Hypertrophic pyloric stenosis is an idiopathic hypertrophy, or thickening, of the circular muscle bundles of the stomach pylorus. This hypertrophy is progressive, resulting in gradual development of gastric outlet obstruction. The etiology of the condition remains unknown, although postulated mechanisms include abnormal myenteric plexus nerve cells of the pylorus as well as excessive spasm or overactivity of the pylorus leading to eventual muscular hypertrophy.

The age at presentation is typically 2-8 weeks; however, cases of infants presenting at birth or after 8 weeks have been described. There are even case reports of hypertrophic pyloric stenosis diagnosed in utero by prenatal ultrasonography. The incidence of the condition is approximately 2-5 cases per 1,000 births per year, and it affects males with a frequency of 4-5 times that of females.

The presentation of hypertrophic pyloric stenosis usually consists of one or more of the following:

Hypertrophic pyloric stenosis diagnosis is not a pediatric emergency, and the diagnosis and documentation of the condition may wait in an overnight situation if necessary.

Hypertrophic pyloric stenosis is an evolving lesion. If initial imaging studies are borderline or negative, but clinical symptoms persist, patients should be reimaged 1-2 weeks later (preferably with ultrasonography).

Ultrasonography is the imaging method of choice, given its degree of accuracy, its ability to directly visualize the anatomy and function of the pylorus, and its lack of ionizing radiation.

Using a linear high-frequency transducer, start with locating the gallbladder in the right upper quadrant, and then turn obliquely sagittal to the body to visualize the pylorus of the stomach in a longitudinal orientation. The pylorus should also then be visualized in the transverse orientation. Once the pyloric sphincter has been visualized, fluid can be given by mouth to assess for pyloric opening, transpyloric flow, and gastric emptying.