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Key Features

  • Typical presentation is infantile encephalopathy; later onset presentations are common with cyclic vomiting or encephalopathy with illness or protein load

  • Diagnosis is suspected with the finding of hyperammonemia frequently with minimal other laboratory findings

Clinical Findings

  • Patients with severe defects usually present in infancy with severe hyperammonemia, vomiting, and encephalopathy, which is rapidly fatal

  • Patients with milder genetic defects may present with vomiting, encephalopathy, or liver failure after increased protein ingestion or infection


  • Blood ammonia should be measured in any acutely ill newborn in whom a cause is not obvious

  • Early hyperammonemia is associated with hyperventilation and respiratory alkalosis

  • Prenatal diagnosis is most commonly done by molecular methods


  • During treatment of acute hyperammonemic crisis, protein intake should be stopped, and glucose and lipids should be given to reduce endogenous protein breakdown from catabolism

  • Long-term treatment includes

    • Low-protein diet

    • Oral administration of arginine or citrulline

    • Sodium benzoate or sodium phenylbutyrate (a prodrug of sodium phenylacetate)

  • Hemodialysis or hemofiltration is indicated for severe or persistent hyperammonemia

  • Liver transplantation

    • May be curative

    • Indicated for patients with severe disorders

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