What Is Hyperammonemia?

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  • Written By: Andy Josiah
  • Edited By: Nancy Fann-Im
  • Last Modified Date: 15 August 2019
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Hyperammonemia is a metabolic disorder that is caused by an excess amount of ammonia in the blood. Ammonia is a substance composed of nitrogen and hydrogen, and it is a major component of the urea cycle, which is a process that concerns the transformation of ammonia to a less toxic organic compound called urea. Thus an excess of ammonia represents an increased level of toxic effects in the body. This particularly affects the central nervous system, which forms the main component of the nervous system, comprising the brain and the spinal cord.

The disease is split into two types: primary hyperammonemia and secondary hyperammonemia. Both are caused by a group of genetic ailments characterized by metabolic dysfunction brought about by reduced enzyme activity and consequent rise of toxicity. They are collectively known as inborn errors of metabolism, a phrase coined by 19th- and 20th-century British physician Archibald Garrod.

With primary hyperammonemia, however, these errors occur in the urea cycle. An example of its associated disorders is citrullinemia, which involves the accumulation of ammonia in the blood due to the deficiency of argininosuccinic acid synthetase. Other examples include N-acetylglutamate synthetase deficiency, ornithine translocase deficiency and arginase deficiency, each of which is named after the enzyme missing in the urea cycle. Secondary hyperammonemia’s errors occur outside the urea cycle. Two examples are methylmalonic acidemia and propionic academia, which belong to a class of amino-acid metabolic disorders known as organic academia.


There are very few specific physical symptoms of hyperammonemia. Some medical researchers, however, have noticed that infant patients usually exhibit dehydration, lethargy, rapid breathing and reduced muscle strength. Also, they tend to develop larger-than-usual fontanelles, which are soft spots on the babies’ skulls where the bone has not yet completely formed.

Physicians treat people with hyperammonemia by increasing the excretion of ammonia while restricting its intake through the prescription of pharmacologic agents such as phenylacetate and sodium benzoate. Left unattended, hyperammonemia can lead to encephalopathy, which is a brain disorder.

The most common form of encephalopathy induced by hyperammonemia is hepatic encephalopathy. Also known as portosystemic encephalopathy, this condition is marked by liver failure as a result of the organ being unable to handle the excess ammonia among other toxic elements. Hepatic encephalopathy can lead to a coma and, ultimately, death. For such a condition, a liver or liver cell transplant is always recommended.


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