Pediatric Abdomen Pediatric patients often present to the ED for evaluation of vomiting and/or abdominal pain In infants less than months of age, there is often concern for hypertrophic pyloric stenosis (HPS), requiring ultrasound for definitive diagnosis In the toddler age group, colicky abdominal pain with associated emesis can be signs of intussusception, a pathology that is also diagnosed with sonography In children of all ages, abdominal pain that is localized to lower right side of the abdomen raises concern for appendicitis Ultrasound has become the first-line diagnostic modality for appendicitis but sensitivity depends on patient characteristics and sonographer skill Recent literature has demonstrated the ability of pediatric emergency medicine providers to identify pyloric stenosis, intussusception, and appendicitis on bedside sonography Hypertrophic Pyloric Stenosis Anatomy The gastric outlet in infants abuts the medial portion of the hepatic contour The pyloric channel connects the antrum of the stomach to the first portion of the duodenum and is surrounded by muscle The position of the pylorus is dependent upon the fullness of the stomach, but is typically adjacent to the gall bladder and anteromedial to the right kidney Technique A high-frequency linear probe, preferably with a large footprint, positioned on the upper abdomen to the right of midline, will allow visualization of the pylorus as it extends from the stomach, deep to the liver The probe is oriented longitudinally with the marker angled slightly toward the right shoulder for the short-axis view and then rotated 90 degrees in the transverse plane to obtain the long-axis view In the long axis, the inner channel appears as a narrow canal flanked by the linear hyperechoic walls that are surrounded by the hypoechoic muscular pylorus (Fig 131.20 ) As the pylorus thickens, it often extends superiorly toward the gall bladder and when the channel has become obstructed, absence of passage of liquids can be seen as retrograde peristalsis into the stomach Measurements of the pylorus should be performed to assess for hypertrophy with an abnormal muscle thickness measuring greater than mm and an abnormal channel length measuring greater than 17 mm Pitfalls