A 2-day-old term neonate develops refractory seizures, persistent hiccups, and apnea within 36 hours of birth. EEG shows burst suppression pattern. Plasma glycine is elevated at 1200 micromol/L, CSF glycine is 180 micromol/L, and the CSF-to-plasma glycine ratio is 0.15 (normal <0.02). Which enzyme complex is deficient?
- A Sarcosine dehydrogenase
- B Serine hydroxymethyltransferase
- C Alanine aminotransferase
- D Glycine cleavage system (GCS) ✓
Explanation
Nonketotic hyperglycinemia (glycine encephalopathy) results from deficiency of the glycine cleavage system (GCS), a four-protein mitochondrial enzyme complex (P, H, T, and L proteins). The diagnosis rests on an elevated CSF-to-plasma glycine ratio above 0.08 (often above 0.15), which distinguishes it from organic acidemias that cause ketotic hyperglycinemia. The classic presentation includes neonatal encephalopathy, hiccups, myoclonic seizures, apnea, and a burst suppression EEG. Prognosis is poor; treatment with sodium benzoate and dextromethorphan offers limited benefit. Serine hydroxymethyltransferase interconverts glycine and serine but its deficiency is a distinct, rarer entity.
Reference: Nelson Textbook of Pediatrics, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.