10.2 Non-Hepatic Hyperammonemia & Metabolic Encephalopathy
Key Takeaways
Valproate-induced hyperammonemic encephalopathy can occur with normal liver enzymes and a therapeutic valproate level; treatment is drug withdrawal plus L-carnitine 100 mg/kg IV.
An ammonia level above roughly 150 to 200 micromoles per liter with encephalopathy is treated as a neurologic emergency, because sustained elevation causes cerebral edema.
High-flux intermittent hemodialysis clears ammonia faster than continuous kidney replacement therapy and is preferred for life-threatening levels when hemodynamics allow.
Nitrogen scavenger therapy uses sodium phenylacetate with sodium benzoate; carglumic acid is specific for N-acetylglutamate synthase deficiency and valproate toxicity.
Ammonia samples must be drawn without a tourniquet or fist clenching, placed on ice, and run promptly, because delays and hemolysis falsely elevate the result.
When Ammonia Is High and the Liver Looks Normal
Ammonia is produced by gut bacteria and by glutamine metabolism in enterocytes and kidney, and it is cleared by the hepatic urea cycle. Encephalopathy follows when astrocytes convert ammonia and glutamate into glutamine, which acts as an osmolyte and causes astrocyte swelling, raised intracranial pressure, and impaired neurotransmission.
Non-hepatic causes to recognize:
| Category | Mechanism and clues |
|---|---|
| Valproate | Inhibits carbamoyl phosphate synthetase I and depletes carnitine. Encephalopathy occurs with normal transaminases and a therapeutic level; risk rises with topiramate, phenobarbital, and carnitine deficiency |
| Urea cycle disorders | Partial ornithine transcarbamylase deficiency is the commonest in adults, often unmasked by postpartum state, surgery, steroids, high protein load, or starvation. Look for a respiratory alkalosis with a normal anion gap and low BUN, with a family history of unexplained encephalopathy |
| Urinary diversion | Ureterosigmoidostomy or an obstructed ileal conduit lets urease-splitting bacteria return ammonia to the circulation |
| Urease-producing infection | Proteus, Klebsiella, Corynebacterium urealyticum, and Ureaplasma (hyperammonemia syndrome after lung transplantation) |
| Post-bariatric and nutritional | Gastric bypass with protein malnutrition, zinc deficiency, and small-bowel bacterial overgrowth |
| High nitrogen load | Massive transfusion, total parenteral nutrition, gastrointestinal bleeding, severe catabolism, seizures, and rhabdomyolysis |
| Drugs and toxins | Glycine from transurethral irrigation, asparaginase, salicylates, carbonic anhydrase inhibitors, 5-fluorouracil, glufosinate |
| Organ failure | Portosystemic shunt with preserved synthetic function (including congenital shunts and post-TIPS), severe heart failure |
Measuring Ammonia Correctly
Pre-analytic error is common and causes needless treatment:
- Draw without a tourniquet and without fist clenching, because muscle releases ammonia.
- Avoid hemolysis; erythrocytes contain far more ammonia than plasma.
- Place the sample on ice and run it within about 15 to 20 minutes.
- Note that the degree of elevation correlates poorly with encephalopathy in chronic liver disease but correlates strongly in acute, non-hepatic hyperammonemia.
Emergency Management
Treat an ammonia level above about 150 to 200 micromoles per liter with encephalopathy, or any rapidly rising level, as an emergency.
- Stop all nitrogen intake: hold protein and amino acid infusions for 24 to 48 hours, then reintroduce protein at a low dose to avoid driving catabolism.
- Stop the offending drug: valproate, topiramate, and asparaginase.
- Reverse catabolism: give dextrose 10% with insulin as needed (aim for roughly 1.5 times basal calories) and intralipid, because starvation breaks down muscle and generates nitrogen.
- Nitrogen scavengers:
- Sodium phenylacetate and sodium benzoate (for example, Ammonul) conjugate glycine and glutamine for renal excretion. Watch for hypokalemia and volume overload.
- L-arginine or citrulline supports urea cycle flux in specific enzyme defects.
- Carglumic acid replaces N-acetylglutamate, the activator of carbamoyl phosphate synthetase I. It is specific for N-acetylglutamate synthase deficiency and is used in valproate-induced hyperammonemia.
