[Clinical presentation of inborn metabolic diseases in the neonatal period].

Inborn metabolic diseases (IMDs) that can start in the neonatal period include various defects in numerous metabolic pathways. Such diseases are due to the genetic deficiency of an enzyme or a transporter. From a physiopathological point of view, the metabolic disorders can be divided into 3 diagnostically useful groups of diseases. The first group is due to the accumulation of endogenous toxic metabolites and includes inborn errors of amino acid metabolism, organic acidemias, urea cycle disorders, and sugar intolerances. The second one includes IMDs of intermediary metabolism causing a disturbance in energy production or utilization resulting from a defect in the liver, the muscles, the myocardium, or the brain (fatty acid oxidation defects, congenital lactic acidosis, etc.). The third group includes diseases that disturb the synthesis or the catabolism of complex molecules (lysosomal or peroxisomal disorders, etc.). IMDs are individually rare, but collectively numerous. Therefore, it is difficult to acquire extensive experience in the management of these diseases. However, the neonate has a limited repertoire of responses to severe illness and, at first, presents with nonspecific symptoms that could be easily attributed to infection or some other common cause. An IMD must be suspected in all situations of neonatal distress for which there is no apparent reason and that does not respond to symptomatic therapy. The priority is given to IMDs that are amenable to treatment, and emergency management has to be scheduled as soon as the diagnosis is suspected, even if the precise diagnosis is still unknown. In fact, emergency treatment must be undertaken in parallel with metabolic investigations, to prevent any delay in the management of the disease. The neonatologist must be able to recognize the neonatal distresses that suggest the possibility of an IMD. In such situations, an adequate diagnostic approach can be based on the proper use of only a few screening tests, which will also be useful to schedule adequate emergency treatment.