Developmental anomalies and vascular disorders of the GI tract

A) Developmental Disorders

1. Atresia (GI tract)

• Complete failure of development of the intestinal lumen
• Most common: duodenal atresia

2. Stenosis

• Narrowing of the intestinal lumen with incomplete obstruction
• Normally no lumen at start of embryonic period → apoptosis creates it; if apoptosis is defective → stenosis

3. Duplication

• Well-formed saccular to tubular cystic structures; may or may not communicate with the lumen of the small intestine

4. Diverticulosis of the Colon

• Herniation of colonic mucosa and submucosa through the intestinal muscular wall → cystic expansion in adventitial tissue
• Diverticula develop at sites of muscle wall weakness, secondary to increased colon pressure from a low-fiber diet
• Complication: hemorrhage from the diverticula

5. Meckel Diverticulum — most common and most harmless anomaly

• Failure of involution of the vitelline duct → persistent blind-ended tubular protrusion in the ileum (proximal to ileocecal valve)
• Complication: bacterial overgrowth → depletion of vitamin B12 → megaloblastic anemia

6. Omphalocele

• Congenital defect of periumbilical abdominal musculature → membranous sac into which intestines herniate

7. Gastroschisis

• Congenital hole in the abdominal wall beside the belly button → intestines (and sometimes other organs) exit through the hole

8. Malrotation

• Malrotation of the developing bowel → intestines do not assume normal intra-abdominal positions (e.g. cecum may end up anywhere in abdomen)

9. Hirschsprung Disease (Congenital Megacolon)

• Distention of colon >6–7 cm in diameter (megacolon); congenital or acquired
• Migration of neural crest-derived cells along alimentary tract arrests before reaching the anus
• → aganglionic segment (lacks Meissner submucosal + Auerbach myenteric plexuses) → ↓ peristalsis → functional obstruction → colon distention
• Morphology: wall upstream may be thinned by distention or thickened by compensatory muscle hypertrophy
• Complications: superimposed enterocolitis, perforation of distended portion

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B) Vascular Disorders

Ischemic Bowel Disease

• Ischemic lesion may affect small intestine, large intestine, or both
• Acute occlusion of one of the 3 major intestinal arteries (celiac, superior/inferior mesenteric) → infarction of extensive intestinal segments
• Slowly progressing obstruction may be asymptomatic due to rich anastomoses
• Obstruction within end-arteries → small focal ischemic lesions

3 Forms of Infarction:

1. Transmural infarction
   • Involves all visceral layers
   • Caused by acute occlusion of a major mesenteric artery
   • Dark red hemorrhagic appearance (blood re-flows into damaged area)
   • Ischemic injury begins in mucosa → extends outward; within 24h: fibrinous exudate over serosa
   • Clinical: sudden abdominal pain + bloody diarrhea → shock + vascular collapse; high mortality
2. Mural infarction
   • Involves mucosa and submucosa
3. Mucosal infarction
   • Does not extend deeper than muscularis mucosa
   • Mural + mucosal: from physiological hypoperfusion or anatomic defects; multi-focal lesions interspersed with spared areas
   • Clinical: unexplained abdominal distention or GI bleeding; not fatal if perfusion re-established

Predisposing factors:

• Arterial thrombosis: severe atherosclerosis, vasculitis, dissecting aneurysm, hypercoagulable states, oral contraceptives
• Arterial embolism: cardiac vegetations (infective endocarditis), MI + mural thrombosis, aortic atheroembolism
• Venous thrombosis: hypercoagulable states, vascular invasive neoplasms, cirrhosis

Angiodysplasia

• Tortuous dilations of submucosal and mucosal blood vessels; usually in cecum or ascending colon
• Prone to rupture and bleeding → chronic intermittent hemorrhage → severe anemia
• Occur after the 6th decade of life
• Mechanism: penetrating veins are occluded intermittently during peristalsis, but thicker-walled arteries remain patent → venous distention

Hemorrhoids

• Variceal dilations of the anal and perianal submucosal plexuses
• Common after age 50; from persistently elevated venous pressure in hemorrhoidal plexus
• Causes: chronic constipation, venous stasis in pregnancy, liver cirrhosis (portal HTN)
• Internal hemorrhoids: inside rectum; dilation of superior and middle hemorrhoidal veins
• External hemorrhoids: outside distal anal canal; dilation of inferior hemorrhoidal veins
• Complications: frequent bleeding, thrombosis, prolapse → trapped by anal sphincter → painful edematous hemorrhagic enlargement

💡 High-yield: Hirschsprung = neural crest arrest + aganglionic segment → megacolon. Meckel = vitelline duct remnant; ileum; vitamin B12 depletion. Omphalocele = periumbilical defect + membranous sac. Transmural infarction = all layers; hemorrhagic; acute occlusion; high mortality. Angiodysplasia = cecum/ascending colon; elderly; chronic GI bleeding. Hemorrhoids: internal = superior/middle veins; external = inferior veins.