projectile Emesis tends to worsen in frequency and severity over days to weeks In the past, diagnosis was made by clinical history and palpation of the hypertrophied pyloric muscle, or “olive” in the abdomen Infants often presented with dehydration and electrolyte abnormalities caused by repeated vomiting, typically a hypokalemic, hypochloremic metabolic alkalosis However, with earlier presentation and diagnosis, fewer patients have an olive palpated at diagnosis, and the majority not have the classic electrolyte derangements at presentation HPS is associated with erythromycin use in the first two weeks of life; however, no association has been found between macrolide use during pregnancy or breastfeeding and HPS HPS should be suspected in the young infant presenting with progressively worsening nonbilious emesis, and can be diagnosed using ultrasound A hypertrophied pylorus with muscle wall thickness of mm or greater and length of 15 mm or greater with no passage of gastric contents into the small intestine confirms the diagnosis Treatment is surgical, with laparoscopic pyloromyotomy Prior to surgery, the patient should be well hydrated and any electrolyte abnormalities should be corrected Intussusception occurs when one portion of the bowel telescopes into its distal segment, commonly the terminal ileum into the cecum The peak incidence for intussusception is between months and years of age, although it remains one of the most common causes of obstruction up to years of age Patients typically present with intermittent episodes of abdominal pain, during which they may cry or pull up the legs Children may be lethargic between episodes, and infants may present only with lethargy and without the classic episodes of pain Bilious emesis and blood-tinged “currant jelly” stools may be seen However, the classic triad of abdominal pain, vomiting, and bloody stools is seen in less than a quarter of children with intussusception, so a high level of clinical suspicion must be maintained when any of these symptoms are present (refer Chapter 53 Pain: Abdomen ) Less common causes of obstruction should also be considered in infants presenting to the ED with vomiting A thorough clinical examination should be performed to evaluate for signs of incarcerated inguinal or umbilical hernias Meckel diverticulum, which results from incomplete obliteration of the omphalomesenteric duct, is the most common congenital anomaly of the GI tract in children While the majority of patients are asymptomatic, children can occasionally present with obstruction Enteric duplication cysts can also lead to intestinal obstruction or can act as lead points for intussusception