Pyloric Stenosis Ultrasound: Diagnosis Criteria and Scanning Technique

Hypertrophic pyloric stenosis (HPS) is a common surgical emergency in infants, typically presenting between 2 and 8 weeks of age with projectile non-bilious vomiting. Ultrasound is the gold standard diagnostic test — with the right technique, it's fast, accurate, and avoids radiation.

Clinical Presentation

• Non-bilious projectile vomiting (bile = bilious → not pyloric stenosis, think obstruction below ampulla)
• Typically presents at 3–6 weeks of age
• More common in first-born males
• Progressive worsening over days
• "Olive sign" palpable in the epigastrium (rare but pathognomonic)
• Metabolic alkalosis on labs (hypochloremic, hypokalemic)

Equipment and Setup

Probe: 7–15 MHz linear transducer. Use the highest frequency that gives you adequate depth.

Patient position: Supine or right lateral decubitus (right side down). The right lateral decubitus position rotates the pylorus toward you and is often easier to scan.

Feeding before the scan: Many centers give the infant a small amount of glucose water or breast milk during the scan. This fills the stomach with fluid, which acts as an acoustic window and lets you watch peristalsis and fluid transit through (or not through) the pylorus.

Preparation: Allow the infant to be calm. A pacifier helps. Don't rush — a crying infant with a tense abdomen is much harder to scan.

Finding the Pylorus

This is the technical challenge of the exam. The pylorus connects the gastric antrum to the duodenal bulb, and it lies in the right upper quadrant, slightly posterior.

Steps:

1. Start with the liver in view to orient yourself
2. Identify the gallbladder — the pylorus is just medial and inferior to the gallbladder
3. Follow the gastric antrum rightward — the antrum leads to the pylorus
4. The right portal vein is a useful posterior landmark — the pylorus lies anterior to it
5. Fluid in the stomach (if fed) will help — watch for peristaltic activity

In transverse: The pylorus appears as a target sign — an echogenic center (mucosa) surrounded by a hypoechoic ring (muscle).

Measurements

Once you've found the pylorus, obtain the following in the optimal plane:

1. Pyloric Muscle Thickness (PMT)

Measured in transverse cross-section of the pylorus:

• Place calipers at the outer hypoechoic muscular wall to the inner mucosa-muscle interface
• Measure only the muscle, not the echogenic mucosa
• Measure on both sides (right and left wall) — report the larger or average

Normal: < 3 mm Abnormal (HPS): ≥ 3–4 mm (most institutions use ≥ 3 mm; ≥ 4 mm is highly specific)

2. Pyloric Channel Length (PCL)

Measured in longitudinal section of the pylorus:

• Length of the elongated muscle channel from gastric end to duodenal end
• The muscle creates a "shoulder" on each end — measure between the shoulders

Normal: < 15 mm Abnormal (HPS): ≥ 16–17 mm (most use ≥ 16 mm or ≥ 17 mm)

3. Pyloric Diameter (optional)

Measured in transverse:

• Outer diameter of the pylorus
• Abnormal: > 13–15 mm

What HPS Looks Like

Transverse view: Target sign — echogenic mucosal center, thick hypoechoic muscular ring. In HPS, the muscle ring is visibly thickened.

Longitudinal view: The pyloric channel is elongated. The muscle "shoulders" create a beak-like appearance at the gastric end — the gastric antrum is dilated and funnels into the narrow, elongated pyloric channel. Mucosa is prolapsed into the antrum at the gastric end (antral nipple sign — not always present).

Dynamic observation: Watch for 3–5 minutes with fluid in the stomach. In HPS, peristaltic waves hit the pylorus and fail to pass through — the stomach continues to contract without emptying. In a normal pylorus, fluid passes freely into the duodenum.

Pylorospasm vs HPS

Pylorospasm (functional narrowing) can mimic HPS and cause elevated measurements. Distinguish by:

Feature	HPS	Pylorospasm
Muscle thickness	Consistently ≥ 3–4 mm	Variable, may normalize
Channel length	Consistently ≥ 16–17 mm	May be borderline
Fluid transit	Absent	Present (even if delayed)
Muscle appearance	Uniformly hypoechoic, fixed	May vary during observation

Key: If you see any fluid passing through the pylorus into the duodenum during 3–5 minutes of observation, HPS is much less likely. A fixed, non-opening pylorus with consistent measurements = HPS.

Pitfalls

Can't find the pylorus: Start at the gallbladder and trace medially. Feed the baby if you haven't — fluid fills the antrum and makes it much easier to follow distally to the pylorus.

Measuring the mucosa in PMT: Only the hypoechoic muscle counts. The bright echogenic mucosa is separate. Easy to overcall if you include it.

Oblique measurement of channel length: If your longitudinal view is even slightly oblique, the channel length is artificially shortened. Make sure you have the longest possible view.

Collapsed pylorus: A contracted, empty pylorus can look thick and elongated. Observe after feeding to see it open — true HPS won't open.

Reporting Language

• "Pyloric muscle thickness 1.8 mm, channel length 12 mm. No evidence of hypertrophic pyloric stenosis. Pylorus opens freely with feeding."
• "Pyloric muscle thickness 4.2 mm (bilateral), pyloric channel length 19 mm. Findings are diagnostic of hypertrophic pyloric stenosis."
• "Pyloric muscle thickness borderline at 3.0 mm, channel length 15 mm. Pylorus does not open during observation. Findings are indeterminate — clinical correlation recommended. If high clinical suspicion persists, UGI series or clinical reassessment may be warranted."

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SonoBuddy's Measurements section includes pyloric stenosis diagnostic thresholds. Pediatric protocols are in the Protocols section.