Pyloric Stenosis

Hypertrophic pyloric stenosis (HPS) is a condition of infancy in which progressive hypertrophy and hyperplasia of the pyloric sphincter muscle causes gastric outlet obstruction. The pylorus is the muscular valve between the stomach and the duodenum; as the circular muscle layer progressively thickens, the pyloric channel narrows until it becomes nearly completely obstructed, preventing gastric contents from passing into the duodenum. This results in forceful, projectile, non-bilious vomiting (the obstruction is proximal to the ampulla of Vater where bile enters the duodenum, so vomited material contains no bile). The condition typically presents between 2-8 weeks of age (most commonly at 3-5 weeks), with firstborn males affected 4-5 times more often than females. The infant is characteristically hungry immediately after vomiting ('hungry vomiter') because they are not nauseated — the stomach simply cannot empty. Progressive vomiting leads to dehydration, weight loss, and a characteristic metabolic alkalosis: loss of hydrochloric acid (H⁺ and Cl⁻) from vomiting causes hypochloremic metabolic alkalosis, and the kidneys paradoxically excrete potassium and hydrogen ions to retain sodium ('paradoxical aciduria'). On physical examination, an 'olive-shaped' mass may be...