A 4-week-old male presents to outpatient clinic with vomiting, which began two weeks ago and has become progressively worse. He is now vomiting after every feed. The vomit is projectile, non-bilious, and non-bloody. He seems hungry but over the past day, he has become fussy and urine output has decreased. He has no fever, respiratory symptoms, diarrhea, or rashes.

On examination, the infant appears alert but mildly dehydrated. His exam reveals fullness in the epigastric region. A quiet, a peristaltic wave is noted (Figure 57-1).

Ultrasound of the abdomen confirms the diagnosis of pyloric stenosis. Laboratory studies reveal hypochloremic, hypokalemic, and metabolic alkalosis. He is admitted for intravenous hydration and undergoes laparoscopic pyloromyotomy the following day. He is discharged home a day later, tolerating breast milk with resolution of his vomiting.

Pyloric stenosis is defined as the progressive thickening and elongation of the pyloric channel ultimately resulting in partial or complete gastric outlet obstruction.

Infantile hypertrophic pyloric stenosis (IHPS).

• IHPS occurs in 0.5–4/1000 live births. Estimates vary by region and surveillance method.¹

• There is a clear gender bias with males affected more than females 4 to 5: 1.^1–4

• A genetic component is theorized with increased rates in twins and other relatives.^1,2

• First-born children are affected more often.¹

• Maternal race and/or ethnicity alter prevalence. IHPS is more common in white and Hispanic families than African or Asian descent.¹

• IHPS is the most common condition requiring surgery in early infancy.²

• The underlying etiology of IHPS remains unknown.

• Causes are likely multifactorial with genetic and environmental contributions.

• Pyloric sphincter function is abnormal. The muscle fails to relax due to decreased nitric oxide synthase production and abnormal smooth muscle receptors.¹

• Over time and with activation of local growth factors, the pylorus gradually thickens producing outlet obstruction. In compensation, the stomach dilates, thickens, and peristalses leading to emesis with every feed.

• Genetic factors clearly play a role. There is a 200-fold increase in monozygotic twins and a 20-fold increase in dizygotic twins and siblings, as compared to individuals without affected relatives.²

• Erythromycin treatment used for pertussis prophylaxis increases risk for IHPS up to 10-fold perhaps due to its action as a motilin agonist.^1,4

• Additional relative risk factors including maternal smoking during pregnancy, preterm delivery, small weight for gestational age, cesarean section, congenital malformations, and bottle feeding have been recently implicated.^3,5

• Smith-Lemli-Opitz and Cornelia de Lange syndromes confer a great risk of IHPS.⁴

Clinical Features

• Symptoms classically occur between 3 to 6 weeks of age, rarely starting after 12 weeks.

• The patient develops projectile, nonbilious, nonbloody emesis immediately following feeds.

• The patient remains vigorous and hungry until hydration is compromised.

• When the abdomen is relaxed following emesis, a pyloric “olive” or mass may be palpable by the skilled examiner.

• With dehydration and weight loss, a gastric peristaltic wave may be visualized moving from left to right in the upper abdomen prior to emesis (Figure 57-1).

• Jaundice is seen in 5 to 14 percent of cases.^4,6