Neonatal Hyperammonemia With Respiratory Alkalosis
A 3-day-old term boy is admitted with poor feeding, vomiting, and lethargy progressing to tachypnea. He was discharged at 36 hours. Examination shows a sleepy infant with mild hepatomegaly, normal tone, and no dysmorphism. Ammonia is 450 µmol/L, pH 7.48, bicarbonate 24 mmol/L, glucose 85 mg/dL, lactate 2.1 mmol/L, and BUN 4 mg/dL. Urine ketones are negative. Newborn screening results are not yet available. He has had one brief tonic seizure.
1. What immediate interventions do you order, and which should be started before confirmatory tests return?
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I would treat this as a metabolic emergency before confirmatory results. I would stop all protein and enteral feeds, start 10% dextrose intravenously at approximately 1.5 times maintenance to suppress catabolism, and obtain urgent plasma amino acids, urine organic acids, acylcarnitine profile, lactate, and repeat ammonia. The respiratory alkalosis, low BUN, and absent ketones point toward a urea cycle disorder rather than an organic acidemia. I would give arginine and nitrogen scavengers such as sodium benzoate and sodium phenylbutyrate under metabolic guidance, correct dehydration carefully, and treat the seizure. If ammonia exceeds 500 µmol/L or encephalopathy progresses, I would arrange urgent hemodialysis, not just pharmacologic scavenging. I would avoid bolusing protein or lipid before the diagnosis is clarified.
2. How do plasma amino acids and urine organic acids distinguish urea cycle disorder from organic acidemia?
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In a urea cycle disorder, plasma amino acids typically show elevated glutamine and low or absent citrulline or argininosuccinate depending on the enzyme, and urine organic acids show increased orotic acid in ornithine transcarbamylase deficiency. The blood gas often shows respiratory alkalosis, and the anion gap is not markedly elevated. In organic acidemia, I would expect a high-anion-gap metabolic acidosis, ketosis, elevated urinary organic acids such as methylmalonic or propionic acid, and secondary hyperammonemia. The absence of ketones and the respiratory alkalosis in this infant favor a urea cycle defect. I would still send both tests because early treatment cannot wait and mixed or secondary disorders occur. The pattern, not a single ammonia value, drives the diagnosis.
3. What long-term management and monitoring plan do you discuss with the family?
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Long-term care requires a metabolic team, a low-protein diet with essential amino acid supplementation, nitrogen scavengers, and arginine or citrulline depending on the specific defect. I would monitor plasma ammonia, amino acids, growth, and neurodevelopment, and teach the family a sick-day plan: stop protein, give glucose, and seek urgent care for vomiting or lethargy. I would discuss that hyperammonemic crises can cause permanent cognitive and motor injury, and that severe urea cycle disorders may require liver transplantation. Genetic counseling and carrier testing are important because the inheritance depends on the enzyme; ornithine transcarbamylase deficiency is X-linked. The family should understand that newborn screening may be normal in the first days, so a normal screen does not exclude the diagnosis.