PassFinals
Paediatrics

Pyloric stenosis

Progressive hypertrophy of the pyloric muscle causes gastric-outlet obstruction in a young infant, producing worsening non-bilious vomiting and dehydration; fluid and electrolyte correction must precede pyloromyotomy.

In a nutshell

Infantile hypertrophic pyloric stenosis causes progressive gastric-outlet obstruction: worsening forceful non-bilious vomiting, a hungry infant, dehydration and often hypochloraemic metabolic alkalosis. Ultrasound confirms the diagnosis when examination is uncertain. Involve paediatric surgery, keep nil by mouth, decompress and correct fluids/electrolytes before pyloromyotomy; bile-stained vomiting is an alternative-obstruction emergency.

Classic presentation

A young infant develops progressively forceful non-bilious vomiting after feeds, remains hungry afterwards and develops fewer wet nappies or faltering weight; an olive or visible peristalsis may be found but is not required.

Key points

  • Projectile vomiting is typically non-bilious; clearly bile-stained vomiting requires urgent assessment for malrotation/volvulus or another distal obstruction.
  • The classic biochemical consequence is hypochloraemic, hypokalaemic metabolic alkalosis from gastric acid loss and volume-depletion compensation, but early disease can have normal bloods.
  • Ultrasound is the usual confirmatory test; do not rely on a palpable olive or visible peristalsis being present.
  • Resuscitation, NG decompression and correction of dehydration and acid-base/electrolyte abnormalities precede anaesthesia and pyloromyotomy.
  • Pyloromyotomy is definitive, but early paediatric-surgical involvement is needed while the infant is being optimised.
  • Small postoperative vomits may occur during recovery; persistent, worsening or bile-stained vomiting needs reassessment.

First-line investigation

Blood gas/electrolytes for dehydration and metabolic alkalosis plus abdominal ultrasound to confirm hypertrophic pyloric stenosis when clinical assessment is not definitive.

Management

Stabilise and call surgery

  • Assess perfusion, glucose and hydration, obtain IV access, keep nil by mouth and contact paediatric surgery early; bile-stained vomiting requires a different urgent obstruction pathway.3,2

Confirm and quantify

  • Check blood gas and electrolytes, arrange ultrasound using local paediatric radiology criteria and place an NG tube for gastric decompression according to the surgical protocol.3,1,2

Correct before anaesthesia

  • Replace dehydration and gastric losses with the local neonatal fluid pathway and repeat blood gas/electrolytes until the acid-base and electrolyte disturbance is corrected.3,4

Perform pyloromyotomy

  • Proceed to open or laparoscopic pyloromyotomy once the infant is clinically and biochemically optimised; early surgical involvement remains essential even while the operation is delayed for correction.2,1

Feed gradually and safety-net

  • Restart feeds gradually, monitor hydration and weight, review the wound and arrange follow-up; recurrent inability to keep feeds down, dehydration, fever, wound change or bile-stained vomit needs urgent reassessment.2,1

Exam traps

  • Do not require an olive-shaped mass: it may be absent, especially early or in a distressed infant.
  • Do not call all yellow vomit bile; milk mixed with gastric acid can look yellow, but true bile staining needs an alternative-obstruction pathway.
  • Normal electrolytes do not exclude pyloric stenosis, particularly early in the course.
  • A pyloromyotomy is not performed before correcting dehydration and acid-base/electrolyte abnormalities.
  • Persistent postoperative vomiting is not automatically normal: assess severity, bile staining, hydration and surgical complications.

Illustrations

Hypertrophied pylorus mechanismDiagram comparing a normal pylorus with hypertrophied pyloric muscle narrowing the gastric outlet and showing how forceful gastric contractions produce vomiting.PassFinals · original
Ultrasound of pyloric stenosisUltrasound image demonstrating the thickened pyloric muscle used to support diagnosis; label the need for local paediatric radiology criteria.Hariadhi, Wikimedia Commons · CC-BY-SA-4.0
Test-feed examinationIllustration of a supervised test feed showing assessment for visible gastric peristalsis and palpation of an olive-shaped pyloric mass, with a note that neither finding is required.PassFinals · original

Key sources

  1. Great Ormond Street Hospital: Pyloric stenosis (Specialist paediatric information on progressive projectile vomiting, dehydration, ultrasound, fluid correction, pyloromyotomy and postoperative care.)
  2. Cambridge University Hospitals: Pyloric stenosis – information for parents, approved September 2025 (Current NHS paediatric-surgical information on presentation, ultrasound, fluid and electrolyte correction, NG decompression, pyloromyotomy, postoperative feeding and follow-up.)
  3. NHSGGC/Right Decisions: Hypertrophic pyloric stenosis RHC 1229, reviewed 2025 (UK paediatric-surgical pathway covering assessment, ultrasound, fluid prescription, NG management, biochemical correction targets and postoperative feeding.)
  4. Royal Cornwall Hospitals NHS Trust: Pyloric stenosis diagnosis and management in children, clinical guideline V5.0 (UK trust pathway supporting blood-gas correction, NG loss replacement and the requirement for normalisation before pyloromyotomy.)

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.