Key Facts

Infantile hypertrophic pyloric stenosis (IHPS) is caused by progressive hypertrophy and hyperplasia of the pyloric smooth muscle, causing gastric outlet obstruction. It is the most common surgical cause of vomiting in infancy and is a surgical emergency requiring prompt diagnosis and treatment.

Epidemiology

Incidence is approximately 2-4 per 1,000 live births in the UK. Strong male predominance (4-5:1 M:F). More common in firstborn children. Familial clustering exists — the risk is approximately 20% if mother was affected and 5% if father was affected. Higher incidence in Caucasian populations.

Aetiology

The exact cause is unknown. Considered multifactorial:

• Genetic: Polygenic inheritance; associated loci identified
• Environmental: Erythromycin/macrolide exposure in first 2 weeks of life (OR ~3-7); maternal smoking; bottle-feeding
• Abnormal nitric oxide: Reduced neuronal nitric oxide synthase (nNOS) in pyloric muscle → impaired relaxation

Pathophysiology

Progressive hypertrophy of the circular muscle layer of the pylorus → narrowing and lengthening of the pyloric canal → functional gastric outlet obstruction. The mucosa is compressed, creating a mechanical barrier to gastric emptying. Persistent vomiting of acidic gastric contents leads to loss of H⁺, Cl⁻, K⁺, and Na⁺ → hypochloraemic hypokalaemic metabolic alkalosis. The kidneys initially compensate by excreting bicarbonate, but as K⁺ depletes, paradoxical aciduria occurs (renal H⁺/K⁺ exchange).

Typical Presentation

• Non-bilious projectile vomiting (may be forceful enough to travel across the room)
• Age of onset: 2-8 weeks (peak 3-5 weeks)
• Infant is hungry and feeds eagerly after vomiting
• Progressive worsening over days
• Weight loss or poor weight gain
• Constipation (reduced stool frequency — 'starvation stools')
• Dehydration signs: dry mucous membranes, reduced urine output, sunken fontanelle

Examination Findings

• Palpable pyloric mass ('olive'): Firm, ~2cm, in the right upper quadrant/epigastrium — best felt during a feed; pathognomonic but present in only ~70% of cases
• Visible gastric peristalsis: Left-to-right peristaltic waves across the upper abdomen
• Dehydration signs: Reduced skin turgor, dry mouth, sunken fontanelle

Red Flags

• Bilious (green) vomiting — NOT pyloric stenosis; consider malrotation/volvulus (surgical emergency)
• Severe dehydration or shock
• Weight loss >5% of birth weight
• Persistent hypokalaemia or alkalosis resistant to correction

Bedside

• Test feed: Observe a feed — may see visible peristalsis and palpate pyloric olive; projectile vomiting after feed
• Weight: Compare to birth weight and recent weights
• Capillary blood gas: Metabolic alkalosis (pH >7.45, raised HCO₃⁻, low Cl⁻, low K⁺)

Bloods

• U&Es: Hypokalaemia, hyponatraemia, hypochloraemia, raised bicarbonate
• Blood gas (venous/capillary): Metabolic alkalosis with respiratory compensation
• Chloride: Characteristically low (<95 mmol/L); must be corrected pre-operatively

Imaging

• Abdominal ultrasound: Gold standard — diagnostic criteria:
  • Pyloric muscle thickness ≥3mm (transverse)
  • Pyloric canal length ≥15mm (longitudinal)
  • Sensitivity and specificity both >95%
• Upper GI contrast study: Rarely needed; shows 'string sign' (elongated narrowed pyloric canal) or 'shoulder sign'

Special Tests

• Not routinely required; genetic testing only in research settings

Non-pharmacological

• Nil by mouth once diagnosis confirmed
• Nasogastric tube: Decompress stomach; free drainage

Pharmacological

• Pre-operative fluid resuscitation: This is critical and takes priority over surgery
  • 0.9% NaCl with 5% dextrose initially; add KCl 20-40 mmol/L once urine output established
  • Aim: Cl⁻ >100 mmol/L, K⁺ >3.5 mmol/L, HCO₃⁻ <26 mmol/L before proceeding to surgery
  • Typically takes 24-48 hours to correct
• Atropine: Medical management with IV/oral atropine has been described but is rarely used; not standard of care in the UK

Surgical/Interventional

• Ramstedt's pyloromyotomy: Definitive treatment
  • Longitudinal incision through the hypertrophied pyloric muscle down to (but not through) the mucosa
  • Can be performed open (RUQ incision or periumbilical) or laparoscopically
  • Success rate: effectively 100%
  • Post-operative: graded feeds starting 4-6 hours post-op; some vomiting initially is normal
  • Discharge usually within 24-48 hours
  • Complications: mucosal perforation (<1%), wound infection, incomplete myotomy (rare)

Referral Criteria

• Any infant with suspected pyloric stenosis — urgent paediatric surgical referral
• Must correct electrolytes before surgery — joint medical/surgical management

• Pyloromyotomy is curative with effectively 100% success rate
• Operative mortality: <0.1% in developed countries
• Post-operative vomiting: Common in first 24-48 hours; resolves spontaneously
• Mucosal perforation: <1%; requires repair if identified intraoperatively
• Incomplete myotomy: Rare (<2%); may require repeat procedure
• Long-term outcomes: Excellent; no long-term sequelae; normal feeding and growth
• Without treatment: progressive dehydration, alkalosis, and death (historically high mortality before surgical era)

Electrolyte Changes in Pyloric Stenosis

USS Diagnostic Criteria

Pre-operative Targets Before Surgery