- L-carnitine 100 mg/kg IV (then 15 mg/kg every 4 to 6 hours) for valproate toxicity.
- Extracorporeal clearance: ammonia is a small, water-soluble molecule that dialyzes efficiently.
- High-flux intermittent hemodialysis has the highest clearance and is preferred for life-threatening levels when hemodynamics allow.
- CRRT follows for ongoing control and in unstable patients.
- Lactulose and rifaximin act slowly on gut production and are not adequate alone for severe non-hepatic hyperammonemia.
- Neuroprotection: treat raised intracranial pressure, avoid hyponatremia and hyperthermia, control seizures, and keep the head elevated.
- Find the cause: plasma amino acids, urine orotic acid and organic acids, acylcarnitine profile, valproate and carnitine levels, urine culture for urease producers, and imaging for portosystemic shunts.
Orotic acid helps separate urea cycle defects: it is high in ornithine transcarbamylase deficiency and low in carbamoyl phosphate synthetase I and N-acetylglutamate synthase deficiency.
Other Metabolic Encephalopathies in the ICU
| Cause | Discriminating features | Treatment |
|---|---|---|
| Uremic encephalopathy | Asterixis, myoclonus, seizures with severe azotemia | Kidney replacement therapy; avoid rapid correction in severe azotemia (dialysis disequilibrium) |
| Hypoglycemia | Rapid onset, focal deficits possible | Dextrose; thiamine first if malnourished |
| Hyponatremia or hypernatremia | Rate of change matters more than the number | Controlled correction (see the dysnatremia section) |
| Hypercapnia | Headache, somnolence, papilledema | Improve ventilation; avoid oxygen-induced hypoventilation in chronic hypercapnia |
| Thiamine deficiency (Wernicke) | Confusion, ophthalmoparesis, ataxia | High-dose IV thiamine before glucose |
| Hypothyroid or adrenal crisis | Hypothermia, hypoglycemia, hyponatremia | Hormone replacement |
| Hypercalcemia | Confusion, constipation, polyuria | Saline, calcitonin, bisphosphonate |
| Cefepime neurotoxicity | Myoclonus, nonconvulsive status, kidney impairment with unadjusted dosing | Stop the drug; dialysis in severe cases |
| Serotonin syndrome or neuroleptic malignant syndrome | Hyperthermia with specific motor signs | See the toxicology sections |
The practical rule in the ICU is to measure what is correctable (glucose, sodium, calcium, ammonia, CO2, thyroid and cortisol when suspected), screen for drugs, and get an EEG early, because nonconvulsive status epilepticus mimics metabolic encephalopathy.
A 44-year-old woman with epilepsy on valproate and topiramate becomes progressively lethargic over 3 days. Transaminases and bilirubin are normal, her valproate level is therapeutic, and ammonia is 210 micromoles per liter. Beyond stopping valproate, which specific therapy is most appropriate?
Rifaximin 550 mg twice daily with lactulose titrated to three stools per day
Intravenous L-carnitine 100 mg/kg followed by maintenance dosing
Intravenous N-acetylcysteine using the acetaminophen protocol
Intravenous thiamine 500 mg three times daily for 3 days
A 31-year-old woman is admitted 5 days postpartum with confusion progressing to coma. Ammonia is 480 micromoles per liter, liver enzymes and INR are normal, arterial blood gas shows a respiratory alkalosis, and BUN is 4 mg/dL. She is intubated and hemodynamically stable. Which intervention provides the fastest reduction in ammonia?
Lactulose 30 mL every 2 hours by nasogastric tube
Continuous venovenous hemofiltration at 25 mL/kg/h
High-flux intermittent hemodialysis
Oral sodium benzoate with a 40% protein-restricted diet
An ICU nurse reports an ammonia level of 95 micromoles per liter in an alert postoperative patient with normal liver function and no encephalopathy. The sample was drawn with a tourniquet while the patient pumped their fist, then sent by pneumatic tube and run 50 minutes later. What is the most appropriate next step?
Start lactulose and rifaximin and obtain urgent neuroimaging
Repeat the sample without a tourniquet, on ice, and run it promptly
Begin sodium phenylacetate and sodium benzoate while awaiting repeat testing
Arrange urgent hemodialysis for ammonia clearance
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