May 15, 2008 -- For parents of children with autism, the knowledge that some unknown chemical caused their child's developmental disorder can weigh heavily on their minds.
Researchers think that a complex combination of genetic and environmental factors contributes to autism, but they don't know what all those environmental factors are.
A preliminary study out of California might have uncovered at least one chemical worthy of investigation: pyrenthrin, a type of pesticide found in common products, from pet shampoos to household bug killers.
Mothers of more than 500 young children (some autistic, some not) participated in the study, reporting long lists of products they remembered using from a few months before conception until their child turned 1.
Mothers of the 138 children with autism were twice as likely to report using pet shampoos and other household products containing pyrenthrins than other mothers.
But while the findings in the study seem strong, lead author Irva Hertz-Picciotto, professor in the Department of Public Health Sciences at University of California, Davis, and other autism experts agree that there is a lot of work left to make sure pyrenthrin isn't a red herring.
Is It Really the Shampoo?
Asking a busy mother about all the products she used in pregnancy can be tricky, but it becomes especially tricky if she had no reason to remember what she used.
"As soon as they know the child's OK, they're going to forget anything they might have done," said Dr. Ernest Krug, director of the Center for Human Development at William Beaumont Hospital, in Berkeley, Mich. "It's interesting, but then you have to go to the next step."
Krug says a good way to do a second study would be to ask the mothers to keep track of the products they use in the beginning of their pregnancies -- before they know if a child will be autistic.
However, narrowing down pyrenthrin from the slew of other chemicals mothers and infants encounter still would only be part of the job.
Scientists also must determine what pyrenthrins do to the brain, and when.
Weakening Barriers, Killing Brain Cells
People know what pyrenthrins do to insects -- kill them -- and they know it affects rats, but that doesn't mean researchers know everything that pyrenthrins can do to human brains.
In rats, pyrenthrins weaken the blood-brain barrier, which helps to keep potential toxins in the bloodstream out of the brain. "Drawing a conclusion about what the effect will be on the human brain is a bit of a jump," Krug said.
Even the known fact that pyrenthrins kill neurons -- the brain cells responsible for sending signals in the brain -- only raises more questions about the link to autism
"The key there is that pyrenthrins might cause death of neurons, but in autism there's no evidence that the death of neurons is the trigger," said Eric Courchesne, professor of neuroscience at the University of California San Diego School of Medicine.
"In fact just the opposite, it might be an excess of neurons," said Courchesne. A few years ago, Courchesne and his colleagues found that children with autism showed brain overgrowth in the first two years of life, raising the question of how chemicals that kill off brain cells are also responsible for extra brain cells.
As it stands, Courchesne and other doctors aren't sure what it is about the brain growth that triggers autism. "You could have the brain growing too fast, so maybe neurons grow too rapidly, maybe they generate too many connections in some regions, maybe two few in other regions," Courchesne said.
The pyrenthrin study now also intrigues him. "From my perspective, the finding is very interesting and very important," said Courchesne, who explained that while autism is largely due to genetics, most scientists recognize some outside factors contribute too.
"It's clearly important to understand whether there might be environmental agents that interact with genetics to increase the chances of autism," he said. Now Courchesne hopes there will be more animal models with pyrenthrins and brain growth.