Infantile hypertrophic pyloric stenosis (IHPS) results in marked hypertrophy of both the circular and longitudinal muscular layers. The effect of the muscular hypertrophy is a lengthening of the canal and thickening of the pylorus, causing the classic “olive” shape. Hypertrophy of the pyloric muscle occurs during the postnatal period, which becomes clinically apparent between 3 and 6 weeks of age.

In their report, Applegate et al., described the incidence of IHPS to be approximately 2 in 1,000. Boys have a four- to fivefold higher risk of developing IHPS compared to girls, and there appears to be a predilection for the first-born child. The development of IHPS is believed to be multifactorial, with both genetic and environmental factors playing a role.

The classic presentation is an afebrile, first-born male child who develops nonbilious, projectile vomiting immediately following feeding, and who is interested in feeding after episodes of emesis. The diagnosis should be considered in any child when there is emesis worsening over days to weeks, becoming more frequent and increasingly forceful. It is extremely important for the clinician to observe the baby feeding, noting the amount of the feed, onset of emesis following feeding, and the quality and quantity of emesis. Depending on the duration of symptoms, there may be physical exam findings associated with dehydration. Classically, infants with pyloric stenosis have hyponatremia, hypochloremia, hypokalemia, and metabolic alkalosis, resulting from chronic loss of gastric secretions.