Causes of Elevated Ammonia
Elevated ammonia results from either primary urea cycle enzyme deficiencies, secondary urea cycle inhibition from metabolic disorders or drugs (particularly valproic acid), or impaired ammonia metabolism/excretion due to liver or kidney disease. 1
Primary Causes: Urea Cycle Disorders
Congenital deficiency of any of the six urea cycle enzymes causes primary hyperammonemia and represents the most critical differential diagnosis in pediatric populations 1:
- N-acetylglutamate synthase (NAGS) deficiency 1
- Carbamoyl phosphate synthase I (CPS) deficiency 1
- Ornithine transcarbamylase (OTC) deficiency - the most common UCD, occurring in 1 in 56,500 births 1
- Argininosuccinate synthetase (ASS) deficiency 1
- Argininosuccinate lyase (ASL) deficiency 1
- Arginase 1 deficiency 1
UCDs collectively occur in approximately 1 in 35,000 births 1. These disorders cause ammonia accumulation along with substrate buildup specific to each enzyme deficiency 1.
Secondary Causes: Metabolic Disorders
Organic acidemias cause mild to moderate hyperammonemia through competitive inhibition of NAGS 1:
These conditions occur in approximately 1 in 21,000 births and typically produce less severe hyperammonemia than primary UCDs 1.
Drug-Induced Hyperammonemia
Valproic acid is the most commonly implicated medication, causing secondary inhibition of the urea cycle 1, 2. Drug-induced hyperammonemia may arise from either decreased ammonia elimination or increased production 2.
Organ Dysfunction
Acute liver injury or failure impairs ammonia metabolism, as most ammonia enters the urea cycle in hepatocytes 1. The liver is the primary site where ammonia is converted to urea for excretion 1.
Acute kidney injury causes ammonia accumulation due to impaired excretion, as ammonia is normally excreted in urine as urea or converted to glutamine for renal excretion 1.
Age-Specific Considerations
Transient hyperammonemia of the newborn occurs in preterm neonates, characterized by absence of organic acidurias and normal urea cycle enzyme activity 1. This condition typically resolves completely without treatment 1.
Early-onset hyperammonemia presents in neonates within the first few days of life after feeding begins, when they can no longer rely on maternal placental transport for ammonia elimination 1.
Clinical Significance
Ammonia levels >200 µmol/l (341 µg/dl) are associated with poor neurological outcomes due to cerebral edema, neuronal damage, and potential for coma and death if untreated 1. The acute rise in brain ammonia leads to increased extracellular potassium, glutamine accumulation in astrocytes, increased intracellular osmolality, and inflammatory cytokine release 1.