The patient underwent an extramucosal longitudinal myotomy of the pylorus known as the Fredet-Ramstedt procedure, where the pyloric hypertrophy was confirmed.

Hypertrophic pyloric stenosis affects 2 to 5 every 1,000 births with a prevalence in first-born male patients and a peak presentation during the 4th week of life. The definitive surgical treatment is the pyloromyotomy. 

Patients are usually referred on an emergent basis. Clinical presentation includes non-bilious vomiting, often mistaken for the onset of gastroesophageal reflux. The possible subsequent gastric obstruction leads to vomiting after every feeding. 

Diagnosis relies on ultrasonography as the first-line option, with a sensitivity and specificity approaching 100% in experienced hands.

Pyloric ultrasound should be performed using a high-resolution linear-array transducer, positioned in a transverse oblique plane parallel to the right lower costal margin. 

Liquid passes readily from the gastric antrum into the duodenal bulb when the pylorus is normal. If the stomach is empty, liquid can be given orally in order to improve pyloric visualization. 
Persistent thickening of the pyloric muscle is the most important finding. Standard measurements are performed on the long-axis views (see images attached with normal values reported).