Abstract
The urea cycle is a series of metabolic reactions that convert ammonia into urea in order to eliminate it from the body. Urea cycle disorders are characterized by hyperammonemia, which can cause irreversible damages in central nervous system. We report a series of three newborns presenting irritability, poor feeding and tachypnea. Their first gas analysis revealed respiratory alkalosis. Hyperammonemia was confirmed, and three different enzymatic blocks in the urea cycle were diagnosed. Immediate treatment consisted in the removal of ammonia by reduction of the catabolic state, dietary adjustments, use of nitrogen scavenging agents and ultimately hemodiafiltration. Hyperammonemia is a medical emergency whose treatment should not be delayed. This report aims to highlight the importance of suspecting urea cycle disorders in newborns with aspecific signs of hyperammonemia and respiratory alkalosis, and to sum up the broad lines of hyperammonemia management.
Highlights
The urea cycle is a metabolic pathway involved in nitrogen detoxification and arginine synthesis [1]
That is thought to result from astrocyte swelling responsible of cerebral edema caused by the accumulation of ammonia, glutamine and other metabolites when osmoregulation is insufficient [5]
Respiratory alkalosis with normal anion gap and blood glucose in a newborn should prompt immediate plasma ammonia measurement, as hyperammonemia is initially present in 50% of acute urea cycle disorders [1]
Summary
The urea cycle is a metabolic pathway involved in nitrogen detoxification and arginine synthesis [1]. It requires six enzymes and two transporters [2]. Any total or partial enzyme deficiency or transport defect in this pathway leads to a urea cycle disorder (UCD), resulting in acute hyperammonemia except for the arginase deficiency [3]. Hyperventilation in UCDs is a common early finding which causes respiratory alkalosis. That is thought to result from astrocyte swelling responsible of cerebral edema caused by the accumulation of ammonia, glutamine and other metabolites when osmoregulation is insufficient [5]. Increasing cerebral edema may result in progressive encephalopathy with hypoventilation and respiratory arrest
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