13.1 Pyloric Stenosis & Vomiting Disorders
Key Takeaways
- Hypertrophic pyloric stenosis classically presents at 2-8 weeks of age (peak 3-6 weeks) with non-bilious projectile vomiting in a hungry infant, an epigastric olive mass, and visible peristaltic waves.
- The signature lab pattern is hypochloremic, hypokalemic metabolic alkalosis with paradoxical aciduria once dehydration becomes severe.
- Abdominal ultrasound is diagnostic when pyloric muscle thickness is approximately 3-4 mm or greater and channel length is approximately 14-17 mm or greater on a relaxed pylorus.
- Fluid and electrolyte correction always precedes pyloromyotomy; operating on an alkalotic, hypokalemic infant risks post-operative apnea.
- Bilious vomiting in any infant is a surgical emergency until malrotation with volvulus is excluded, regardless of how reassuring the rest of the exam looks.
Hypertrophic Pyloric Stenosis
Hypertrophic pyloric stenosis (HPS) is progressive hypertrophy and hyperplasia of the circular muscle layer of the pylorus, narrowing the gastric outlet until it obstructs. It is the most common surgical cause of vomiting in infancy, occurring in roughly 1-3 per 1,000 live births, with a male-to-female ratio near 4:1 and higher risk in firstborn infants and those with a family history of HPS. Exposure to oral macrolide antibiotics (particularly erythromycin) in the first two weeks of life is a recognized precipitant, so a stem describing a neonate treated for pertussis prophylaxis or chlamydial conjunctivitis who then develops projectile vomiting is testing this association.
Symptoms classically begin between 2 and 8 weeks of age, peaking at 3-6 weeks; presentation before 1 week or after 12 weeks should prompt reconsideration of the diagnosis. The hallmark is non-bilious, projectile vomiting immediately after feeds in an infant who remains hungry and eager to re-feed right afterward — the "hungry vomiter." Vomiting is never bile-stained because the obstruction lies proximal to the ampulla of Vater. As obstruction progresses, the infant develops poor weight gain and dehydration, and visible peristaltic waves cross the epigastrium left to right just before an emesis.
The pathognomonic physical finding is a firm, mobile, olive-shaped mass palpable in the right upper quadrant or epigastrium, representing the hypertrophied pyloric muscle. The "olive" is best felt after the stomach has been emptied by vomiting or nasogastric aspiration, with the infant calm and the abdominal wall relaxed — a detail exam stems use to explain why the mass was "not found" on a prior exam.
Metabolic Derangement
Repeated vomiting of gastric contents (HCl- and potassium/sodium-rich fluid) produces a classic and testable acid-base picture:
| Finding | Mechanism |
|---|---|
| Hypochloremia | Direct loss of chloride-rich gastric HCl |
| Hypokalemia | Renal potassium wasting as the kidney conserves sodium and hydrogen |
| Metabolic alkalosis | Loss of H+ in vomitus plus volume contraction |
| Paradoxical aciduria | With severe hypovolemia the kidney reabsorbs Na+ with H+ (sparing K+) to preserve volume, acidifying urine despite systemic alkalosis |
This hypochloremic, hypokalemic metabolic alkalosis is a signature exam pattern — any question describing a vomiting infant with a low chloride and low potassium on a basic metabolic panel is pointing toward HPS.
Diagnosis and Management
Abdominal ultrasound is the imaging study of choice, with sensitivity and specificity both exceeding 95% in experienced hands. Diagnostic thresholds, measured on a relaxed, non-contracting pylorus, are approximately: pyloric muscle wall thickness at or above 3-4 mm and pyloric channel length at or above 14-17 mm. An elongated, thick-walled pylorus that fails to open during observation confirms the diagnosis; a contrast upper GI study showing the "string sign" (a thin trickle of contrast through the narrowed channel) is reserved for equivocal ultrasounds.
Management is never immediate surgery. The exam-critical sequence is: correct the fluid and electrolyte derangement first, then operate. Infants receive isotonic IV fluid boluses for dehydration, followed by maintenance dextrose-saline with added potassium once urine output is established, normalizing chloride, potassium, and pH before anesthesia. Operating on an alkalotic infant risks post-operative apnea, because the respiratory center compensates for metabolic alkalosis by hypoventilating, and anesthetic agents can unmask this. Once electrolytes normalize, definitive treatment is pyloromyotomy (the Ramstedt procedure) — an incision through the hypertrophied muscle down to, but not through, the submucosa, performed open or laparoscopically. It is curative, and infants typically resume feeds within 24-48 hours.
Differential Diagnosis of Infant Vomiting
Not every vomiting infant has HPS, and distinguishing benign from dangerous causes is a recurring exam theme.
Gastroesophageal reflux (GER) is physiologic regurgitation from an immature lower esophageal sphincter, seen in a majority of healthy infants under 6 months. The "happy spitter" feeds well, gains weight normally, and has no respiratory or growth complications; management is reassurance, upright positioning after feeds, and smaller, more frequent feeds. In contrast, the infant with hypertrophic pyloric stenosis continues to vomit progressively larger volumes and fails to gain weight despite appearing hungry — poor weight gain alongside non-bilious projectile vomiting is the key discriminator from the happy spitter, who tracks growth curves normally. Gastroesophageal reflux disease (GERD) is diagnosed when reflux causes complications — poor weight gain, esophagitis, feeding refusal, arching and irritability (Sandifer syndrome), or recurrent respiratory symptoms — and may warrant acid suppression or further workup.
Cow's milk protein allergy (CMPA) presents in the first weeks to months of life with regurgitation or vomiting plus clues that separate it from simple reflux: blood-streaked or mucousy stools, eczema, family history of atopy, and irritability. Diagnosis is largely clinical, confirmed by resolution on an extensively hydrolyzed or amino-acid formula (or maternal dairy elimination if breastfeeding) and recurrence on reintroduction.
The single most important exam rule for infant vomiting is: bilious vomiting is a surgical emergency until proven otherwise. Green or yellow-green vomitus implies obstruction distal to the ampulla of Vater — malrotation with volvulus, jejunoileal atresia, or another mechanical obstruction — and mandates urgent evaluation (an upper GI contrast study) rather than reassurance, no matter how well the infant otherwise looks. A stem pairing "bilious" with a benign-sounding exam is testing whether the candidate reflexively escalates rather than anchors on reassuring findings.
A 4-week-old male infant has non-bilious projectile vomiting after every feed but remains hungry immediately afterward. A basic metabolic panel is most likely to show which pattern?
Ultrasound confirms hypertrophic pyloric stenosis in a dehydrated 5-week-old with a serum chloride of 82 mmol/L and potassium of 2.9 mmol/L. What is the correct next step?
A previously well 3-week-old develops sudden bilious vomiting. The abdomen is soft and non-tender. What is the most appropriate next step?
A 2-month-old has recurrent regurgitation with blood-streaked, mucousy stools, mild eczema, and a strong family history of atopy. Symptoms resolve on an extensively hydrolyzed formula. Which diagnosis best fits this pattern?