A rise in autism?
When the prevalence of autism was first studied in 1966, a rate of 4 per 10,000 was found. Today’s best estimate of the prevalence of autism is 60 to 70 per 10,000 (Fombonne et al, 2009). The discrepancy between early rates and more current numbers has been the focus of much debate. Making sense of this increase poses a challenge for researchers who must provide explanations to an increasingly concerned public.
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.
About 30 years ago, finer distinctions in the autism diagnosis began to be described. Today, the umbrella term “autism spectrum disorder” (ASD) and its synonym “pervasive developmental disorder” (PDD) are inclusive of a range of diagnoses. Definitions have broadened to include individuals with relatively mild symptoms, including individuals without cognitive delays, and with a fewer number of symptoms and/or severity in symptoms. When trying to interpret prevalence rates, changes in nomenclature must be taken into account. Current epidemiological studies, using a broader definition of autism, are likely casting a much wider net.
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).
Awareness of autism has also changed over time, and more children are being referred to health care professionals for autism evaluations. In the face of this demand, the clinical and research community have responded by making concerted efforts to standardize and refine the approach to assessment, as well as push the lower boundaries of the age of diagnosis. Although availability of services is far from ideal, there are many more specialized professionals and much better access to services than in previous decades. At the same time, standardized measures to assess autism have been developed and professionals are now in a better place to distinguish autism from other developmental disorders. Increases in referral rates must be understood in the context of many factors, including changes in awareness, availability of services, and better diagnostic tools.
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.
It would be important to look at environmental factors if there were an increase in incidence. Even though no such increase has been established, some environmental factors have been proposed as explanations for why more children are now being given an autism label. An alleged link between the Measles Mumps Rubella (MMR) vaccine and autism has been proposed. A related hypothesis, alleging links between thimerosal, vaccine preservative containing ethyl mercury, has also sparked public debate. Causal hypotheses such as these seem to offer much explanatory power and have the appeal of an “easy answer” to why autism happens. However, the evidence to date argues strongly against these theories. There is no clear association between autism rates and stopping or starting the MMR vaccine, or exposure to thimerosal. If there are true environmental risk factors for autism, vaccines are unlikely candidates (Rutter, 2005).
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.
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.