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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Pyloric stenosis classically presents at 2-8 weeks with progressive projectile nonbilious vomiting after feeds
- Vomiting is nonbilious because obstruction is proximal to the ampulla of Vater
- Classic labs: hypochloremic, hypokalemic metabolic alkalosis from gastric acid loss
- Diagnosis: abdominal ultrasound showing thickened/elongated pylorus
- Management: correct dehydration and electrolytes first, then pyloromyotomy
Overview
Infantile hypertrophic pyloric stenosis is gastric outlet obstruction caused by hypertrophy and hyperplasia of the pyloric muscle. It is a high-yield cause of vomiting in early infancy. The infant is often hungry after vomiting and may develop weight loss, dehydration, and electrolyte derangements. Surgical treatment is definitive, but the child must be medically stabilized before anesthesia.
Epidemiology
Pyloric stenosis is more common in males, firstborn infants, and infants with family history. Associations include macrolide exposure in early infancy and maternal smoking. It typically presents between 2 and 8 weeks of age, rarely at birth.
Clinical Features
Symptoms
Progressive nonbilious projectile vomiting shortly after feeds
Persistent hunger after vomiting
Weight loss, poor weight gain, decreased urine output
No fever or diarrhea in typical cases
Bilious vomiting suggests malrotation with volvulus, not pyloric stenosis
Signs
Dehydration: sunken fontanelle, dry mucosa, delayed capillary refill
Palpable olive-shaped epigastric/RUQ mass after vomiting
Visible gastric peristaltic waves from left to right
Lethargy or shock in severe dehydration
Jaundice may occur from decreased glucuronyl transferase activity
Investigations
First-line
Abdominal ultrasoundDiagnostic test of choice; shows pyloric muscle thickening and channel elongation
BMP/venous blood gasHypochloremic hypokalemic metabolic alkalosis is classic
Weight and hydration assessmentDetermines urgency of fluid/electrolyte resuscitation
Second-line
Upper GI seriesIf ultrasound is equivocal or malrotation/other obstruction is suspected
GlucosePoor intake/vomiting may cause hypoglycemia in young infants
Specialist
Pediatric surgeryDefinitive pyloromyotomy after correction of dehydration and electrolytes
AnesthesiaMetabolic alkalosis and dehydration must be corrected before surgery to reduce perioperative risk
1
Stabilize first
- NPO, IV access, isotonic fluid bolus if dehydrated
- Correct hypochloremia, hypokalemia, and metabolic alkalosis before surgery
- Place NG tube if persistent vomiting or significant distension
2
Confirm diagnosis
- Ultrasound is first-line imaging
- If bilious emesis or atypical age/features are present, evaluate urgently for malrotation/volvulus
3
Definitive treatment
- Ramstedt pyloromyotomy is definitive
- Feeds are gradually restarted postoperatively
- Mild postoperative vomiting can occur transiently; persistent vomiting needs reassessment
Complications
- Severe dehydration: Can progress to shock and AKI if diagnosis delayed
- Electrolyte disturbance: Hypochloremic metabolic alkalosis and hypokalemia increase anesthesia risk
- Failure to thrive: Poor intake retention leads to weight loss
- Aspiration: Recurrent vomiting increases risk before stabilization
USMLE Step 2 CK Exam Tips
- 1Projectile nonbilious vomiting at 2-8 weeks = pyloric stenosis
- 2Nonbilious because obstruction is proximal to the ampulla of Vater
- 3Hypochloremic hypokalemic metabolic alkalosis is the classic lab pattern
- 4Ultrasound is the diagnostic test of choice
- 5Correct fluids and electrolytes before pyloromyotomy
- 6Bilious vomiting in an infant is malrotation with volvulus until proven otherwise
- 7Olive mass is classic but often absent on exams and in real practice
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