Ultrasound Findings in Congenital Hypertrophic Pyloric Stenosis
The definitive ultrasound diagnosis of hypertrophic pyloric stenosis requires pyloric muscle thickness >3-4 mm and pyloric channel length >15-17 mm, with ultrasound being the diagnostic modality of choice with 95-100% sensitivity and specificity. 1
Primary Diagnostic Criteria
Pyloric muscle wall thickness >3-4 mm is the main diagnostic criterion for hypertrophic pyloric stenosis on ultrasound. 1, 2 The American College of Radiology establishes this as the definitive measurement threshold, though the exact cutoff varies slightly by institution (3-4 mm). 1
Pyloric channel length >15-17 mm indicates abnormal elongation characteristic of HPS. 1, 2 Some sources define this as >12 mm, but the more conservative 15-17 mm threshold provides higher specificity. 2
Key Imaging Features
"Target sign" or "doughnut sign" on transverse view - the hypertrophied pyloric muscle appears as a thick hypoechoic ring surrounding the echogenic mucosa in the center. 1, 2
Failure of the pyloric canal to relax - real-time observation shows persistent narrowing without normal opening and closing during the examination. 2
Lack of gastric emptying - fluid remains in the stomach without passage through the pylorus during the study. 3
Critical Age-Related Considerations
In infants younger than 21 days, lower thresholds should be applied - muscle thickness >3.5 mm may be diagnostic rather than the standard 4 mm cutoff used in older infants. 4 This is crucial because mean measurements in proven HPS cases for infants ≤21 days were significantly lower (MT 3.7 mm vs 4.6 mm in older infants). 4
Important Pitfalls to Avoid
Pylorospasm can mimic HPS measurements during portions of the examination, with 53% of pylorospasm cases showing muscle wall thickness ≥4 mm and 18% showing pyloric length ≥18 mm at some point during scanning. 5 The key distinguishing feature is measurement variability - pylorospasm shows considerable fluctuation in measurements throughout the study, while true HPS maintains unchanged measurements. 5
If initial ultrasound is negative but clinical suspicion remains high, repeat ultrasound in 48 hours rather than dismissing the diagnosis, as HPS can evolve over days. 3
Role of Point-of-Care Ultrasound (POCUS)
The European Society of Paediatric and Neonatal Intensive Care acknowledges that POCUS may recognize HPS, though definitive diagnosis should be confirmed by a pediatric radiologist. 6 This reflects disagreement in the guidelines (quality of evidence D), suggesting that while POCUS can identify HPS, formal radiology confirmation is prudent before surgical intervention. 6
Ultrasound Technique Considerations
Systematic scanning of the antropyloric region should be performed with the infant in the right lateral decubitus or supine position, using a high-frequency linear transducer to optimize visualization of the pyloric muscle layers. 2 Real-time observation over several minutes is essential to distinguish true HPS from transient pylorospasm. 5