Differential Diagnosis for 5-Week Infant with Vomiting and Poor Weight Gain

• Single Most Likely Diagnosis
  • Pyloric stenosis: This condition is characterized by the narrowing of the pylorus, the passage between the stomach and small intestine, leading to projectile vomiting after feedings. The symptoms worsened over 3 days, and the presence of a scaphoid abdomen, which is a common finding in pyloric stenosis due to the stomach being emptied frequently, supports this diagnosis. The low sodium, potassium, and chloride levels with normal bicarbonate also point towards metabolic alkalosis, a common complication of pyloric stenosis.
• Other Likely Diagnoses
  • Gastroesophageal reflux disease (GERD): Although GERD typically presents with bilious vomiting, it can sometimes cause non-bilious vomiting, especially in infants. The irritability and poor weight gain could be attributed to discomfort and difficulty feeding due to reflux.
  • Congenital adrenal hyperplasia: This condition can lead to electrolyte imbalances, including low sodium and potassium levels, due to the impaired production of aldosterone. However, it would typically be associated with other signs such as ambiguous genitalia in females or virilization.
• Do Not Miss Diagnoses
  • Intussusception: Although less common in this age group and typically presenting with bilious vomiting and abdominal tenderness, intussusception can lead to severe complications if not promptly diagnosed and treated. The absence of bilious vomiting and tenderness does not rule out this condition entirely.
  • Hirschsprung's disease: This congenital condition affects the colon and can cause severe constipation, vomiting, and failure to thrive. The scaphoid abdomen could be misleading, as Hirschsprung's disease often presents with a distended abdomen. However, the absence of bilious vomiting and the specific electrolyte imbalance might make this less likely.
• Rare Diagnoses
  • Congenital chloride diarrhea: A rare genetic disorder leading to chronic diarrhea and metabolic alkalosis due to the excessive loss of chloride ions. While it could explain the electrolyte imbalance, the primary symptom of diarrhea is not mentioned.
  • Other metabolic disorders: Various inborn errors of metabolism can present with vomiting, electrolyte imbalances, and failure to thrive. These are rare and would typically require specific diagnostic tests to confirm.