A metabolic disturbance characterised by an excess of ammonia in the blood. It is a dangerous condition that may lead to brain injury and death. Ammonia is a substance that contains nitrogen, a product of the catabolism of protein.

Symptoms can range from mild, such as irritability, headache, and vomiting, to severe with encephalopathy, seizures, ataxia, and coma.

It is one of the metabolic derangements that contributes to hepatic encephalopathy (an altered level of consciousness as a result of liver failure), which can cause swelling of astrocytes and excitotoxicity by overstimulation of NMDAreceptors in the brain.