Hypertrophic Pyloric Stenosis

Hypertrophic pyloric stenosis is an infantile condition in which the circular muscle of the pylorus becomes abnormally thick. The pylorus is the muscular outlet between the stomach and duodenum. As its channel narrows, milk and gastric secretions cannot pass normally into the small intestine, creating a functional gastric outlet obstruction. The stomach responds by contracting more forcefully, so vomiting becomes increasingly forceful and may be projectile. The vomit is classically non-bilious because it comes from the stomach, before bile enters the gastrointestinal tract. The infant may remain eager to feed after vomiting because little nutrition has reached the intestine. Repeated loss of hydrochloric acid and fluid produces dehydration and a characteristic metabolic pattern: hypochloremia, hypokalemia, and metabolic alkalosis. Reduced circulating volume activates the renin–angiotensin–aldosterone system, which promotes further renal potassium loss and bicarbonate retention. This is why a baby who initially appears hungry can later become weak, sleepy, tachycardic, and oliguric. The immediate threat is usually not the narrowed pylorus itself. It is the combination of inadequate intake, ongoing gastric losses, and electrolyte disturbance. Surgery corrects the obstruction, but the...