Purpose or Case Report: To standardize the measurements of the pyloric muscle and to evaluate the stratified appearance of the pylorus with the current high frequency transducers in normal pylorus and hypertrophic pyloric stenosis (HPS).
Methods & Materials: We retrospectively analyzed all abdominal ultrasounds in infants with suspected HPS from Jan 2019-Dec 2020. We excluded studies with poor quality, those without linear transducers or transducers under 12MHz, and those from outside. We evaluated the layers of the pylorus from outer to inner starting with the muscularis propria, submucosa, muscularis mucosa and mucosal interface. Two pediatric radiologists measured the muscle thickness of the pylorus independently by two methods. Measurement (a) includes the outer layer of muscularis propria till the inner layer of muscularis mucosa. Measurement (b) only includes the muscularis propria.

We divided the cases into normal cases and HPS while assessing the stratified appearance of the pylorus for all. The mean, range of the muscle thickness and the diagnostic accuracy of the two sets of measurements were calculated.
Results: We included 300 infants (114F:186M). 59 cases of HPS were confirmed by surgery. 241 cases were normal, discharged without surgery or re-admission. There was a strong agreement between the two readers. Measurement (a), mean thickness is 2.5 mm (range 1.5 mm – 4.0 mm) in normal cases and 4.9 mm (range 3.0 mm-7.0 mm) in HPS. Measurement (b), mean thickness is 1.5 mm (range 0.6 mm–3.0 mm) in normal cases and 3.9 mm (range 2.0 mm - 5.8 mm) in HPS. Measurement (a) has an accuracy of 84.5% (95%CI: 79.7%,88.6%) with 87.5% sensitivity and 83.5% specificity while measurement (b) has an accuracy of 93.5% (95%CI:90%,96.1%) with 76.4% sensitivity and 99.5% specificity.

The pylorus stratification is preserved in all normal cases and 31 cases of HPS. There was partial loss of stratification in 18 cases of HPS and complete loss of stratification in 10 cases HPS. The muscularis propria was more hyperechogenic in all HPS in contrast to normal cases showing similar echogenicity to the muscularis mucosa.
Conclusions: Measuring the muscularis propria solely has a better diagnostic accuracy and specificity, decreasing the need of unnecessary follow ups. Determining the best method of measuring the pylorus decreases the overlap of negative and positive cases. The loss of pyloric muscle stratification is only seen in HPS. The radiological diagnosis of HPS needs a combination of quantitative, qualitative and functional assessment.