**7. Hypertrophic pyloric stenosis**

Hypertrophic pyloric stenosis (HPS) is the most common cause of surgery in vomiting infants due to the failure of relaxation of the pyloric sphincter of stomach [39]. The disease usually appears between 2nd and 12th week of life and commonly affects white males [2, 39]. The typical complaint is non-bilious, projecting vomiting by a previously healthy infant after feeding. HPS is not an actual emergency unless severe dehydration or excessive electrolyte loss occur. HPS can be palpated as a pyloric mass in the epigastrium on physical examination (olive sign) [22]. Preoperative US is the gold standard radiologic modality for the diagnosis of HPS with sensitivity, specificity and accuracy of approximately 100% if adequate equipment is provided [40].

The US scanning begins with placing the baby in a supine or right lateral decubitus position. A high-frequency (10–18 MHz), linear-array transducer should be applied from sub-xiphoid area to the right paramedian area to search for pylorus [1]. If adequate fluid is not present in the stomach, breast feeding or oral sugar contained water can be given in order to displace the air in the stomach and to see the passage of the fluid [41]. Normal position of the pylorus can be demonstrated between the liver and the head of the pancreas, medial to the gallbladder. If abundant air present in the gastric antrum, the patient should be moved into the right lateral decubitus position, to displace air into the fundus and to move pylorus anteriorly [3]. To confirm the HPS, pyloric canal length and thickness of the pyloric muscle should be measured [1]. Pyloric muscle thickness > 3 mm, canal length > 17 mm, and antero-posterior diameter of pylorus >12 mm confirm the diagnosis of HPS with high accuracy (**Figure 2**) [1, 3, 22]. By the way, pylorus is a dynamic structure and muscle thickness may change due to peristalsis during a real-time US examination. Therefore, imaging for a sufficient time is needed to exclude pylorospasm from HPS, which is a transient phenomenon [41]. Other ancillary findings to diagnose HPS are the prolapsed mucosa into the gastric antrum (antral nipple or cervix sign) and trapped fluid within the crevices of mucosa. The main reason of false-negative result is the overdistention of stomach that moves antra-pyloric canal posteriorly [3]. To overcome this issue, gastric content can be aspirated via nasogastric or orogastric tube.

### **Figure 2.**

*(A) Normal appearance of pylor in a 25-day-old baby and (B) hypertrophic pyloric stenosis in a 30-day-old boy. Hypertrophic pylor (arrows) is thicker and longer than normal (dashed arrows) that does not permit the passage of gastric content into the duodenum.*
