The human species is altricial in comparison to other large mammals, including large primates, and has only a modest supply of motor and cognitive skills at birth. The anatomic substrates of cognition, behavior, and movement are largely set down in utero, but their full functional expression takes many years. Alterations in patterns of in-utero brain development, whether fashioned by genetic abnormalities, by damage from environmental exposures, or by the interaction of these two forces, can damage this anatomic substrate without altering early developmental trajectories in any substantial way. Children born lacking major cortical structures, as in hydranencephaly, are often incapable of expressing the full repertoire of typical human newborn behavior.1
Disorders of development, which by convention refer nearly exclusively to development of the nervous system, are thus almost never diagnosed at birth, but are recognized when milestones expected of typical human behavior are not met. Unlike diagnostic categories in adults, or those involving organ systems in children that do not have as prolonged a postnatal developmental trajectory as the brain, recognition of developmental disorders, and assessment of their impact on the child, is entirely age dependent. Widely used measures such as IQ have no real meaning until at least early childhood, and the firm diagnosis of cerebral palsy (CP) requires a child to be at least 18–24 months old. Learning disabilities, by definition, cannot be diagnosed until school age.
The topic of this chapter is developmental disabilities. As is appropriate for a text-book of public health, the focus of this chapter is on those developmental disorders that are not just present, but that are disabling, meaning that they exact a price in terms of human functionality, that represent a personal, familial, and societal burden, and that require a substantial investment of familial and societal resources for their amelioration and prevention. Thus, I exclude from consideration in this chapter a number of disorders of development that do not reach the threshold of having public health implications, either because of their rarity or because they do not convey major disability to affected individuals. I focus on six disorders, three of which (severe intellectual disability [ID], autism spectrum disorder [ASD], and CP) are clearly disabling, while the other three (mild ID, learning disability [LD], and attention deficit/hyperactivity disorder [ADHD]) occur with relatively high frequency in the population, and interfere in a measurable way with quality of life.2,3
One way of looking at developmental disabilities is to distinguish the conditions that are rare and severe, and those that are fairly common and present somewhat fewer difficulties to affected individuals and their families. CP and severe ID together rarely exceed a prevalence of 5–6/1000 live births, but, as will be detailed below, they entail heavy burdens in terms of medical expenditure, family caregiving needs, and foregone earnings. ASD is more common, perhaps affecting as many as 1–1.5% of the population, and also carries a ...