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Urea Cycle Disorders

  • Argininosuccinic acidemia
  • Citrullinemia

Description - Urea cycle disorders are characterized by a defect in the urea cycle resulting in the accumulation of ammonia and its precursor amino acids. Citrullinemia (ASD) and argininosuccinic acidemia (ALD) are two of the urea cycle disorders.

Citrullinemia is caused by a defect in the urea cycle enzyme argininosuccinic acid synthetase. Deficiency of this enzyme results in markedly elevated blood ammonia, citrulline and glutamine levels, as well as urine citrulline and orotic acid and decreased arginine production.

Argininosuccinic acidemia is caused by a defect in the production of the urea cycle enzyme argininosuccinase. Deficiency in this enzyme results in markedly elevated blood ammonia, citrulline and glutamine levels, as well as urine argininosuccinic acid. Elevations of argininosuccinic acid are not found in blood due to its efficient excretion by the kidneys.

Clinical Features - The newborn appears normal at birth. Clinical symptoms of citrullinemia and argininosuccinic acidemia become apparent between 1 and 3 days of age, when the newborn undergoes a rapid neurological deterioration. As ammonia levels increase, newborns quickly develop anorexia, vomiting, hypothermia, irritability, lethargy, seizures, apnea and cerebral edema. Without intervention, they become comatose and die.

In addition to disease with neonatal onset, there is a late onset presentation that can occur any time from the newborn period to adulthood. Patients with partial residual activity of the defective enzyme exhibit a variety of different clinical pictures, but typically late onset presentation is characterized by vomiting, lethargy, irritability, agitation, disorientation, ataxia and occasionally coma. Episodes are frequently associated with a change in formula and/or diet with an intercurrent illness or infection. While seizures, delayed growth, developmental delay and cerebral atrophy have been described in patients with late onset presentation, others may have normal intelligence.

Treatment - The goal of treatment of citrullinemia and argininosuccinic acidemia is to provide sufficient protein and arginine for growth, development and energy while preventing hyperammonemia and hyperglutaminemia. Protein restriction and caloric supplementation with protein-free powder; arginine supplementation to promote synthesis and excretion of citrulline, and supplementation of sodium phenylbutyrate to promote alternate nitrogen excretion pathways.

Patients are optimally managed by a team of specialists including those from genetics, metabolic genetics, pediatric gastroenterology and pediatric neurology.

 


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Last Modified: Monday, 28-Jul-14 09:35:03