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Concept
Hyperammonemia
Summary
Hyperammonemia is 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. It may be primary or secondary.
Ammonia is a substance that contains nitrogen. It is a product of the catabolism of protein. It is converted to the less toxic substance urea prior to excretion in urine by the kidneys. The metabolic pathways that synthesize urea involve reactions that start in the mitochondria and then move into the cytosol. The process is known as the urea cycle, which comprises several enzymes acting in sequence. It is greatly exacerbated by common zinc deficiency, which raises ammonia levels further.
Hyperammonemia is one of the metabolic derangements that contribute to hepatic encephalopathy, which can cause swelling of astrocytes and stimulation of NMDA receptors in the brain. Overstimulation of NMDA receptors induces excitotoxicity.
Primary hyperammonemia is caused by several inborn errors of metabolism that are characterised by reduced activity of any of the enzymes in the urea cycle. The most common example is ornithine transcarbamylase deficiency, which is inherited in an X-linked fashion.
Secondary hyperammonemia is caused by inborn errors of intermediary metabolism, which are characterised by reduced activity of enzymes that are not part of the urea cycle or dysfunction of cells that make major contributions to metabolism. Examples of the former are propionic acidemia and methylmalonic acidemia, and examples of the latter are acute liver failure and hepatic cirrhosis with liver failure.
Acquired hyperammonemia is usually caused by diseases that result in either acute liver failure, such as overwhelming hepatitis B or exposure to hepatotoxins, or cirrhosis of the liver with chronic liver failure. Chronic hepatitis B, chronic hepatitis C, and excessive alcohol consumption are common causes of cirrhosis. The physiologic consequences of cirrhosis include shunting of blood from the liver to the inferior vena cava, resulting in decreased filtration of blood and removal of nitrogen-containing toxins by the liver, and then hyperammonemia.
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