Hyperammonemia

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.

Hyperammonemia
Other namesHyperammonaemia
Ammonia
SpecialtyEndocrinology 

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.[1]

Signs and symptoms

Complication

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.

Diagnosis

Primary vs. secondary

Acquired vs. congenital
  • Acquired hyperammonemia is usually caused by diseases that result in either acute liver failure, such as overwhelming hepatitis B or exposure to hepatoxins, 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. This type of hyperammonemia can be treated with antibiotics to kill the bacteria that initially produce the ammonia, though this doesn't work as well as removal of protein from the colon prior to its digestion to ammonia, achieved by lactulose administration for frequent (3-4 per day) bowel movements.
  • Medication induced hyperammonemia can occur with valproic acid overdose, and is due to a deficiency in carnitine. Its treatment is carnitine replacement.
  • Severe dehydration and small intestinal bacterial overgrowth can also lead to acquired hyperammonemia.
  • Glycine toxicity causes hyperammonemia, which manifests as CNS symptoms and nausea. Transient blindness can also occur. [2]
  • Congenital hyperammonemia is usually due to genetic defects in one of the enzymes of the urea cycle, such as ornithine transcarbamylase deficiency, which leads to lower production of urea from ammonia.

Specific types

The following list includes such examples:

Treatment

Treatment centers on limiting intake of ammonia and increasing its excretion. Dietary protein, a metabolic source of ammonium, is restricted and caloric intake is provided by glucose and fat. Intravenous arginine (argininosuccinase deficiency), sodium phenylbutyrate and sodium benzoate (ornithine transcarbamoylase deficiency) are pharmacologic agents commonly used as adjunctive therapy to treat hyperammonemia in patients with urea cycle enzyme deficiencies.[3] Sodium phenylbutyrate and sodium benzoate can serve as alternatives to urea for the excretion of waste nitrogen. Phenylbutyrate, which is the product of phenylacetate, conjugates with glutamine to form phenylacetylglutamine, which is excreted by the kidneys. Similarly, sodium benzoate reduces ammonia content in the blood by conjugating with glycine to form hippuric acid, which is rapidly excreted by the kidneys.[4] A preparation containing sodium phenylacetate and sodium benzoate is available under the trade name Ammonul. Acidification of the intestinal lumen using lactulose can decrease ammonia levels by protonating ammonia and trapping it in the stool. This is a treatment for hepatic encephalopathy.

Treatment of severe hyperammonemia (serum ammonia levels greater than 1000 μmol/L) should begin with hemodialysis if it is otherwise medically appropriate and tolerated.[2]

See also

References
  1. Riggio, O.; Merli, M.; Capocaccia, L.; Caschera, M.; Zullo, A.; Pinto, G.; Gaudio, E.; Franchitto, A.; Spagnoli, R.; D'Aquilino, E. (September 1992). "Zinc supplementation reduces blood ammonia and increases liver ornithine transcarbamylase activity in experimental cirrhosis". Hepatology (Baltimore, Md.). 16 (3): 785–789. doi:10.1002/hep.1840160326. ISSN 0270-9139. PMID 1505922.
  2. Chapter 298 – Inborn Errors of Metabolism and Continuous Renal Replacement Therapy Archived 2013-07-01 at the Wayback Machine in: John J. Ratey MD; Claudio Ronco MD (2008). Critical Care Nephrology: Expert Consult - Online and Print. Philadelphia: Saunders. ISBN 978-1-4160-4252-5. ISBN 9781416042525
  3. eMedicine - Hyperammonemia: Article by Kazi Imran Majeed
  4. "Ammonul (Sodium Phenylacetate and Sodium Benzoate Injection) clinical pharmacology - prescription drugs and medications at RxList". Archived from the original on 2008-06-16. Retrieved 2008-06-26.
Classification
External resources
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