Hypertrophic pyloric stenosis !
- Malformation
The pyloric muscle thickens in the first weeks of life and closes the stomach outlet: the baby vomits forcefully and is still hungry.
Updated 2026-11-01
See it in 3D: Stomach · Duodenum
What it is
It is hypertrophy of the muscle layer of the pylorus that narrows the gastric outlet in young infants. It is not a failure of embryonic formation: the pylorus forms normally and its muscle overgrows after birth, for multifactorial reasons. It is the most common surgical cause of non-bilious vomiting in the first weeks of life and an exam classic for its picture and its metabolic disturbance.
Embryological cause
- The pylorus forms normally from the foregut; the disorder appears after birth.
- The pyloric muscle fibres, mainly the circular ones, hypertrophy and lengthen the pyloric channel until it obstructs.
- The cause is multifactorial and the exact mechanism is unknown: heredity, male sex, postnatal macrolide exposure, bottle feeding and maternal smoking all play a part.
How it presents
- Increasingly forceful, projectile, non-bilious vomiting after feeds from the third week of life.
- A hungry infant who loses weight and becomes dehydrated.
- A palpable pyloric olive in the epigastrium and peristaltic waves moving left to right.
- Some infants have mild jaundice.
Diagnosis
- Ultrasound: thickened pyloric muscle and an elongated pyloric channel.
- Blood gas and electrolytes: hypochloremic, hypokalemic metabolic alkalosis, now less frequent because diagnosis is earlier.
- If a contrast study is done: the string sign through the narrowed pyloric channel.
Test yourself
Think of the answer, then tap the question to see it.
Why is the vomit in pyloric stenosis free of bile?
Because the obstruction is at the pylorus, before the bile duct opens into the duodenum.
Which acid–base disturbance is typical?
Hypochloremic, hypokalemic metabolic alkalosis, from loss of hydrochloric acid in the vomit.
Which antibiotics are linked to a higher risk in newborns?
Macrolides such as erythromycin given in the first weeks of life.
What is the imaging test of choice?
Abdominal ultrasound, which shows the thickened pyloric muscle and the elongated channel.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Foregut: esophagus, stomach rotation and duodenum
- Duodenal atresia and stenosis
- Malrotation and midgut volvulus
Related conditions
- Infantile hypertrophic pyloric stenosis
- Metabolic alkalosis
- Hypokalaemia
- Dehydration (fluid volume deficit)
Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.
Sources: Pyloric Stenosis (StatPearls Publishing (NCBI Bookshelf), 2023); Pyloric stenosis in infants (MedlinePlus, U.S. National Library of Medicine, 2025).
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Representative model, for educational purposes only.