Applying Hill's criteria to the study of Autism Spectrum Disorders and exposure to heavy metals

Purpose. Autism Spectrum Disorders (ASDs) are neuropsychiatric disorders that include Autistic Disorder, Asperger syndrome and Pervasive Developmental Disorders – not otherwise specified (PDD-NOS). Prevalence rates of ASDs are reported to have increased in the last decade, raising concern amongst researchers, service providers, policymakers, and families. While current aetiological research is exploring the interface between genetic and environmental factors, exposure to heavy metals, in particular mercury has retained public interest.

Methods. A systematic review of publications from 1980 to 2010 inclusive was used to examine the hypothesized link between ASD and mercury exposure. Hill’s criteria for causation were applied to critically appraise the reviewed studies.

Results. Reviewed studies failed to demonstrate strength and consistency of association as well as establish a temporal link between the onset of ASD symptoms and mercury exposure.

Conclusions. The risk of developing and being diagnosed with an ASD as a result of mercury exposure remains unclear because of methodological flaws in studies conducted to date. Rigorous research is needed to provide adequate information to families, clinicians and decision-makers. They must be provided with up-to-date, critically appraised information to help
them make informed decisions.