Autism spectrum disorder (ASD) is
a complex neurobehavioral disorder
that is believed to be multifactorial in origin. As the incidence
of ASD is rising along with industrialization, and because certain
metals have been linked to neurological problems, it is important
to consider whether such metals may play a role in the development
of ASD. Previously, we performed a meta-analysis of existing literature
to examine the potential link between inorganic arsenic and lead exposure
and ASD. This is a continuation of that study investigating the association
of the exposure to aluminum (Al), cadmium (Cd), and mercury (Hg) and
ASD. These metals were chosen because they are abundant in our environment,
are known to cause neurological problems in humans, and have multiple
published studies examining their potential links with ASD. Following
the same approach as our previous paper, we conducted a systematic
review of the existing literature and performed a meta-analysis to
evaluate the current evidence regarding these metals and their potential
relationship with autism. We reviewed 18 studies on Al, 18 on Cd,
and 23 on Hg, and the individual studies showed inconsistent results.
When the measurements were integrated into the meta-analysis, we found
significant associations between all the metals and ASD, but the associations
were not always in the same direction. Levels of Hg in hair, urine,
and blood were all positively associated with ASD. Levels of Al in
hair and urine were positively associated with ASD, while levels of
Al in blood were negatively associated. In comparison, levels of Cd
in hair and urine were negatively associated with ASD. These results
imply that, while these metals are all neurotoxic, their impact on
the development of ASD and their modes of action could be different.
Further research is warranted to examine the longitudinal effects
of these toxic metals on the risk of ASD, to assess the critical period
when exposure may affect development, and to investigate potential
factors that may enhance or ameliorate the effect of metals. Overall,
these findings support policies that advocate limiting exposure to
neurotoxic metals, particularly for pregnant women and young children,
in order to help reduce the rising incidence of ASD.