Diagnosis and treatment of urea cycle disorders

Abstract

Urea cycle disorders (UCDs) comprise a group of inherited defects of metabolism affecting the detoxification of excess nitrogen and, hereby, leading to hyperammonemia. The urea cycle consists of six consecutive enzymatic steps and is located both in the mitochondria and cytosol of periportal hepatocytes. In addition, two transporters are required for the urea cycle function. Besides the detoxification of excess nitrogen, the endogenous synthesis of the amino acid arginine is the second role of the urea cycle. The clinical presentation of UCDs comprises a continuum ranging from early onset hyperammonemic decompensation in severe defects to late onset presentation in less severe affected patients at any age. Since ammonia is primarily toxic to the central nervous system, signs and symptoms of UCDs are mainly neurological but, unfortunately, highly unspecific. Therefore, it can be challenging to clinically diagnose patients suffering from UCDs. If a UCD is suspected, ammonia should be measured immediately. To confirm the diagnosis, a variety of biochemical, enzymatic and genetic tools exist. Most UCD patients need a strict treatment protocol consisting of dietary protein restriction, nitrogen scavenger drugs, and vitamin and amino acid supplementations. Treatment must be followed lifelong unless liver transplantation is performed. Patients with UCDs are still affected by a poor outcome quoad vitam et sanitam. To improve both survival rates as well as the quality of life in surviving patients will largely depend on an increased awareness of all medical professionals towards this group of inherited metabolic defects.