Pyloric Stenosis

Pyloric Stenosis – High-Yield Study Guide for Medical Students

Definition

Pyloric stenosis, more precisely infantile hypertrophic pyloric stenosis (IHPS), is a condition characterized by progressive hypertrophy and hyperplasia of the pyloric circular muscle leading to functional gastric outlet obstruction. It typically presents in early infancy with projectile, non-bilious vomiting and may rapidly cause hypochloremic, hypokalemic metabolic alkalosis.

Epidemiology

Understanding the epidemiology helps narrow the differential in a vomiting infant:

• Age: Classically presents between 2–8 weeks of life; rare after 12 weeks.
• Sex: Strong male predominance (≈4:1 male:female).
• Ethnicity: More common in White infants; lower incidence in Black and Asian populations.
• Family history: Increased risk in siblings and offspring, suggesting a genetic component.
• Risk factors:
  • First‑born male.
  • Exposure to macrolides (e.g., erythromycin) in the first weeks of life.
  • Possibly associated with bottle feeding and certain maternal smoking histories.

Pathophysiology

The key lesion is concentric hypertrophy of the pyloric circular smooth muscle with associated hyperplasia, which leads to progressive narrowing of the pyloric channel between the stomach and duodenum.

Important mechanisms and concepts:

• Muscle hypertrophy and elongation: The pyloric canal becomes elongated, thickened, and stiff, creating a fixed gastric outlet obstruction.
• Functional obstruction: Initially partial but progresses to complete obstruction of gastric emptying; this explains the progressive nature of vomiting.
• Gastric physiology changes:
  • Persistent vomiting leads to loss of H⁺, Cl⁻, water, and some K⁺.
  • Results in classic hypochloremic, hypokalemic metabolic alkalosis.
  • Compensatory mechanisms (renal retention of HCO₃⁻, exchange of Na⁺ for H⁺/K⁺) further affect electrolytes.
• Neurohormonal factors: Abnormalities in gastrin, nitric oxide synthase, and innervation have been reported, but the precise etiologic mechanism remains multifactorial.

Clinical Presentation

Pyloric stenosis is a classic exam topic; the presentation is stereotypical but may be incomplete early on.

Typical history

• Age: 2–8 weeks old, previously feeding well.
• Vomiting:
  • Progressive, projective, non‑bilious vomiting after feeds.
  • Typically occurs shortly after feeding; infant may seem hungry immediately afterward ("hungry vomiter").
  • Non‑bilious because the obstruction is proximal to the ampulla of Vater.
• Feeding behavior:
  • Initially very hungry, eager to feed.
  • With progression, may become lethargic and less interested in feeds due to dehydration.
• Weight and hydration:
  • Poor weight gain or weight loss.
  • Dehydration signs: decreased urine output, dry mucous membranes, sunken anterior fontanelle.

Physical examination

• General:
  • May appear dehydrated and irritable initially, later lethargic.
  • Failure to thrive if delayed presentation.
• Abdominal findings:
  • Visible peristaltic waves moving from left to right across the upper abdomen after feeding (enhanced gastric peristalsis against obstruction).
  • Palpable olive: a firm, mobile, 2–3 cm "olive-shaped" mass in the right upper quadrant or epigastrium, best felt after vomiting or with relaxed infant.
  • Epigastric fullness due to gastric distension.
• Signs of dehydration and electrolyte disturbance:
  • Decreased skin turgor.
  • Tachycardia, delayed capillary refill in more severe cases.
  • Constipation from reduced intestinal transit.

Differential diagnosis of non-bilious vomiting in early infancy

• Physiologic gastroesophageal reflux (usually not projectile, infant appears well).
• Gastroesophageal reflux disease with poor weight gain.
• Milk protein allergy or intolerance (often associated with diarrhea, blood/mucus in stool).
• Sepsis, UTI, meningitis (systemic signs, fever, irritability).
• Inborn errors of metabolism (metabolic acidosis, lethargy, seizures).

Diagnosis

Diagnosis is based on a combination of clinical suspicion, laboratory evaluation, and imaging. High-yield features are essential for exam preparation.

Laboratory findings

• Electrolytes and blood gas:
  • Hypochloremic, hypokalemic metabolic alkalosis is classic but may be absent early.
  • Low serum Cl⁻ (< 98 mmol/L), low K⁺, elevated HCO₃⁻.
  • Paradoxical aciduria may be seen (acidic urine despite systemic alkalosis) due to renal Na⁺ reabsorption in exchange for H⁺ when K⁺ is depleted.
• Hemoconcentration may indicate dehydration (elevated hematocrit, BUN).

