A 6-month-old girl from Delhi presents with recurrent episodes of projectile vomiting for 3 weeks. The infant appears dehydrated with visible peristaltic waves moving from left to right across the epigastrium. On palpation, a firm, olive-shaped mass is felt in the epigastric region. Abdominal ultrasound is ordered. What is the expected ultrasound finding?

Explanation

## Infantile Hypertrophic Pyloric Stenosis (IHPS) **Key Point:** IHPS is the most common cause of gastric outlet obstruction in infants, presenting with projectile vomiting between 2–8 weeks of age (peak 3–5 weeks). The condition results from smooth muscle hypertrophy and hyperplasia of the pylorus. ### Clinical Features | Feature | Details | |---------|----------| | **Age of onset** | 2–8 weeks (peak 3–5 weeks) | | **Sex predilection** | Male > Female (4:1) | | **Presentation** | Projectile vomiting, hungry infant, visible peristalsis | | **Palpable mass** | Olive-shaped, epigastric, firm | | **Metabolic derangement** | Hypochloremic, hypokalemic metabolic alkalosis | **Clinical Pearl:** The classic description is a "hungry vomiter" — the infant appears hungry immediately after vomiting and feeds again, leading to repeated vomiting cycles. ### Ultrasound Diagnostic Criteria (Gold Standard) **High-Yield:** Ultrasound is the imaging modality of choice with >95% sensitivity and specificity. **Mnemonic:** **PHAT** = Pyloric Hypertrophy Assessment Tool - **P**yloric muscle thickness: **>4 mm** (measured in transverse plane) - **H**ypertrophied pyloric channel length: **>14 mm** (measured in longitudinal plane) - **A**ntral wall thickness: >3 mm - **T**arget sign: Concentric hypoechoic muscle surrounding echogenic mucosa ### Ultrasound Technique - **Transverse plane:** Measure pyloric muscle thickness (outer to outer margin) - **Longitudinal plane:** Measure pyloric channel length - **Dynamic assessment:** Observe lack of pyloric opening (failure of pyloric relaxation) - **Associated findings:** Dilated antrum (>3 cm), delayed gastric emptying ### Pathophysiology Smooth muscle hypertrophy leads to: 1. Increased pyloric resistance 2. Gastric outlet obstruction 3. Gastric distension and hyperperistalsis 4. Projectile vomiting and dehydration 5. Metabolic alkalosis (loss of HCl) **Warning:** Do not confuse IHPS with gastroesophageal reflux (GER) — GER does not produce projectile vomiting or a palpable mass, and ultrasound is normal. ### Management - **Preoperative:** Fluid and electrolyte resuscitation; correct metabolic alkalosis - **Definitive treatment:** Pyloromyotomy (Ramstedt procedure) - **Imaging role:** Confirm diagnosis; rule out other causes of vomiting