Background Hyperammonemia caused by a disorder of the urea cycle is a rare cause of metabolic encephalopathy that may be underdiagnosed by the adult intensivists because of its rarity. Urea cycle disorders are autosomal recessive diseases except for ornithine transcarbamylase deficiency (OTCD) that is X-linked. Optimal treatment is crucial to improve prognosis. Main body We systematically reviewed cases reported in the literature on hyperammonemia in adulthood. We used the US National Library of Medicine Pubmed search engine since 2009. The two main causes are ornithine transcarbamylase deficiency followed by type II citrullinemia. Diagnosis by the intensivist remains very challenging therefore delaying treatment and putting patients at risk of fatal cerebral edema. Treatment consists in adapted nutrition, scavenging agents and dialysis. As adults are more susceptible to hyperammonemia, emergent hemodialysis is mandatory before referral to a reference center if ammonia levels are above 200 µmol/l as the risk of cerebral edema is then above 55%. Definitive therapy in urea cycle abnormalities is liver transplantation. Conclusion Awareness of urea cycle disorders in adults intensive care units can optimize early management and accordingly dramatically improve prognosis. By preventing hyperammonemia to induce brain edema and herniation leading to death.

Purpose of reviewHyperammonaemia is almost always develops in patients with severe liver failure and this remains the commonest cause of elevated ammonia concentrations in the ICU. Nonhepatic hyperammonaemia in ICU presents diagnostic and management challenges for treating clinicians. Nutritional and metabolic factors play an important role in the cause and management of these complex disorders.Recent findingsNonhepatic hyperammonaemia causes such as drugs, infection and inborn errors of metabolism may be unfamiliar to clinicians and risk being overlooked. Although cirrhotic patients may tolerate marked elevations in ammonia, other causes of acute severe hyperammonaemia may result in fatal cerebral oedema. Any coma of unclear cause should prompt urgent measurement of ammonia and severe elevations warrant immediate protective measures as well as treatments such as renal replacement therapy to avoid life-threatening neurological injury.SummaryThe current review explores important clinical considerations, the approach to testing and key treatment principles that may prevent progressive neurological damage and improve outcomes for patients with hyperammonaemia, especially from nonhepatic causes.

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Ammonia is a neurotoxic molecule that causes cerebral edema and encephalopathy. Ammonia is either produced in excess or poorly purified during severe hepatic insufficiency, poisoning, infection, and inborn errors of metabolism. During continuous renal replacement therapy, ammonia clearance is determined by the dialysate flow rate and the dialyzer surface area. Extra-renal blood purification for ammonia clearance has been studied in neonates with urea cycle disorders. Prognostic factors affecting patient outcome are thought to be the duration of coma, the patient’s clinical status prior to dialysis, and the ammonia removal rate. In this review, we discuss the various dialytic modalities used for ammonia clearance as well as the thresholds for initiating dialysis and the better strategy ensures rapid patient protection from cerebral edema and herniation induced by hyperammonemia.