Prevalence of Pervasive Developmental Disorders among Children at the English Montreal School Board

Article excerpt

Objectives: The prevalence of pervasive developmental disorders (PDDs) has increased. There has been speculation regarding a role of thimerosal-containing vaccines (TCVs) in this trend. Our objectives were to determine prevalence rates of PDDs among school-aged children, and to evaluate the impact of discontinuation of thimerosal use in 1996 in routine childhood vaccines on PDD rates.

Method: Children (n = 23 635) attending kindergarten to Grade 11 were surveyed in 71 schools from the English Montreal School Board. For children with PDD, information was obtained about their diagnostic subtype, age, sex, grade, and school. Prevalence rates were calculated for the entire school population and for each grade. Prevalence rates were also compared for children born before or after 1996.

Results: Students (n = 187; male to female ratio: 5.4:1) with PDD were identified, corresponding to a prevalence of 79.1/10 000 (95% CI 67.8 to 90.4/10 000). The prevalence was 25.4, 43.6, 9.7, and 0.4 for autistic disorder, PDD not otherwise specified, Asperger syndrome, and childhood disintegrative disorder, respectively. During the study period, there was a significant linear increase in prevalence (OR 1.17 per year; 95% CI 1.12 to 1.23). The trend in prevalence of PDDs was unrelated to the discontinuation of TCVs.

Conclusion: Our study provides additional evidence that the PDD rate is close to 1%. We estimate that at least 11 500 Canadian children aged 2 to 5 years suffer from a PDD. The reasons for the upward trend in prevalence could not be determined with our methods. Discontinuation of thimerosal use in vaccines did not modify the risk of PDD.

Can J Psychiatry. 2010;55(11):715-720.

Clinical Implications

* This prevalence study confirms our previous study performed in Montreal 5 years ago. The slightly higher prevalence rate of 0.8% is consistent with recent figures obtained by the Center for Disease Control in the United States.

* The prevalence estimates for each subtype of PDD (autistic disorder, PDD not otherwise specified, childhood disintegrative disorder, Asperger syndrome) are also highly consistent with recent reviews of prevalence studies of autistic spectrum disorders (ASDs).

* As in other surveys, prevalence rates have been increasing in recent birth cohorts. Our study provides preliminary evidence that in the youngest school-aged birth cohorts, aged 5 to 9 years, the prevalence may have stabilized around figures of 1.2% to 1.4%


* Although many of the children included in our study have been diagnosed in our McGill tertiary pediatric hospital, no clinical validation of the administrative definition of the cases could be performed.

* The lack of association between increasing rates of ASD and changes in the exposure to TCV during our study was based on aggregated data. Therefore, the usual limitations of ecological analyses apply to our findings of no association.

* Some special schools in Montreal that provide services to children with mental retardation, sometimes associated with PDD, were not included in our study, leading to a potential underestimation of the true population rate.

Key Words: epidemiology, autism, pervasive developmental disorder, prevalence, school-aged, immunizations, vaccines

Abbreviations used in this article

ASD autism spectrum disorder

CDD childhood disintegrative disorder

EMSB English Montreal School Board

MEQ Ministry of Education of Quebec

NOS not otherwise specified

PDD pervasive developmental disorder

TCV thimerosal-containing vaccines

Pervasive developmental disorders are a group of disorders beginning in early childhood that cause marked impairments in language and communication, social interaction, and the presence of repetitive or stereotypic patterns of behaviour. The group of disorders is comprised of ASD, PDD-NOS, Asperger syndrome, and CDD. …