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# Pyloric stenosis: infantile hypertrophic and acquired

- Lesion

In some infants the muscle of the stomach outlet grows too thick and blocks it; in adults scarring or cancer can do the same.

Updated Nov 12, 2026

## Key points

- Presents at about three to six weeks with projectile non-bilious vomiting and a hungry baby.
- A firm olive-shaped mass in the epigastrium and visible peristaltic waves; ultrasound measures the muscle thickness.
- Males several times more often; first-born; family history; Turner syndrome and trisomy 18 raise risk.
- Erythromycin or azithromycin in the first two weeks of life is associated with it.
- Vomiting of acid leads to hypochloraemic hypokalaemic metabolic alkalosis.
- Adult (acquired) form: peptic scarring or distal gastric and pancreatic cancers.

## What it is

Infantile hypertrophic pyloric stenosis is a thickening of the muscularis propria of the pylorus that obstructs gastric emptying in the first weeks of life. It runs in families, is far more common in boys, and has been linked to early exposure to macrolide antibiotics. In adults, the pylorus narrows for other reasons: scarring from chronic antral gastritis or ulcers near the pylorus, or infiltration by gastric or pancreatic cancer.

## Structure

- Gross: a firm, elongated, olive-like pylorus with a narrow canal.
- Micro: hypertrophy and hyperplasia of the smooth muscle of the muscularis propria.
- Mucosal and submucosal oedema and inflammation may tighten the canal further.

## Step by step

- The pyloric muscle coat thickens in the first weeks after birth.
- The canal lengthens and narrows; feeds cannot leave the stomach.
- Forceful non-bilious vomiting follows each feed.
- Loss of gastric acid and fluid causes alkalosis, low chloride and low potassium.
- Surgical splitting of the muscle (pyloromyotomy) relieves the obstruction.

## Clinical notes

- The baby is hungry and wants to feed again soon after vomiting.
- In an adult with gastric outlet obstruction, biopsy is needed to exclude cancer.

## Memory aid

An olive at the outlet, a projectile and a hungry baby: muscle, not mucosa, is the problem.

## Test yourself

Think of the answer, then tap the question to see it.

## Which layer is thickened in infantile hypertrophic pyloric stenosis?

The smooth muscle of the pyloric muscularis propria.

## What acid-base disturbance does persistent vomiting in pyloric stenosis cause?

Hypochloraemic, hypokalaemic metabolic alkalosis.

## What are the usual causes of acquired pyloric stenosis in adults?

Scarring from antral gastritis or peptic ulcers near the pylorus, and infiltrating distal gastric or pancreatic cancer.

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## Elsewhere in DurgiPedia

- [Hypertrophic pyloric stenosis](https://durgical.com/en/embryology/hypertrophic-pyloric-stenosis/)

## Connections

## Related 5

- [Congenital anomalies of the gut](https://durgical.com/en/general-pathology/congenital-anomalies-of-the-gut/)
- [Peptic ulcer](https://durgical.com/en/general-pathology/peptic-ulcer/)
- [Gastric adenocarcinoma: intestinal vs diffuse](https://durgical.com/en/general-pathology/gastric-adenocarcinoma-intestinal-vs-diffuse/)
- [Hypertrophy: physiologic and pathologic](https://durgical.com/en/general-pathology/hypertrophy-physiologic-and-pathologic/)
- [Hyperplasia: physiologic and pathologic](https://durgical.com/en/general-pathology/hyperplasia-physiologic-and-pathologic/)

## Related conditions 3

- [Infantile hypertrophic pyloric stenosis](https://durgical.com/en/conditions/infantile-hypertrophic-pyloric-stenosis/)
- [Peptic ulcer disease](https://durgical.com/en/conditions/peptic-ulcer-disease/)
- [Gastric cancer](https://durgical.com/en/conditions/gastric-cancer/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Pyloric Stenosis](https://www.ncbi.nlm.nih.gov/books/NBK555931/) (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/pyloric-stenosis-infantile-hypertrophic-and-acquired/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/pyloric-stenosis-infantile-hypertrophic-and-acquired/