Outline of metabolic diseases in adult neurology

Inborn errors of metabolism (IEM) are traditionally defined by enzymatic deficiencies or defects in proteins involved in cellular metabolism. Historically discovered and characterized in children, a growing number of IEM are described in adults, and especially in the field of neurology. In daily practice, it is important to recognize emergency situations as well as neurodegenerative diseases for which a metabolic disease is likely, especially when therapeutic interventions are available.

Here, the goal is to provide simple clinical, imaging and biochemical tools that can first orientate towards and then confirm the diagnosis of IEM. General guidelines are presented to treat the most common IEM during metabolic crises - acute encephalopathies with increased plasma ammonia, lactate or homocystein, as well as rhabdomyolysis. Examples of therapeutic strategies currently applied to chronic neurometabolic diseases are also provided - GLUT1 deficiency, adrenoleukodystrophy, cerebrotendinous xanthomatosis, Niemann-Pick type C and Wilson disease. Genetic counseling is mandatory in some X-linked diseases - ornithine transcarbamylase deficiency and adrenoleukodystrophy - and recommended in maternally inherited mitochondrial diseases - mutations of mitochondrial DNA.

Besides these practical considerations, the contribution of metabolism to the field of adult neurology and neurosciences is much greater: first, with the identification of blood biomarkers that are progressively changing our diagnostic strategies thanks to lipidomic approaches, as illustrated in the field of spastic paraplegia and atypical psychiatric presentations; and second, through the understanding of pathophysiological mechanisms involved in common neurological diseases thanks to the study of these rare diseases.