A 3-day-old male neonate born to non-consanguineous parents presents with lethargy, poor feeding, and vomiting. Serum ammonia is 180 µmol/L (normal <50 µmol/L). Plasma amino acid profile shows elevated glutamine and alanine. Urine orotic acid is markedly elevated. Genetic testing confirms ornithine transcarbamylase (OTC) deficiency. What is the most appropriate immediate management?

Explanation

## Immediate Management of Acute Hyperammonemia in OTC Deficiency ### Clinical Context This is a neonatal presentation of OTC deficiency (the most common urea cycle disorder), manifesting with hyperammonemia, elevated plasma glutamine, and markedly elevated urinary orotic acid (due to carbamoyl phosphate shunting to pyrimidine synthesis). ### Correct Management Strategy **Key Point:** The immediate goal is rapid ammonia reduction via two mechanisms: (1) **ammonia scavenging** with sodium benzoate and sodium phenylbutyrate, and (2) **protein restriction** to reduce nitrogen load. **High-Yield:** Sodium benzoate conjugates glycine to form hippurate (excreted in urine), removing 1 mole of nitrogen per mole of benzoate. Sodium phenylbutyrate is metabolized to phenylacetate, which conjugates with glutamine to form phenylacetylglutamine (removes 2 moles of nitrogen per mole of phenylbutyrate). ### Dosing and Protocol - **Sodium benzoate:** 250 mg/kg/day (divided doses) - **Sodium phenylbutyrate:** 400–600 mg/kg/day (divided doses) - **Protein restriction:** 0.5–1.0 g/kg/day in acute phase; titrate based on ammonia levels and clinical response - **L-arginine:** 100–200 mg/kg/day (essential amino acid in OTC deficiency; also promotes ammonia removal via urea cycle) ### Monitoring - Serum ammonia every 4–6 hours until <100 µmol/L - Plasma amino acids (glutamine, alanine) - Neuroimaging if altered mental status persists **Clinical Pearl:** Early, aggressive ammonia reduction within the first 24–48 hours is critical to prevent hyperammonemic encephalopathy and permanent neurological damage. ### Long-term Management - Protein-restricted diet (1.0–1.5 g/kg/day) - Continued benzoate and phenylbutyrate - L-arginine supplementation - Liver transplantation is definitive cure but reserved for recurrent hyperammonemic crises or poor dietary control ## Why Other Options Are Incorrect | Option | Why Wrong | |--------|----------| | High-dose corticosteroids | No role in acute hyperammonemia management; delays definitive ammonia-lowering therapy | | Emergency hemodialysis alone | Removes ammonia poorly; phenylbutyrate and benzoate are more effective and faster | | L-arginine alone | Insufficient monotherapy; must combine with benzoate/phenylbutyrate for rapid ammonia reduction | [cite:Harrison 21e Ch 355]