A term newborn develops hypotonia, hiccups, myoclonic jerks, and apnea on day 2. Plasma ammonia is normal and there is no metabolic acidosis. CSF glycine is 900 micromol/L with plasma glycine of 600 micromol/L. What is the most likely diagnosis?
- A Non-ketotic hyperglycinemia ✓
- B Propionic acidemia
- C Maple syrup urine disease
- D Transient hyperammonemia of prematurity
Correct answer: A. Non-ketotic hyperglycinemia
Explanation
Non-ketotic hyperglycinemia (glycine encephalopathy) arises from defective mitochondrial glycine cleavage enzyme. Glycine acts as an excitatory neurotransmitter in cortex and brainstem, producing myoclonus, hiccups, and apnea with a normal anion gap and normal ammonia. A CSF to plasma glycine ratio above 0.08 confirms it. Propionic acidemia causes ketoacidosis, which this child lacks.
Reference: Nelson Textbook of Pediatrics, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.