There is increased recognition that the so-called "small" or subclinical changes in neurobehavioural function are frequently irreversible.

Clinical Considerations

Developmental neurotoxicity is assessed by experimental testing for end points that span from structural down to functional problems, including fetal death, malformations, altered growth and, lastly, functional abnormalities.

We know much more than ever before about the exquisite vulnerability of the developing nervous system and the subtle degrees of impairment that can occur with even low level exposures to certain contaminants in the environment. However, our knowledge is limited as to understanding the exact causal mechanisms of most neurodevelopmental disorders, particularly with respect to the functional and behavioural deficits and regarding individual clinical cases.

A recent report by the Greater Boston’s Physicians for Social Responsibility (GBPSR), In Harm’s Way,5 identifies a number of fundamental challenges to diagnosing the precise cause of observed cognitive and behavioural disorders:

• Prime among these is the fact that cognitive functions and behaviour are influenced by the "integration of multiple structural and functional factors with cultural and social forces…(making) it exceedingly difficult to study the contribution of each factor independently."6 Determining the role that environmental factors play toward that development adds an additional layer of complexity.
• In addition, there may be a long lag time between exposure and symptoms, perhaps even a generation. For example, a mother’s exposure while pregnant may produce effects seen only in the child’s later development. Also, exposures to neurotoxicants early in life may not lead to appreciable changes until adulthood. Parkinson’s disease is one such disease speculated to have an early environmental origin.

The effects from neurotoxicants are chiefly not as dramatic as gross malformations, but are subclinical in nature and reflect conditions that are difficult to accurately diagnose.

There is increased recognition that the so-called "small" or subclinical changes in neurobehavioural function are frequently irreversible. These changes may adversely influence IQ, the ability to learn or adjust socially, and the long-term results on a broader level, such as higher school drop-out rates, increased rates of violent behaviour and lack of success, are devastating to us as a society.

The adverse effects from a number of these substances have been determined largely from animal experiments and limited epidemiological studies. They are often subtle, although measurable. Effects are mainly subclinical, and it may be that a physician discovers them incidentally.

The authors of In Harm’s Way describe an "epidemic of developmental, learning and behavioral disabilities"8 among American children, estimating that nearly 17% (12 million) suffer from some form of neurodevelopmental or neurobehavioural condition.

There are no comparable figures for the prevalence of such conditions among Canadian children, although there is a sense among schooleachers and physicians that learning disabilities and attention deficit disorders have increased substantially in recent decades.9 (A recent Health Canada survey indicates that 80% of a sample [n=636] of Canadian physicians reported having seen a child with ADHD at one time.) It is not clear whether there is indeed a true increase and, if so, what has caused it; however, health researchers have turned to an examination of the potential influence of various chemicals in the environment on children’s neurodevelopment.