn's disease, intestinal TB, typhoid perforation | Skip lesions vs transverse ulcers |
High-yield: Adhesions (post-operative) are the commonest cause of small bowel obstruction in adults; hernia is the commonest cause in patients who have never had abdominal surgery; intussusception is the commonest cause in children 3 months–2 years.
Meckel's diverticulum
Definition & embryology
Meckel's diverticulum is the persistence of the vitellointestinal (omphalomesenteric) duct. It is a true diverticulum — it contains all layers of the bowel wall — arising from the antimesenteric border of the ileum. It is the commonest congenital anomaly of the GI tract.
The rule of twos (must memorise)
- 2% of the population
- 2 feet (≈60 cm) proximal to the ileocaecal valve
- 2 inches (≈5 cm) long
- 2 types of heterotopic mucosa: gastric (commonest) and pancreatic
- 2:1 male predominance for complications
- presents commonly by 2 years of age
- 2% become symptomatic
Clinical features
Most are silent. When symptomatic:
- Children: painless lower GI bleeding (brick-red/maroon stools) from peptic ulceration of ectopic gastric mucosa — the commonest symptomatic presentation in children.
- Adults: obstruction (commonest complication in adults — via volvulus around a band, intussusception, or Littre's hernia) and diverticulitis mimicking appendicitis.
High-yield: A Meckel's diverticulum containing the appendix-like inflammation that mimics appendicitis is classic; a Meckel's in a hernial sac is a Littre's hernia.
Diagnosis — investigation of choice
- Technetium-99m pertechnetate scan ("Meckel's scan") is the investigation of choice in a bleeding child — the isotope is taken up by ectopic gastric mucosa. Pre-treatment with pentagastrin, cimetidine or glucagon increases sensitivity by enhancing/retaining uptake.
- In obstruction, CT/contrast studies are used; the diagnosis is often intra-operative.
Management
- Symptomatic: diverticulectomy or segmental small-bowel resection (resection preferred if the diverticulum is broad-based, the base is inflamed, or palpable ectopic tissue is at the base).
- Incidental (asymptomatic) Meckel's: controversial. Resect in young patients, narrow-necked/long diverticula, palpable ectopic mucosa, or fibrous bands — otherwise leave alone in older asymptomatic patients.
Intussusception
Definition
Telescoping of one segment of bowel (the intussusceptum) into the lumen of the adjacent distal segment (the intussuscipiens), dragging the mesentery and causing venous congestion → oedema → ischaemia.
Epidemiology & etiology
- Peak age 3 months–2 years; idiopathic in children (often follows a viral illness/Peyer's patch hypertrophy; rotavirus vaccine association historically).
- The commonest type is ileocolic.
- In adults, ~90% have a pathological lead point (tumour, e.g. lipoma, GIST, metastasis; or Meckel's) — adult intussusception is treated by resection without reduction.
Clinical features — classic triad
Colicky abdominal pain (drawing up legs) → "red-currant jelly" stools → palpable sausage-shaped mass (usually right upper quadrant, with emptiness in the RIF = Dance's sign). The infant is often pale and lethargic between spasms.
Diagnosis — investigation of choice
- Ultrasound is the investigation of choice: target / doughnut sign (transverse) and pseudokidney sign (longitudinal).
- Plain film may show paucity of gas in the RIF.
Management — flow
Resuscitate (IV fluids, NG decompression) → confirm on USG → if stable & no peritonitis: non-operative reduction (pneumatic/air enema preferred, or hydrostatic/contrast enema under fluoroscopy/USG) → if reduction fails, peritonitis, perforation or shock: surgery (manual reduction; resect if non-viable or a lead point is found).
High-yield: Air (pneumatic) enema is both diagnostic and therapeutic and is the preferred reduction technique (success ~80–90%). Contraindications to enema reduction: peritonitis, perforation, free air, and haemodynamic instability/shock.
Acute mesenteric ischaemia (AMI)
Definition & types
Sudden compromise of intestinal blood flow, usually involving the superior mesenteric artery (SMA) territory. A true surgical emergency with high mortality.
| Type | Frequency | Key feature |
|---|---|---|
| SMA embolism | Commonest (~50%) | AF / recent MI; embolus lodges distal to the middle colic artery (spares proximal jejunum) |
| SMA thrombosis | ~25% | Pre-existing atherosclerosis; preceding "intestinal angina"/weight loss |
| Non-occlusive (NOMI) | ~20% | Low-flow state (shock, digoxin, vasopressors, dialysis); patent vessels |
| Mesenteric venous thrombosis | ~10% | Hypercoagulable states; more indolent onset |
Clinical features
- Pain out of proportion to physical findings is the hallmark (early, severe, poorly localised pain with a soft, non-tender abdomen).
