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Pyloric stenosis, occurring in approximately 1 out of 600 live births, is one of the most common gastrointestinal abnormalities of the newborn. It usually manifests within the first few weeks of life and affects males three to four times more often than females, and firstborn males are more likely to be affected than their siblings. Pyloric stenosis consists of hypertrophied circular and longitudinal fibers of the muscularis propria of the pylorus that results in gastric outlet obstruction. It may occur in association with Turner syndrome, Edward’s syndrome (trisomy 18), or esophageal atresia.

Cardinal features include persistent, projectile, nonbilious vomiting, visible peristalsis, and a hypochloremic metabolic alkalosis. Jaundice, due to caloric deprivation and hepatic glucuronyl transferase deficiency, occurs in less than 5% of children. The diagnosis is usually confirmed by abdominal ultrasonography, with the classic physical examination feature of a palpable olive-sized mass in the upper abdomen or distal pylorus. Infants may also be jaundiced as a result of starvation decreasing glucuronyl transferase activity (Table 140-1).

TABLE 140-1Summary of Key Characteristics for Pyloric Stenosis


Pyloric stenosis usually results in metabolic alkalosis with associated hypochloremia and hypokalemia due to a loss of hydrogen and chloride ions from vomiting gastric contents. However, if the dehydration is severe enough, paradoxical aciduria may occur. Dehydration also produces a lactic acidosis and ketosis due to starvation. Thus, a mixed metabolic acidosis may be present.

Compensatory mechanisms for metabolic alkalosis arise from the respiratory system and the renal system. An alkalotic condition results in chemoreceptors within the brainstem to trigger hypoventilation and a subsequent respiratory acidosis. The renal system “wastes” bicarbonate, producing alkaline urine as an increased bicarbonate load is presented to the kidney and the capacity of the proximal convoluted tubule to reabsorb bicarbonate is overwhelmed. Sodium follows bicarbonate within the nephron unit, resulting in volume depletion, heightened aldosterone secretion, and subsequent sodium retention and kaliuresis. Furthermore within the distal convoluted tubule, potassium is exchanged with hydrogen ion through the hydrogen–potassium countertransport pump in an effort to retain hydrogen ions and balance the pH in a more favorable acidotic condition. This exacerbates the body’s loss of potassium. If the hypokalemia is severe, sodium exchange is preferentially substituted with hydrogen in the distal convoluted tubule, producing the “paradoxically” acidic urine.


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