ABSTRACT
Distinct disorders involve the enzymes of every step in the urea cycle (Figure 28.1) [1,2]. These include ornithinetranscarbamylase deficiency (Chapter 29), citrullinemia (Chapter 31), carbamyl phosphate synthetase deficiency (Chapter 30), argininosuccinic aciduria (Chapter 32) and argininemia (Chapter 33). In addition, there is a syndrome of transient hyperammonemia of the newborn [3], in which the early clinical manifestations mimic those of the severe defects of urea cycle enzymes and may be fatal, but if the patient can get through the first five days of life the problem disappears and
prognosis is good. The hyperammonemic syndrome is also characteristic of the HHH (hyperammonemia, hyperornithinemia and homocitrullinuria) syndrome (Chapter 34), which is caused by defective transport of ornithine into the mitochondria. Lysisuric protein intolerance (Chapter 35) is also associated with episodic hyperammonemia, but its major expression is as extreme failure to thrive.
