# Infantile Hypertrophic Pyloric Stenosis

> Infantile hypertrophic pyloric stenosis for NEET-PG Paediatrics, covering projectile vomiting, the olive sign, metabolic alkalosis, ultrasound and surgery.

- Canonical URL: https://prepelephant.com/topics/neet-pg/paediatrics/infantile-hypertrophic-pyloric-stenosis
- Exam / course: NEET-PG · Subject: Paediatrics
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-15
- Last updated: 2026-10-15
- Reference basis: Bailey and Love's Short Practice of Surgery; Nelson Textbook of Pediatrics
- How to cite: "Infantile Hypertrophic Pyloric Stenosis", PrepElephant, https://prepelephant.com/topics/neet-pg/paediatrics/infantile-hypertrophic-pyloric-stenosis

## Direct answer

Infantile hypertrophic pyloric stenosis is hypertrophy and hyperplasia of the pyloric muscle that narrows the gastric outlet, usually presenting from about 3 to 6 weeks of age with projectile non-bilious vomiting. It causes hypochloraemic, hypokalaemic metabolic alkalosis, and ultrasound measuring pyloric muscle thickness and channel length confirms the diagnosis. Electrolyte correction is followed by Ramstedt pyloromyotomy.

## What you must remember

- **Presentation:** projectile, non-bilious vomiting from about 3 to 6 weeks, often after feeds, in a baby who remains hungry.
- **Who gets it:** first-born males predominate, and early macrolide exposure such as erythromycin is a recognised risk factor.
- **Examination:** visible gastric peristalsis moving from left to right, and a palpable olive-sized mass in the epigastrium or right upper abdomen.
- **Biochemistry:** hypochloraemic, hypokalaemic metabolic alkalosis, with paradoxical aciduria in the urine.
- **Ultrasound:** pyloric muscle thickness and channel length; thresholds of about 3 to 4 mm and 14 to 16 mm are commonly quoted and vary between centres.
- **Resuscitation first:** correct dehydration, chloride and potassium before surgery, without unnecessary delay.
- **Definitive treatment:** Ramstedt pyloromyotomy, open or laparoscopic, divides the thickened muscle and leaves the mucosa intact.
- **Differential diagnosis:** gastro-oesophageal reflux, which is non-projectile and has no olive mass, and malrotation, which causes bilious vomiting.

## From the first projectile vomit to surgery

A four-week-old first-born boy vomits forcefully after every feed and remains hungry afterwards. The steps below show how the diagnosis is confirmed and the metabolic state is corrected before surgery.

1. **Recognise the pattern.** Projectile non-bilious vomiting in a hungry infant of this age is pyloric stenosis until proven otherwise.
2. **Assess hydration.** Look for dry mucosa, reduced urine output and a sunken fontanelle, and weigh the infant.
3. **Check electrolytes.** A blood gas and electrolytes usually show hypochloraemic, hypokalaemic alkalosis.
4. **Confirm with ultrasound.** Measure pyloric muscle thickness and channel length in transverse and longitudinal views.
5. **Correct fluids and electrolytes.** Replace fluid with chloride and potassium until the biochemistry is stable.
6. **Operate.** Perform Ramstedt pyloromyotomy, open or laparoscopically, leaving the mucosa intact.
7. **Restart feeds.** Feeds resume after surgery once the infant tolerates them, and vomiting usually settles quickly.

Each step moves from the mechanical problem to the physiological problem it has caused. Surgery is timed by the biochemistry rather than by the clock, which is the decisive teaching point of this topic.

## Where the exam tries to catch you

Most pyloric stenosis questions test whether you can separate the vomit pattern from the biochemistry.

- **Trap:** choosing bilious vomiting as the expected feature. **Correct:** the obstruction lies proximal to the entry of bile, so vomit contains gastric contents only, and bilious vomiting points to malrotation.
- **Trap:** expecting metabolic acidosis from persistent vomiting. **Correct:** the classic picture is hypochloraemic, hypokalaemic metabolic alkalosis, with paradoxical aciduria in the urine.
- **Trap:** operating immediately on a dehydrated and alkalotic infant. **Correct:** fluid and electrolyte correction comes first, then pyloromyotomy.
- **Trap:** diagnosing reflux because the baby is thriving and hungry. **Correct:** projectile vomiting, an olive mass and metabolic alkalosis point to pyloric stenosis even in a growing infant.

## Frequently asked questions

### Why is the vomit non-bilious in infantile hypertrophic pyloric stenosis?

The obstruction lies at the pylorus, proximal to the point where bile enters the duodenum, so the vomit contains gastric contents only. Bilious vomiting instead points to obstruction distal to the ampulla, such as malrotation with volvulus.

### What biochemical abnormality is typical of infantile hypertrophic pyloric stenosis?

Infantile hypertrophic pyloric stenosis typically produces hypochloraemic, hypokalaemic metabolic alkalosis because gastric hydrochloric acid is lost with vomiting. Paradoxical aciduria, in which urine is acidic despite alkalosis, reflects renal conservation of sodium at the expense of hydrogen ions.

### Which ultrasound measurements support a diagnosis of pyloric stenosis?

Pyloric muscle thickness and channel length are the key measurements, with a thickness of about 3 to 4 mm or more and a length of about 14 to 16 mm or more used in many centres. Because thresholds vary between centres, a borderline result should be interpreted alongside the clinical picture.

### Why is fluid and electrolyte correction done before pyloromyotomy?

Correcting dehydration, chloride and potassium before pyloromyotomy lowers the risk of anaesthetic complications and arrhythmias from hypokalaemia. Surgery is then performed once the biochemistry has improved, and the operation is usually curative.