Neonatal Hyperammonemia
Steven M. Donn MD, FAAP1
Raul C. Banagale MD, FAAP1
1 Assistant Professor, Department of Pediatrics, Section of Newborn Services, University of Michigan Hospitals, Box 007, L3018 Women's Hospital, Ann Arbor, MI 48109
Neonatal hyperammonemia is a life-threatening complication of several conditions seen in the newborn infant (Table 1). Some of these disorders are potentially reversible; others are amenable to long-term therapy if early diagnosis and aggressive management in the neonatal period can prevent the catastrophic neurologic deterioration that accompanies massive hyperammonemia.
Normal plasma ammonia concentrations for both term and preterm infants vary according to different laboratories and methods, from 50 to 150 µg/dl,1 but values greater than 75 µg/dl in the term infant and 150 µg/dl in the preterm infant are considered abnormal and should be investigated. Most of the neonatal hyperammonemic conditions produce ammonia concentrations that are several-fold greater than normal, so small interlaboratory differences are less important.
INCIDENCE
The true incidence of hyperammonemic conditions in the general population is not known. Many infants probably die before a diagnosis is made. Ornithine transcarbamylase deficiency and argininosuccinate lyase deficiency are the two most common of the five urea cycle defects, with the frequency of argininosuccinate aciduria estimated to be 1/70,000.2 When all of the disorders are taken together, the incidence approaches 1/30,000 births. The increased awareness of transient neonatal hyperammonemia in recent years has led to an increase in diagnosis, and, in our experience, it is more common than the inherited urea cycle enzyme defects.
