Inborn errors of metabolism (IEM) - a big actor
IEM is often a major cause of "treatment-resistant" psychiatric, neurological and physical disorders
An IEM means that a genetic mutation causes a block in a metabolic pathway resulting either in accumulation of toxic substances or deficiency of substances needed for normal organ function.
Most IEMs affect the nervous system causing behavioral and neuropsychiatric abnormalities that include ASD, learning disorders, social deficits, hyperactivity, aggression, catatonia, psychosis and depression.
Symptoms of an IEM may be acute, appearing in the first 2-3 years of life, or chronic. Symptoms of a chronic IEM may come to medical attention at any time in life. A well known example of an IEM is phenylketonuria or PKU. The frequency of IEMs is approximately 1 in 500-800 births.
Many IEMs are treatable by nutrient or vitamin supplementation, or withdrawal of certain nutrients. However, deciding on which nutrients should be withheld and which should be added to the diet is only possible if the exact nature of the IEM is known. Because of a large number (thousands) of genetic errors that could result in IEMs and, consequently, symptoms, it is not possible to diagnose the actual problem from a clinical examination alone.
At this time, Whole Exome and Whole Genome DNA sequencing are the only tools that can accurately identify the majority of IEMs.
(From Wikipedia, with modifications)