THE “AUTISM EPIDEMIC” AND KNOWN CAUSES OF AUTISM

Objective: To review epidemiological surveys of autism and evaluate trends over time with respect to environmental (mainly immunizations) and genetic risk factors.

Methods: Epidemiological studies of pervasive developmental disorders (PDD) have accumulated recently. We will review the design, sample characteristics and main findings of 42 surveys published between 1966 and 2005. Focused hypotheses on immunization exposures as risk factors for autism/PDD that could account for the rise in diagnoses of autism will be examined.

Results: Recent surveys suggest that the rate for all forms of PDDs are as high as 60–70/10,000. The rate for Asperger disorder is not well established, and a preliminary conservative figure is 2.5/10,000. Childhood disintegrative disorder is extremely rare with a pooled estimate across studies of 0.2/10,000. There is evidence that changes in case definition, improved awareness and new policies in special education services explain much of the upward trend of prevalence rates in recent decades. However, available epidemiological surveys do not provide an adequate test of the hypothesis of a changing incidence of PDDs. At present, epidemiological studies that relied on a combination of designs (prospective, case-control, ecological) and performed by independent groups of investigators from different countries do not support an association between autism and exposure to combined measles-mumps-rubella vaccines or to thimerosal-containing vaccines. Few other environmental exposures have been identified that could account for a substantial proportion of cases of autism. Genetic factors have been robustly identified as increasing the risk of autism/PDD but molecular genetic studies have yet to identify genes accounting for large effects.

Conclusion: Increased numbers of children diagnosed with PDD cannot be readily interpreted as indicative of a true epidemic. Exposure to MMR or thimerosal does not appear to raise the risk of autism.