The Medical Medium, Anthony William, describes the menace of heavy metal toxicity buildup, especially in the midline cerebral canal, as a cause of ADHD and autism. Is that the true, actual cause in all cases, or a major contributor?

This is not the cause in all cases but is the major cause. The reason for the lack of total causation is that autism is a relatively ill-defined state of being and there are a number of minor causal events that can result in a perturbation of function that renders a person diagnosable as falling on the so-called autism spectrum. So there will be some lack of precision and some confusing elements exhibited by various individual patients making it impossible to make a cut and dried assessment in some instances, and there will be a minor percentage of causation from other situations and influences, but those will be a minor percentage. Fully 90% of ADHD and autism are the result of heavy metal toxicity as described.