A 4-day-old term neonate develops lethargy, poor feeding, and myoclonic jerks progressing to apnea. Plasma ammonia is normal, anion gap is normal, glucose is normal, and CSF analysis shows a glycine level five times the plasma glycine level. EEG shows burst-suppression. Most likely diagnosis?

• A Maple syrup urine disease
• B Propionic acidemia
• C Nonketotic hyperglycinemia ✓
• D Methylmalonic acidemia

Explanation

Nonketotic hyperglycinemia (glycine encephalopathy) results from defective glycine cleavage complex. Glycine acts as an excitatory neurotransmitter in cortex and brainstem, producing hiccups, seizures, myoclonus, and apnea. Diagnostic clues are normal anion gap, normal ammonia, and raised CSF to plasma glycine ratio above 0.08. MSUD, propionic, and methylmalonic acidemias all show ketoacidosis with elevated anion gap, which is absent here.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.