Hyperammonemia is a metabolic disease caused by excess ammonia in the blood, leading to toxic effects on the central nervous system. It is divided into primary and secondary types, both caused by genetic diseases. Treatment involves limiting ammonia intake and increasing excretion, while severe cases can lead to hepatic encephalopathy and require a liver transplant.
Hyperammonemia is a metabolic disease caused by too much ammonia in the blood. Ammonia is a substance composed of nitrogen and hydrogen, and is an important component of the urea cycle, which is a process that involves the transformation of ammonia into a less toxic organic compound called urea. So 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, including the brain and spinal cord.
The disease is divided into two types: primary hyperammonemia and secondary hyperammonemia. Both are caused by a group of genetic diseases characterized by metabolic dysfunction due to reduced enzymatic activity and consequent increase in toxicity. They are collectively known as inborn errors of metabolism, a phrase coined by 19th- and 20th-century British physician Archibald Garrod.
With primary hyperammonaemia, 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 argininosuccinic acid synthetase deficiency. Other examples include N-acetylglutamate synthetase deficiency, ornithine translocase deficiency, and arginase deficiency, all of which are named after the missing enzyme in the urea cycle. The errors of secondary hyperammonemia 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 noted that newborn patients usually exhibit dehydration, lethargy, rapid breathing, and reduced muscle strength. They also tend to develop larger-than-usual fontanelles, which are weak spots on babies’ skulls where the bone hasn’t fully formed yet.
Doctors treat people with hyperammonemia by increasing the excretion of ammonia and limiting its intake through the prescription of pharmacological agents such as phenylacetate and sodium benzoate. If left unattended, hyperammonemia can lead to encephalopathy, which is a disease of the brain.
The most common form of hyperammonemia-induced encephalopathy is hepatic encephalopathy. Also known as portosystemic encephalopathy, this condition is characterized by liver failure due to the organ’s inability to handle excess ammonia among other toxic elements. Hepatic encephalopathy can lead to coma and ultimately death. For this condition, a liver or liver cell transplant is always recommended.
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