White matter abnormalities in amino acid disorders and organic acidurias

Inborn errors of metabolism (IEMs) are traditionally the domain of pediatricians and internists for metabolic diseases. In general, neurologists only become involved when these disorders are complicated by neurologic symptoms such as seizures, developmental delay, or motor problems. However, in recent years and mainly due to the successes of next-generation sequencing, the number of IEMs primarily presenting with neurologic symptoms and not detected by classic biochemical testing has grown significantly. This in particular relates to disorders in the biosynthesis of amino acids. Therefore, I will start by discussing defects in the synthesis pathways of the amino acids serine, glutamine, proline, and asparagine. In these disorders, the amino acid can be low in body fluids with biochemical testing, but more frequently are completely normal and although are in different metabolic pathways, they share many clinical features such as hypomyelination and white matter abnormalities. Next, I will discuss classic amino acid disorders and organic acid disorders due to defects in breakdown pathways characterized by elevations of key metabolites in body fluids and associated with neurologic abnormalities and white matter changes on MRI.