- Later: peritonism, bloody diarrhoea, distension, shock — these signal infarction.
Diagnosis — investigation of choice
- CT angiography (CTA) is the investigation of choice.
- Conventional/catheter mesenteric angiography is the gold standard and allows intra-arterial therapy.
- Labs: raised lactate (sensitive marker of bowel ischaemia), metabolic acidosis, leucocytosis, raised amylase.
High-yield: "Pain out of proportion to examination" + AF/recent MI → think SMA embolism. Serum lactate elevation is the most useful biochemical clue to intestinal infarction.
Management — flow
Resuscitate + broad-spectrum antibiotics + systemic anticoagulation (heparin) → CTA → if no peritonitis: endovascular therapy (embolectomy/thrombolysis/stent) → if peritonitis or non-viable bowel: emergency laparotomy with embolectomy/revascularisation + resection of dead bowel → second-look laparotomy at 24–48 h to reassess borderline bowel. NOMI is managed by correcting the precipitating cause and intra-arterial papaverine (a vasodilator).
Short bowel syndrome (SBS)
Definition
Malabsorption resulting from loss of functional small-bowel length, classically when < 200 cm of viable small bowel remains. Commonest causes in adults: mesenteric ischaemia, Crohn's disease, radiation, volvulus, trauma; in neonates: necrotising enterocolitis, gastroschisis, atresia.
Pathophysiology & determinants of outcome
- Presence of an intact ileocaecal valve improves prognosis (slows transit, limits bacterial overgrowth).
- Ileal resection is worse tolerated than jejunal: ileum is the site of vitamin B12 and bile-salt absorption → B12 deficiency, bile-salt diarrhoea, and steatorrhoea with fat-soluble vitamin (ADEK) loss.
- Excess unabsorbed fatty acids bind calcium, freeing oxalate for absorption → enteric hyperoxaluria → calcium oxalate renal stones; loss of bile salts predisposes to cholesterol gallstones.
- Gastric acid hypersecretion occurs early.
Phases & management
- Acute phase: total parenteral nutrition (TPN), fluid/electrolyte correction, PPI for hypersecretion.
- Adaptation (up to 1–2 years): progressive enteral feeding to stimulate mucosal hyperplasia; antidiarrhoeals (loperamide), bile-acid binders (cholestyramine) if colon intact, B12 supplementation.
- Maintenance: wean from TPN if possible; teduglutide (GLP-2 analogue) promotes intestinal adaptation; surgery (STEP/Bianchi lengthening procedures; intestinal transplantation) for TPN failure.
High-yield: Preserving the ileocaecal valve and even a short length of colon dramatically improves the chance of weaning off TPN. Teduglutide (GLP-2 analogue) is the recognised drug to enhance adaptation.
Carcinoid (neuroendocrine) tumour of the small bowel
Definition & origin
Arise from enterochromaffin (Kulchitsky) cells. The appendix is the commonest GI site overall, but the ileum is the commonest small-bowel site and the one most likely to metastasise and cause the syndrome. They are argentaffin/chromogranin-A positive.
Carcinoid syndrome
Occurs when vasoactive mediators (chiefly serotonin) reach the systemic circulation — typically only after liver metastases (because hepatic first-pass metabolism otherwise inactivates serotonin).
- Flushing, diarrhoea, bronchospasm (wheeze), and right-sided valvular heart disease (tricuspid regurgitation/pulmonary stenosis — TIPS: Tricuspid Insufficiency, Pulmonary Stenosis).
- Niacin/tryptophan diversion can cause pellagra.
Diagnosis — investigation of choice
- 24-hour urinary 5-HIAA (5-hydroxyindoleacetic acid) — the biochemical test of choice; serum chromogranin A is a useful tumour marker.
- Localisation: Octreotide scan (somatostatin-receptor scintigraphy) / Ga-68 DOTATATE PET.
Management — drug of choice
- Octreotide (somatostatin analogue) controls symptoms and is given to prevent carcinoid crisis peri-operatively.
- Resection of the primary ± hepatic metastasis debulking; avoid catecholamine/morphine triggers.
High-yield: Carcinoid syndrome implies liver metastases. Urinary 5-HIAA is diagnostic. Cover all interventions with octreotide to prevent carcinoid crisis (profound hypotension/bronchospasm).
