Causes of Elevated Ammonia
Elevated ammonia results from either primary urea cycle defects, secondary inhibition of the urea cycle (from organic acidemias, drugs like valproic acid, or liver disease), acute liver or kidney injury impairing ammonia metabolism/excretion, or infections with urease-producing organisms. 1
Primary Causes: Urea Cycle Disorders
Congenital deficiency of any of the six urea cycle enzymes causes primary hyperammonemia and represents the most common non-hepatic etiology in pediatric populations 1:
- N-acetylglutamate synthase (NAGS)
- Carbamoyl phosphate synthase I (CPS)
- Ornithine transcarbamylase (OTC) - most common UCD (1 in 56,500 births)
- Argininosuccinate synthetase (ASS)
- Argininosuccinate lyase (ASL)
- Arginase 1
UCDs occur in approximately 1 in 35,000 births overall 1. These can present as early-onset (neonatal) or late-onset forms, with late-onset cases triggered by metabolic stressors in children, adolescents, or adults 2.
Secondary Causes: Metabolic Disorders
Organic acidemias (occurring in ~1 in 21,000 births) cause mild to moderate hyperammonemia through competitive inhibition of NAGS 1:
- Methylmalonic acidemia
- Propionic acidemia
- Isovaleric acidemia
- Multiple carboxylase deficiency
Hepatic Causes
Liver disease remains the most common cause of hyperammonemia in critically ill adults 3:
- Acute liver failure
- Cirrhosis with hepatic encephalopathy
- Severe hepatic dysfunction from any etiology
The liver is the primary site of ammonia detoxification via the urea cycle, so hepatic impairment directly compromises ammonia clearance 1.
Drug-Induced Causes
Valproic acid is specifically highlighted as causing secondary urea cycle inhibition 1. Other medications may also impair ammonia metabolism, though valproate is the most clinically significant.
Renal Causes
Acute kidney injury leads to ammonia accumulation due to impaired excretion 1. The kidneys normally excrete ammonia as urea and glutamine, so renal dysfunction prevents this clearance 1.
Infectious Causes
Urease-producing organisms, particularly Mycoplasma hominis, can hydrolyze urea to ammonia, driving up serum levels 4. This represents a frequently overlooked non-hepatic cause that can be fatal if not recognized promptly.
Neonatal-Specific Causes
Transient hyperammonemia of the newborn occurs in preterm neonates, characterized by:
- Absence of organic acidurias
- Normal urea cycle enzyme activity
- Usually self-resolving without treatment 1
Clinical Context for Diagnosis
A normal ammonia level (≤35 µmol/L or <60 µg/dl) has high negative predictive value and should prompt investigation for alternative causes of encephalopathy 5, 6. This is particularly important because hyperammonemia without liver failure is frequently underrecognized in critically ill patients 7.
Key Diagnostic Pitfall
In any patient with unexplained coma or altered consciousness, ammonia should be measured urgently, as severe elevations (>200 µmol/L or 341 µg/dl) are associated with poor neurological outcomes and can cause fatal cerebral edema 1, 3. Non-hepatic causes are often overlooked, leading to delayed diagnosis and worse outcomes 7, 4.
When to Suspect Non-Hepatic Causes
Consider inherited metabolic disorders when encountering 6:
- Severe hyperammonemia (>100 µmol/L)
- Family history of liver disease or neurological disorders
- Only slightly abnormal liver function tests
- Severe neurological impairment disproportionate to hepatic dysfunction