Autism is a neurodevelopmental disorder characterized by impairments in social and communication skills, with accompanied repetitive or restrictive interests and behaviours. Changes in the definition of autism likely explain at least some of the increase in prevalence rates of this diagnosis. The earliest characterizations of autism reflected, from the current perspective, relatively severe symptom presentations, often associated with significant cognitive delays.
Changes in diagnostic conventions confound our understanding of prevalence rates in other ways. For example, the same children who would have been diagnosed with another developmental disorder in the past may now be more likely to receive an ASD label. Conditions for this “diagnostic substitution” phenomenon become especially ripe when access to services through the public health care system and school may depend on having an autism label. There is now some evidence that children previously diagnosed as having mental retardation or a learning disability are now diagnosed with autism instead (Fombonne et al, 2009).
There is much evidence for an increase in the prevalence of autism diagnoses, but an epidemic of autism cannot be established unless an increase in incidence rates can be substantiated. Prevalence rates refer to the proportion of individuals with a given condition at a given point in time, while incidence rates refer to the number of new cases occurring in a population over a period of time. Incidence rates are more difficult to ascertain. In the case of autism, there are many confounds to consider, including broader diagnostic categories and better ascertainment of cases of autism. The vast majority of research is limited to studies on prevalence, and data on incidence is scarce, and when available, difficult to interpret due to confounding variables. There is the hope that new survey methods used in some recent and ongoing surveillance studies will soon shed light on the question of autism incidence rates.
What we do know is that autism, which is diagnosed in about 1/150 children, is much more common than many childhood disorders, including Type I diabetes, childhood cancer, and cystic fibrosis. These numbers support the imperative to address the needs of the children and families affected by autism. Timely diagnosis and intensive treatments for autism are in high demand. The needs of children with autism and their families require ongoing attention and action within the spheres of public health and educational institutions.
Author :Mandy Steiman, Ph.D. Psychologist Autism Spectrum Disorders Program McGill University Health Centre
Rutter, M. (2005). Incidence of autism spectrum disorders: Changes over time and their meaning. Acta Paediatrica, 94, 2-15.
Fombonne, E., Quirke, S., & Hagen, A. (in press). Epidemiology of pervasive developmental disorders. In Autism Spectrum Disorders. D. G. Amaral, G. Dawson & D. H. Geschwind (Editors). Oxford University Press.