Pyloric stenosis, also known as infantile hypertrophic pyloric stenosis (IHPS), is a condition affecting young infants in which the pylorus muscle becomes abnormally thickened, causing obstruction to gastric emptying. It is most often seen in the first few weeks of life, with the most common age of presentation between 5 and 6 weeks with a range of 2 to 31 weeks.^1,2 Although premature infants may present later than term babies (6 weeks compared with 5 weeks, respectively), their presentation is actually earlier when measured from postmenstrual age (40 weeks vs. 45 weeks, respectively).² Its incidence has been reported to be 3 per 1000 live births in the Western world. Lower incidence ranges have been reported for Asian children, with reports between 0.3 to 0.5 per 1000 infants in Taiwanese infants.^3,4 A recent decline in IHPS has been reported both in the United States and several European countries concurrent with a declining incidence of sudden infant death syndrome, leading to some belief that recommended supine sleeping position may be responsible for both declines.⁵ A genetic predisposition has been noted with an increased incidence of siblings and a strong increased incidence in twins, both monozygotic and dizygotic.^6,7 It is unclear if the increased incidence in twins is due to genetic or environmental causes but genetic tests have identified a number of potential loci, specifically chromosomes 3 and 5, and further studies are ongoing.^8–10 There is nearly a four- to fivefold male predominance, and an increased incidence in premature infants.² The highest incidence reported is in first-born infants, although this may be due to more families having only one child than two or three children.^9,11 Environmental and mechanical factors have been implicated as potential causes. Infants that were breastfed exclusively have a much lower risk of developing IHPS compared with infants that have been bottle-fed or bottle-fed and breastfed.^12,13 Very early exposure to erythromycin has been associated with a nearly eight- to tenfold increased risk of developing IHPS.^14,15 Erythromycin is a motilin agonist, and thus it produces strong gastric and pyloric contractions. Infants exposed to erythromycin through breast milk may also be at risk for developing IHPS, but the data are not as conclusive.¹⁶ Case reports and a more recent study have also demonstrated that exposure to oral azithromycin is associated with up to an eightfold increased risk of IHPS in the first 6 weeks of life, with the greatest risk if exposure occurs within the first 2 weeks of life.^17,18 However, the complications for pertussis outweigh the risk of IHPS, and the American Academy of Pediatrics still recommends the use of azithromycin for the treatment or prophylaxis of pertussis.¹⁹

The key to diagnosing IHPS remains a comprehensive history and physical examination, with confirmation by ultrasound. The diagnosis should be considered in any young infant with a history of repeated vomiting, especially after feeding. The mean duration of symptoms prior to diagnosis is 8 days; however, it can range from 1 day to 13 weeks.^1,2 The classic triad of projectile vomiting, palpable “olive,” and visible peristalsis on feeding has been described with IHPS, though it is rare to simultaneously discover all three.^1,2 The key finding, reported in nearly all cases, is projectile vomiting, typically occurring immediately after feeding. Infants will not have anorexia and are often described as hungry. They generally appear well, depending on the level of hydration, which ranges from normal to severe dehydration depending upon the duration of symptoms. In earlier decades, a palpable olive in the right upper quadrant used to be present in the majority of cases but is now uncommon and present in only 13% in a recent study, likely due to earlier diagnosis.¹ Peristaltic gastric waves may be seen in infants with advanced disease and dehydration, but has recently been reported in only 25% of cases.^2,20 Although the majority of infants have nonbilious vomiting, a small percentage of infants present with bilious emesis and hematemesis.²¹

Electrolyte and blood gas measurements may show classic imbalances including hypochloremia and metabolic alkalosis.²² Similar to the disappearing palpable olive, recent studies demonstrate that laboratory abnormalities are less common than in the past, with hypochloremia now present in 23% and alkalosis present in 21% of patients with IHPS.^1,22 Patients are generally depleted of total potassium as well, although the serum potassium level is usually normal.^22,23