Evolution of the concept

Although the frequency of the diagnosis of AD-HKD has certainly increased rapidly in the second half of the twentieth century, the discovery of AD-HKD is not recent. Excessively hyperactive, inattentive, and impulsive children have been described in the medical literature since the late nineteenth century. (:!,,2) In the early years of the twentieth century, the syndrome was clearly described, along with its various cardinal manifestations, such as soft neurological signs, minor congenital anomalies, and inattentiveness, and labelled a 'defect in moral control'.(34) According to the prevailing social Darwinian theory, moral control was the latest and greatest achievement of evolution, and, as a consequence, was considered particularly susceptible to loss as a result of various brain insults. Only persons with a genetic predisposition to AD-HKD were thought to develop the disorder. The adverse environmental and social circumstances that were characteristic of the situations of many affected persons were considered consequences rather than causes of the disorder.(2)

In many respects, theories of AD-HKD have changed little over the decades. These theories have continued to reflect the belief that the syndrome is a consequence of an interaction between subtle hereditary and acquired traumatic or toxic processes acting on the nervous system during fetal or early child development to cause minimal brain damage or dysfunction.(5)

As knowledge of brain function grew during the century, concepts of the fundamental behavioural and neuropathological deficit of AD-HKD changed. Initial theories emphasized hyperactivity and abnormal arousal arising from the dysfunction of the thalamus. Later theories focused on the deficient attention that results from the dysfunction of the frontal circuits involved in cognition. Current theory favours the role of deficient inhibitory control—the delaying mechanism involved in the timing, initiation, and interruption of action—and neuropathology in the frontal and related regions of the brain. (6)

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