The exact etiology of hypertrophic pyloric stenosis is uncertain, but a genetic component is likely because siblings and offspring of affected people are at increased risk, particularly monozygotic twins. Maternal smoking during pregnancy also increases risk. Proposed mechanisms include lack of neuronal nitric oxide synthase, abnormal innervation of the muscular layer, and hypergastrinemia. Infants exposed to certain macrolide antibiotics (eg, erythromycin) in the first few weeks of life are at significantly increased risk.

Symptoms of hypertrophic pyloric stenosis typically develop between 3 wk and 6 wk of life. Projectile vomiting (without bile) occurs shortly after eating. Until dehydration sets in, children feed avidly and otherwise appear well, unlike many of those with vomiting caused by systemic illness. Gastric peristaltic waves may be visible, crossing the epigastrium from left to right. A discrete, 2- to 3-cm, firm, movable, and olive-like pyloric mass is sometimes palpable deep in the right side of the epigastrium. With progression of illness, children fail to gain weight, and dehydration develops.

Hypertrophic pyloric stenosis should be suspected in all infants in the first several months of life with projectile vomiting.

Diagnosis of hypertrophic pyloric stenosis is by abdominal ultrasonography showing increased thickness of the pylorus (typically to ≥ 4 mm; normal, < 2 mm) along with an elongated pylorus (> 16 mm).

If the diagnosis remains uncertain, ultrasonography can be repeated serially or an upper GI series can be done, which typically shows delayed gastric emptying and a string sign or railroad track sign of a markedly narrowed, elongated pyloric lumen. In rare cases, upper endoscopy is required for confirmation.

The classic electrolyte pattern of an infant with pyloric stenosis is that of hypochloremic metabolic alkalosis (due to loss of hydrochloric acid and simultaneous hypovolemia). About 5 to 14% of infants have jaundice, and about 5% have malrotation.

Initial treatment of hypertrophic pyloric stenosis is directed at hydration and correcting electrolyte abnormalities.

Definitive treatment is a longitudinal pyloromyotomy, which leaves the mucosa intact and separates the incised muscle fibers. Postoperatively, the infant usually tolerates feeding within a day.