A 2-day-old female neonate born to consanguineous parents presents with severe lethargy, poor feeding, and hypotonia. Neonatal screening shows elevated branched-chain amino acids (leucine 850 µmol/L, isoleucine 520 µmol/L, valine 680 µmol/L; normal <200 µmol/L each). Urine organic acids show elevated branched-chain ketoacids. Blood glucose is 45 mg/dL, and arterial pH is 7.22. What is the most likely diagnosis and the immediate next step?

Explanation

## Diagnosis: Maple Syrup Urine Disease (MSUD) ### Clinical Presentation The vignette describes **neonatal MSUD (classic form)**: - **Elevated branched-chain amino acids (BCAAs)**: leucine, isoleucine, valine all >500 µmol/L - **Elevated branched-chain ketoacids** in urine (alloisoleucine also elevated) - **Severe metabolic acidosis** (pH 7.22) with hypoglycemia (45 mg/dL) - **Neurological signs**: lethargy, hypotonia, poor feeding (onset by day 2–3 of life) - **Characteristic "maple syrup" odour** of urine (from branched-chain ketoacids) - **Consanguineous parents** (autosomal recessive inheritance) ### Pathophysiology **Branched-chain α-ketoacid dehydrogenase (BCKDC) deficiency** → accumulation of BCAAs and their ketoacid metabolites → metabolic acidosis, hyperammonemia, cerebral edema, and neurological damage. ### Acute Management Algorithm ```mermaid flowchart TD A["Elevated BCAAs + ketoacidosis in neonate"]:::outcome --> B{"MSUD confirmed?"}:::decision B -->|"Yes"| C["Acute metabolic crisis"]:::urgent C --> D["High-dose dextrose + insulin IV"]:::action D --> E["Stop all protein intake"]:::action E --> F["Correct acidosis & hypoglycemia"]:::action F --> G{"BCAAs not decreasing?"}:::decision G -->|"Yes"| H["Consider hemodialysis or hemofiltration"]:::action H --> I["Remove BCAAs & ketoacids"]:::outcome G -->|"No"| J["Continue IV dextrose + insulin"]:::action J --> K["Transition to BCAA-restricted diet (phenylalanine-free formula)"]:::action ``` ### Acute Phase Management (First 48–72 Hours) 1. **Dextrose infusion (10–15 g/kg/day IV)** + **high-dose insulin (0.1–0.2 U/kg/hr)** → suppresses BCAA catabolism and provides anabolic state 2. **Restrict all protein intake** (NPO or protein-free formula) 3. **Correct acidosis** with sodium bicarbonate if pH <7.2 4. **Hemodialysis or hemofiltration** if: - BCAAs do not decline with medical management - Severe metabolic acidosis persists - Neurological deterioration (cerebral edema, seizures) 5. **Monitor blood glucose, electrolytes, BCAAs, ammonia, arterial blood gas** hourly ### Chronic Management - **BCAA-restricted formula** (e.g., MSUD-1, MSUD-2) with essential amino acid supplementation - **Thiamine supplementation** (50–100 mg daily) in thiamine-responsive MSUD (~10% of cases) - **Protein target**: 1.5–2 g/kg/day (adjusted to maintain BCAAs in target range: 200–400 µmol/L) - **Lifelong dietary adherence** to prevent intellectual disability **Key Point:** MSUD is a **medical emergency**. Neonatal presentation (classic form) with metabolic acidosis and hypoglycemia requires **immediate IV dextrose + insulin + protein restriction ± dialysis**. Delay in treatment leads to irreversible neurological damage and death. **Clinical Pearl:** The **maple syrup odour** is pathognomonic and results from branched-chain ketoacids (especially α-ketoisovalerate) in urine and sweat. This odour may be the first clue to diagnosis before biochemical confirmation. **High-Yield:** MSUD is one of the few amino acidurias that presents as a **medical emergency in the neonatal period** with metabolic acidosis and hypoglycemia. Alkaptonuria and homocystinuria present later in childhood or adulthood.