Metabolic diseases are disorders that stem from a deficiency in the activity of an enzyme. Most commonly they affect an enzyme involved in the synthesis or breakdown of intermediates in the metabolism of carbohydrates, fats or proteins. The field of inherited metabolic diseases has expanded tremendously in the last decade and now also includes defects in the function of subcellular organelles (for example mitochondria, peroxisomes and lysosomes) and defects in the metabolism of additional molecules, such as neurotransmitters.
The symptoms patients with metabolic diseases experience are a consequence of the accumulation of compounds that cannot be degraded, or the lack of an essential compound which cannot be synthesized, depending on the specific enzyme defect. Accordingly, accumulation of a compound which is toxic to the brain is likely to result in developmental retardation, seizures, abnormal muscle tone and instability. Accumulation of a compound which is toxic to muscle will lead to muscle weakness, muscle breakdown or failure of the heart muscle. Other commonly encountered symptoms include disturbed liver and kidney function. As many of these symptoms (particularly in the neurological category) are not specific to metabolic disorders, it is customary to refer patients for metabolic evaluation when their symptoms cannot be accounted for by any other known disorder.