DEFINITION AND EPIDEMIOLOGY
Hypertrophic pyloric stenosis (HPS) is a condition of infancy that occurs between 2 and 10 weeks of age, most commonly between 3 and 6 weeks of age. In premature infants, it presents at a later chronological age, but earlier postconceptional age. It is characterized by hypertrophy of the circular muscle of the pylorus causing gastric outlet obstruction. Its incidence is 1 to 5 per 1000 births in white infants. It is less common in African Americans and least common in the Asian population. It is more common in boys with a male:female ratio of 4:1. Sharp decreases in incidence over the 1990s have been reported from several European countries, while the incidence in the United States appears to be stable but varies widely by state.
Despite advances in the field, HPS is still considered idiopathic. It appears to result from the interplay of genetic predisposition with local tissue factors, as well as pre- and postnatal environmental exposures.
HPS is not congenital, as it has been shown by studying healthy newborns with ultrasound (US) and upper gastrointestinal (UGI) fluoroscopy. All infants had normal studies after birth, but a few of them went on to develop HPS, while most did not. Pyloric stenosis does not develop without initiation of feeds, so the interaction of feeds and other pre- and postnatal micro- and macroenvironmental factors with the developing pyloric muscle is likely the key element to trigger the condition in genetically predisposed individuals. The majority of hypertrophy happens in the circular muscle layer, which thickens and elongates, but thickening of the mucosa is also present. The natural course of HPS is that of resolution as evidenced by the feasibility of nonoperative treatment with atropine and/or parenteral nutrition. If followed by serial US, pyloric muscle caliber returns to normal over the course of 3 to 6 months.
A hereditary component of pyloric stenosis is consistent with a multifactorial, sex-modified threshold model of inheritance, in which 5.5% of sons and 2.5% of daughters of an affected father develop HPS, in comparison to 20% of sons and 7% of daughters of an affected mother. The risk in siblings is 20 to 30 times higher than in the general population. There is no single gene that is responsible for HPS, but genome-wide analyses have revealed several potential contributing loci including the gene encoding the enzyme neuronal nitric oxide synthase (nNOS), a family of genes encoding transient receptor potential cation channels, and a locus adjacent to the apolipoprotein gene cluster.
Complex interactions between nerve and muscle cells and the extracellular matrix of pyloric tissue have been studied along with the influences of hormones, growth factors, and local paracrine mediators. Gastrin and prostaglandin E may be contributing factors, as well as increased levels of insulinlike growth factor-I, platelet-derived growth factor, epidermal growth factor, and transforming growth factor β in pyloric muscle. The ...