ABOUT UCDs
A urea cycle disorder (UCD) is an inherited, inborn error of metabolism. Patients with a UCD lack or are deficient in one of the key enzymes that comprise the urea cycle, the body’s primary vehicle for removing ammonia, a potent neurotoxin, from the bloodstream. Left untreated, UCDs can cause dangerously heightened levels of ammonia in the bloodstream (hyperammonemia) resulting in brain damage, coma, and/or death.
Infants with complete enzyme deficiencies are usually born at term with no prenatal complications, because the maternal circulation detoxifies the accumulating ammonia. Between 1 to 5 days of age, however, they start feeding poorly, vomit frequently, become lethargic and hypotonic, and may hyperventilate. These babies will progressively have tremor, stupor, seizures, apnea, coma, increased intracranial pressure, and death if the hyperammonemia is not diagnosed and treated effectively.1
In patients with partial enzyme deficiencies, the first recognized clinical episode may be delayed for months or years; the hyperammonemia is less servere, and the symptoms are more subtle. In most of these "late onset" urea cycle disorders the hyperammonemic episode is marked by loss of appetite, cyclical vomiting, lethargy, and behavorial abnormalities and is often precipitated by viral illness, high protein diet, stress, or trauma.2
Any episode of acute symptomatic hyperammonemia should be treated as a life-threatening emergency. Treatment of hyperammonemia may require dialysis, preferably hemodialysis, to remove a large burden of ammonia. Uncontrolled hyperammonemia can rapidly result in brain damage or death. Prompt use of all necessary therapies to reduce ammonia levels is essential.
Management of hyperammonemia due to inborn errors of metabolism should be done in coordination with medical personnel familiar with these diseases. The severity of the disorder may necessitate the use of hemodialysis combined with nutritional management and medical support. The multidisciplinary nature of the treatment usually requires the facilities of a tertiary or quaternary care center.
More information on urea cycle disorders can be found at the Web site for the National Institutes of Health (NIH)-sponsored Urea Cycle Disorders Consortium (UCDC).
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References
1. Summar M, Tuchman M. Proceedings of a consensus conference for the management of patients with urea cycle disorders. J Pediatr. 2001;138(suppl):S8.
2. Summar ML, Barr F, Dawling S, et al. Unmasked adult-onset urea cycle disorders in the critical care setting. Crit Care Clin. 2005;21(4suppl):S1-S8.
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