Congenital GI Obstruction & Anomalies

Gastric outlet: hypertrophic pyloric stenosis

Lesion and risk. Hypertrophy and hyperplasia of the pyloric circular smooth muscle develop after birth and progressively narrow the gastric outlet. Symptoms begin at 2–6 weeks: forceful, projectile, nonbilious vomiting after feeds in an infant who stays hungry, with visible gastric peristaltic waves and a firm, mobile, olive-shaped epigastric mass. Firstborn males are most affected (about 4:1), family history raises risk, and so does erythromycin or azithromycin in the first 2 weeks of life; erythromycin is a motilin-receptor agonist whose strong antral contractions are thought to drive the hypertrophy.

Metabolic picture. Vomiting gastric HCl leaves a hypochloremic metabolic alkalosis. Volume contraction activates the renin-angiotensin-aldosterone system, and aldosterone reclaims Na+ in exchange for K+ and H+, adding hypokalemia and, late, a paradoxically acidic urine that perpetuates the alkalosis. Acidosis appears only with severe dehydration and shock.

Duodenal and small-bowel obstruction

Duodenal atresia. Early in development the duodenal lumen is plugged by proliferating epithelium and normally recanalizes by about weeks 8–10. Failure produces atresia, stenosis or a membranous web, usually just distal to the ampulla of Vater. The fetus cannot absorb swallowed amniotic fluid, so polyhydramnios is common; about a third of affected infants have trisomy 21, and cardiac defects, annular pancreas and malrotation travel with it. The newborn vomits bile within hours of the first feed, with little distension because the block is proximal, and x-ray shows the "double bubble" of a gas-filled stomach and proximal duodenum with no distal gas. Treatment is duodenoduodenostomy.

Bilious vomiting in a newborn is malrotation with midgut volvulus until proven otherwise. Distal bowel gas does not exclude it, and an urgent upper GI series is needed because the whole midgut is at risk of infarction.

Annular pancreas. The pancreas forms from a dorsal and a ventral bud. The ventral bud, which gives rise to the uncinate process, part of the head and the proximal main pancreatic duct, normally rotates behind the duodenum to fuse with the dorsal bud. If a bifid ventral bud migrates around both sides, a ring of pancreatic tissue encircles the second part of the duodenum and can obstruct it, causing vomiting (bilious or nonbilious, depending on its level relative to the ampulla). It is associated with Down syndrome and duodenal atresia.

Pancreas divisum. If the ventral and dorsal duct systems fail to fuse (normally around week 8), most pancreatic juice drains through the dorsal duct of Santorini and the small minor papilla. It is the most common congenital pancreatic anomaly, present in about 5–10% of people; most are asymptomatic, but relative outflow obstruction can cause recurrent acute pancreatitis.

Jejunoileal atresia. Atresia beyond the duodenum has a different cause: a fetal mesenteric vascular accident (volvulus, intussusception, thrombosis, or vasoconstrictive exposures such as maternal cocaine use or smoking) infarcts a bowel segment, which is then resorbed. In the "apple-peel" variant, a blind proximal jejunum sits above a distal ileum coiled around a single ileocolic vessel. Infants present with bilious vomiting, abdominal distension and multiple dilated loops.

Vitelline duct anomalies

Meckel diverticulum. The vitelline (omphalomesenteric) duct connects the midgut to the yolk sac and normally obliterates early in gestation. Persistence of its intestinal end leaves a true diverticulum, containing all layers of the bowel wall, on the antimesenteric border of the ileum; Zenker and most colonic diverticula, by contrast, are false (mucosa and submucosa only). It is the most common congenital anomaly of the GI tract and follows a "rule of 2s": about 2% of people, about 2 feet from the ileocecal valve, about 2 inches long, 2 types of ectopic tissue (gastric and pancreatic), symptoms often by age 2 and a 2:1 male predominance. Most carriers never have symptoms.

