hypertrophic pyloric stenosis
Summary
Hypertrophic pyloric stenosis (HPS) is a condition of infancy caused by hypertrophy of the pyloric sphincter smooth muscle, leading to gastric outlet obstruction. It classically presents at 2-8 weeks of life with progressive, non-bilious projectile vomiting after feeds in a hungry infant. It is more common in firstborn males and is treated surgically with pyloromyotomy.
Detail
Hypertrophic pyloric stenosis results from idiopathic hypertrophy and hyperplasia of the circular and longitudinal muscle layers of the pylorus, narrowing the gastric outlet and causing a functional obstruction. Etiology is multifactorial, involving genetic predisposition (family history increases risk), male sex (4:1 male-to-female ratio), firstborn status, and possible environmental factors such as early exposure to macrolide antibiotics (e.g., erythromycin) in neonates or via breastfeeding mothers.
Clinical presentation: Infants are typically well at birth and develop symptoms between 2-8 weeks of age (rarely up to 5 months). The hallmark is non-bilious, projectile vomiting immediately after feeding, with the infant remaining hungry afterward ("hungry vomiter"). Persistent vomiting leads to dehydration and a classic hypochloremic, hypokalemic metabolic alkalosis due to loss of gastric HCl and compensatory renal responses (paradoxical aciduria develops in severe cases due to renal potassium and hydrogen ion wasting to conserve sodium).
Physical exam may reveal a palpable "olive-shaped" mass in the epigastrium/right upper quadrant, representing the hypertrophied pylorus, and visible peristaltic waves across the abdomen after feeding.
Diagnosis is confirmed by abdominal ultrasound, which is the gold standard—showing a thickened pyloric muscle (>3-4 mm) and elongated pyloric channel (>14-17 mm). Upper GI contrast study may show the "string sign" (thin trickle of contrast through the narrowed pyloric channel) if ultrasound is equivocal.
Management involves initial correction of fluid and electrolyte imbalances (IV normal saline with potassium supplementation as needed) prior to surgery, as anesthesia risk is increased with alkalosis. Definitive treatment is surgical pyloromyotomy (Ramstedt procedure), which involves longitudinal incision of the hypertrophied pyloric muscle without cutting the mucosa. Prognosis after surgery is excellent, with rapid resolution of symptoms and normal growth expected.
Key board associations: hypochloremic, hypokalemic metabolic alkalosis; olive-shaped mass; projectile non-bilious vomiting; ultrasound diagnosis; treatment is pyloromyotomy, not medical management.
Sources
- First Aid for the USMLE Step 1
- UWorld Step 2 CK
- Nelson Textbook of Pediatrics
- Schwartz's Principles of Surgery
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