The urea cycle is a process in which waste (ammonia) is removed from the body. When you eat proteins, the body breaks them down into amino acids. Ammonia is produced from leftover amino acids, and it must be removed from the body.
The liver produces several chemicals (enzymes) that change ammonia into a form called urea, which the body can remove in the urine. If this process is disturbed, ammonia levels begin to rise.
Several inherited conditions can cause problems with this waste-removal process. People with a urea cycle disorder are missing a gene that makes the enzymes needed to break down ammonia in the body.
These diseases include:
- Argininosuccinic aciduria
- Arginase deficiency
- Carbamyl phosphate synthetase (CPS) deficiency
- N-acetyl glutamate synthetase (NAGS) deficiency
- Ornithine transcarbamylase (OTC) deficiency
As a group, these disorders occur in 1 in 30,000 newborns. OTC deficiency is the most common of these disorders.
Boys are more often affected by OTC deficiency than girls. Girls are rarely affected. Those girls who are affected have milder symptoms and can develop the disease later in life.
To get the other types of disorders, you need to receive a non-working copy of the gene from both parents. Sometimes parents don't know they carry the gene until their child gets the disorder.
Typically, the baby begins nursing well and seems normal. However, over time the baby develops poor feeding, vomiting, and sleepiness, which may be so deep that the baby is difficult to awaken. This usually occurs within the first week after birth.
- Decreased food intake
- Dislike of foods that contain protein
- Increased sleepiness, difficulty waking up
- Nausea, vomiting
Exams and tests
The health care provider will often diagnose these disorders when the child is still an infant.
Signs may include:
- Abnormal amino acids in blood and urine
- Abnormal level of orotic acid in blood or urine
- High blood ammonia level
- Normal level of acid in blood
Tests may include:
- Genetic tests
- Liver biopsy
- MRI or CT scan
Source: National Library of Medicine (NLM)