Congenital metabolic disorders result from the absence or abnormality of an enzyme or its cofactor, leading to either accumulation or deficiency of a specific metabolite (table 1 and table 2 and table 3 and table 4 and table 5 and table 6).
The major classes of IEM and their characteristic clinical and biochemical features are described below. The epidemiology, pathogenesis, clinical presentation, evaluation, and initial management of IEM are discussed separately, as are specific disorders. (See "Inborn errors of metabolism: Metabolic emergencies" and "Inborn errors of metabolism: Epidemiology, pathogenesis, and clinical features" and "Inborn errors of metabolism: Identifying the specific disorder".)
The traditional classification system for IEM groups the disorders according to the general type of metabolism involved. Some diseases fit into more than one category. These major categories can be further grouped based upon similarities in pathogenesis and/or presenting features (table 1). (See "Inborn errors of metabolism: Epidemiology, pathogenesis, and clinical features" and "Inborn errors of metabolism: Metabolic emergencies".)
Peroxisomal and lysosomal storage disorders, for example, often have characteristic clinical features and permanent, progressive symptoms that are independent of triggering events (eg, anemia, thrombocytopenia, and hepatomegaly in a child of Ashkenazi Jewish ancestry is suggestive of Gaucher disease) . (See "Gaucher disease: Pathogenesis, clinical manifestations, and diagnosis".)
The other categories of IEM (amino acid disorders, organic acidemias, urea cycle disorders, disorders of carbohydrate metabolism, fatty acid oxidation disorders, and mitochondrial disorders) may be considered in two broad categories :