Imaging

• Abdominal ultrasound (test of choice):
  • Highly sensitive and specific, noninvasive, no radiation.
  • Findings:
    • Elongated pyloric channel (typically length > 14–17 mm, depending on reference).
    • Increased pyloric muscle thickness (usually > 3–4 mm).
    • Visualization of "target sign" or "donut sign" in transverse view.
• Upper GI contrast study (if ultrasound equivocal or unavailable):
  • Typically shows string sign (narrowed pyloric channel with thin column of contrast) or "railroad track" sign.
  • Can also rule out malrotation with midgut volvulus when the distinction is unclear.

Diagnostic criteria (typical ultrasound thresholds)

Exact cutoffs may vary slightly by source, but commonly used criteria include:

• Muscle thickness > 3–4 mm.
• Pyloric channel length > 14–17 mm.
• Absent or minimal passage of gastric contents into the duodenum.

Management

Management has two main components: preoperative stabilization and definitive surgical correction. Pyloric stenosis is a medical emergency in terms of fluid/electrolyte management, but the surgery itself is semi-urgent and should be performed after correction of metabolic derangements.

1. Initial stabilization

• NPO (nil per os) to prevent further vomiting and aspiration risk.
• IV fluid resuscitation:
  • Start with isotonic fluids (e.g., 0.9% saline) to correct dehydration and sodium deficits.
  • Add potassium to IV fluids once adequate urine output is confirmed and serum K⁺ is known.
• Correct metabolic alkalosis and electrolyte abnormalities:
  • Target normalization of serum chloride, potassium, and bicarbonate before anesthesia.
  • Frequent monitoring of electrolytes and acid–base status.
• Gastric decompression with nasogastric tube in selected cases with significant gastric distension.

2. Definitive therapy – Pyloromyotomy

• Surgical treatment is required; pyloric stenosis does not resolve with medical therapy alone.
• Ramstedt pyloromyotomy is the standard procedure:
  • Longitudinal incision through the serosa and hypertrophied circular muscle of the pylorus, stopping short of the mucosa.
  • Allows the mucosa to bulge through the split muscle ring, thereby relieving the obstruction.
• Approaches:
  • Open pyloromyotomy: via a right upper quadrant or periumbilical incision.
  • Laparoscopic pyloromyotomy: increasingly used; associated with shorter recovery and better cosmesis in many series.
• Postoperative care:
  • Gradual reintroduction of feeds, typically within 12–24 hours.
  • Small, frequent feeds; occasional mild vomiting in first 24–48 hours can be normal.
  • Monitor hydration and weight gain.

Complications

• Preoperative:
  • Severe dehydration and electrolyte disturbances.
  • Failure to thrive.
• Intraoperative/postoperative:
  • Mucosal perforation during pyloromyotomy (requires repair; may convert to open).
  • Incomplete pyloromyotomy with persistent obstruction (recurrent symptoms; may need reoperation).
  • Wound infection, bleeding, or incisional hernia (less common).

Prognosis

• With timely diagnosis and appropriate management, prognosis is excellent.
• Most infants have complete symptom resolution and normal growth and development.
• Recurrence after adequate pyloromyotomy is rare.

Key Clinical Pearls and Exam Tips

• Presentation window: Think pyloric stenosis in a 2–8 week old first-born male with progressive non-bilious vomiting.
• Nature of vomiting: Projectile, non‑bilious vomiting shortly after feeds is classic.
• Hungry vomiter: Infant is eager to feed again immediately after vomiting, at least early in the course.
• Physical exam buzzword: Palpable olive-shaped mass in the right upper quadrant/epigastrium plus visible gastric peristalsis.
• Metabolic abnormality: Hypochloremic, hypokalemic metabolic alkalosis ± paradoxical aciduria due to prolonged vomiting and renal compensation.
• Imaging of choice: Ultrasound with increased pyloric muscle thickness > 3–4 mm and length > 14–17 mm.
• Stabilize before surgery: Always correct dehydration and electrolyte derangements before pyloromyotomy (high‑yield anesthesia consideration).
• Definitive treatment: Ramstedt pyloromyotomy (open or laparoscopic) is curative in the vast majority of patients.
• Differential with bilious vomiting: Bilious vomiting suggests obstruction distal to the ampulla of Vater (e.g., duodenal atresia, malrotation with volvulus), not pyloric stenosis.
• Risk factor to remember: Early macrolide (erythromycin) exposure in neonates is associated with increased risk of pyloric stenosis.

Summary for Exams

Pyloric stenosis is a common cause of gastric outlet obstruction in early infancy, classically presenting with projectile non‑bilious vomiting, a palpable olive mass, and metabolic alkalosis. Diagnosis is confirmed by abdominal ultrasound, initial management focuses on fluid and electrolyte correction, and definitive therapy is Ramstedt pyloromyotomy. Recognizing this pattern quickly is critical both in clinical practice and on exams.