Other small-bowel conditions worth knowing
| Condition | Key fact |
|---|---|
| Small-bowel adenocarcinoma | Commonest in duodenum; associated with coeliac disease, Crohn's, FAP |
| GIST | c-KIT (CD117) positive; drug = imatinib; risk by size + mitotic count |
| Crohn's disease | Terminal ileum, skip lesions, transmural, cobblestoning, "string sign" of Kantor; non-caseating granulomas |
| Intestinal TB | Ileocaecal, transverse ("girdle") ulcers, caseating granulomas; Napkin-ring/contracted caecum |
| Gallstone ileus | Elderly; Rigler's triad = pneumobilia + SBO + ectopic gallstone; impaction at terminal ileum |
| Typhoid perforation | Terminal ileum; antimesenteric border |
Key differentials at a glance
- Painless lower GI bleed in a child: Meckel's diverticulum vs juvenile polyp vs intussusception.
- Right iliac fossa pain: appendicitis vs Meckel's diverticulitis vs Crohn's vs ileocaecal TB.
- Severe acute abdominal pain with soft abdomen: mesenteric ischaemia vs pancreatitis vs ruptured AAA.
- Flushing + diarrhoea: carcinoid vs VIPoma vs systemic mastocytosis vs medullary thyroid carcinoma.
Complications summary
- Meckel's: bleeding, obstruction (volvulus/intussusception), diverticulitis, perforation, Littre's hernia, rarely neoplasia.
- Intussusception: ischaemia, perforation, peritonitis, recurrence (higher after enema reduction).
- AMI: transmural infarction, short bowel syndrome, sepsis, death.
- SBS: TPN-related sepsis/cholestasis, gallstones, oxalate renal stones, micronutrient deficiencies.
- Carcinoid: carcinoid heart disease, carcinoid crisis, hepatic metastases.
Recently asked / exam angle
- A toddler with intermittent colicky pain, red-currant jelly stools and a sausage-shaped abdominal mass → diagnosis (intussusception); investigation of choice = USG (target sign); first-line treatment = pneumatic/air enema reduction; contraindication asked = peritonitis/perforation.
- Painless rectal bleeding in a 2-year-old with normal colonoscopy → Meckel's diverticulum; best test = Tc-99m pertechnetate scan; structure responsible = ectopic gastric mucosa; embryological remnant = vitellointestinal duct.
- Elderly AF patient with sudden severe abdominal pain and a soft abdomen → SMA embolism; investigation = CT angiography; biochemical clue = raised lactate.
- Flushing, diarrhoea and right-heart valve disease → carcinoid syndrome; test = urinary 5-HIAA; implication = liver metastases; drug to prevent crisis = octreotide.
- One-liner facts: rule of twos; commonest cause of SBO in adults vs children; Rigler's triad of gallseal ileus; ileocaecal valve significance in SBS; c-KIT for GIST.
- Image-based: target/doughnut sign on USG; pneumobilia in gallstone ileus; "string sign" in Crohn's.
Rapid revision
- Meckel's diverticulum = persistent vitellointestinal duct, a true antimesenteric ileal diverticulum; remember the rule of twos.
- Bleeding Meckel's → Tc-99m pertechnetate scan; bleeding is due to ectopic gastric mucosa.
- Commonest SBO cause: adhesions (adults), intussusception (children), hernia (no prior surgery).
- Intussusception triad: colicky pain + red-currant jelly stool + sausage-shaped mass; USG target sign.
- Treat childhood intussusception with air/pneumatic enema; contraindicated in peritonitis/perforation/shock.
- Adult intussusception → almost always a lead point → resect, do not reduce.
- AMI: pain out of proportion; commonest type = SMA embolism (AF); IOC = CT angiography; marker = lactate.
- NOMI → treat cause + intra-arterial papaverine; do a second-look laparotomy for borderline bowel.
- Short bowel = <200 cm; ileocaecal valve preservation improves prognosis; ileal loss → B12 + bile-salt problems; drug = teduglutide.
- SBS stones: calcium oxalate renal stones (enteric hyperoxaluria) and cholesterol gallstones.
- Carcinoid arises from Kulchitsky/EC cells; syndrome means liver mets; IOC = urinary 5-HIAA; cover with octreotide.
- GIST = c-KIT/CD117 positive → imatinib; gallstone ileus = Rigler's triad; Crohn's = caseation absent, TB = caseation present.