Ultrasound Diagnostic Criteria
| Measurement | Normal | HPS (Positive) |
|---|---|---|
| Pyloric muscle thickness (single wall) | <3 mm | ≥4 mm |
| Pyloric channel length | <14 mm | ≥17 mm |
| Pyloric transverse diameter | <13 mm | ≥13 mm |
Measure on a RELAXED pylorus. Active pyloric contraction mimics hypertrophy — the muscle wall thickens transiently during peristalsis. The definitive real-time finding is absent gastric emptying through the pylorus despite active antral peristalsis (the "cervix sign" — stomach contracts but nothing passes). Wait for a relaxed phase before obtaining measurements.
Key Clinical Points
Metabolic consequence: Repeated vomiting of gastric contents → hypochloremic, hypokalemic metabolic alkalosis. This must be corrected with IV fluids (normal saline + KCl) before surgical pyloromyotomy (Ramstedt procedure). Operating on an uncorrected alkalosis risks apnea under anesthesia.
UGI series (if US equivocal): String sign (thin elongated pyloric channel) and shoulder sign (pyloric mass indenting the antrum) — now rarely needed given US accuracy.
Pitfalls
| Pitfall | Explanation |
|---|---|
| Measuring during contraction | Transiently falsely elevated muscle thickness; wait for relaxation |
| Incomplete pyloric visualization | Prone positioning and graded compression helps; off-axis measurements are unreliable |
| Pylorospasm | Functional spasm without true hypertrophy; resolves on serial imaging; no surgical intervention |
| Age outside typical window | HPS very rare before 2 weeks or after 3 months; reconsider diagnosis at the extremes |