Complications. Ectopic gastric mucosa secretes acid that ulcerates the adjacent ileal mucosa, producing painless, often brisk hematochezia (bright red or maroon) in a toddler. A Tc-99m pertechnetate scan, concentrated by gastric mucosa, localizes the diverticulum, whereas a tagged RBC scan detects active bleeding of any cause; symptomatic diverticula are resected. A Meckel diverticulum can also be the lead point of an intussusception, anchor a volvulus around a fibrous band to the umbilicus, become inflamed (diverticulitis mimicking appendicitis) or lie within an inguinal hernia (Littré hernia).

Other remnants. The duct can persist along its whole length or in part:

• Vitelline fistula: a fully patent duct discharges meconium or stool from the umbilicus.
• Vitelline sinus or cyst: a blind umbilical tract or a cyst along the obliterated duct.
• Fibrous cord: a band from ileum to umbilicus that invites volvulus.
• Contrast: urine leaking from the umbilicus means a patent urachus, the remnant of the allantois.

Distal obstruction: Hirschsprung disease and meconium ileus

Pathogenesis. Neural crest cells migrate craniocaudally along the gut to form the myenteric (Auerbach) and submucosal (Meissner) plexuses. When migration stops early, the rectum and a variable, continuous segment above it lack ganglion cells. The aganglionic segment is tonically contracted and cannot relax or propagate peristalsis, so it acts as a functional obstruction, and normal bowel above it dilates into a megacolon. Loss-of-function RET mutations are the main genetic association, Down syndrome is strongly associated, boys outnumber girls about 4:1, and most cases are short-segment (rectosigmoid).

Presentation. Newborns fail to pass meconium within 48 hours and develop abdominal distension and bilious vomiting; milder cases show chronic constipation from infancy. The rectal vault is empty, and a digital exam releases an explosive gush of stool and gas.

Hirschsprung-associated enterocolitis (fever, distension, foul explosive diarrhea) is the leading cause of death and can progress to toxic megacolon.

Meconium ileus. In cystic fibrosis, absent CFTR function leaves meconium dehydrated, protein-rich and sticky, and it obstructs the terminal ileum at birth in about 15–20% of affected newborns; most infants with meconium ileus have CF. Findings are distension, bilious vomiting, no passage of meconium, dilated loops with a "soap-bubble" right lower quadrant and an unused microcolon on contrast enema. A hyperosmolar (Gastrografin) enema can be diagnostic and therapeutic; volvulus, atresia or perforation with meconium peritonitis and calcifications complicate it. Meconium plug syndrome is a milder colonic obstruction linked to the small left colon of infants of diabetic mothers, and it too prompts testing for CF and Hirschsprung disease.

Anorectal malformations and biliary atresia

Imperforate anus. Abnormal partitioning of the cloaca by the urorectal septum, or failed breakdown of the anal membrane, leaves no normal anal opening. The rectum often ends in a fistula to the perineum, the urethra (meconium in the urine) or the vaginal vestibule. It belongs to the VACTERL association (vertebral, anal, cardiac, tracheoesophageal, renal and radial limb defects), so affected infants are screened with spine imaging, echocardiography and renal ultrasound, and a feeding tube is passed to exclude esophageal atresia.

Biliary atresia. A progressive fibro-obliterative inflammation destroys the extrahepatic bile ducts in the first weeks of life. A thriving infant has jaundice persisting beyond 2 weeks with conjugated (direct) hyperbilirubinemia, acholic clay-colored stools, dark urine, hepatomegaly and an elevated GGT. A minority have a syndromic form with polysplenia and situs anomalies.

Any infant still jaundiced after 2 weeks needs a fractionated bilirubin: physiologic and breast milk jaundice are unconjugated, whereas conjugated hyperbilirubinemia with pale stools demands urgent evaluation.

Diagnosis and treatment. Ultrasound shows a small or absent gallbladder, a HIDA scan shows no excretion into the bowel, and biopsy shows ductular proliferation, bile plugs and portal fibrosis; intraoperative cholangiography confirms the diagnosis. Kasai hepatoportoenterostomy, which drains bile from the porta hepatis into a jejunal limb, works best before about 60 days of age. Many children still progress to biliary cirrhosis, and biliary atresia is the most common indication for pediatric liver transplant.