Researchers in California suggest environmental factors may play a larger role in triggering autism than was previously thought. Their study was published Monday in the journal Archives of General Psychiatry.
The study shows a "need to accept that we have to go down the route of environment and genetics " when it comes to studying the causes of autism, says lead author Dr. Joachim Hallmayer. "We have look at both sides of coin."
Scientists at Stanford University in Palo Alto, California, using state records, identified 192 pairs of twins in which at least one of the two had some form of autism. Among these sets, there were 54 pairs of identical and 138 pairs of fraternal twins. The Stanford researchers then examined the children for autism themselves, using standard diagnosing tools.
What they found, using mathematical formulas, was that the genes twins share can increase the risk of getting autism by about 38%, but the environment twins share in the womb and immediately after birth may increase the risk even more an estimated 58%. Hallmayer says these are only estimates, but he believes the environment may play a larger role than previously thought.
Approximately 1 in 110 children in the United States have some form of developmental delay known as an autism spectrum disorder (ASD), according to the Centers for Disease Control and Prevention. ASDs become apparent by the time a child turns 3 and begins to show significant social, communication and behavioral difficulties. The exact causes of these brain disorders which can range from mild to severe are not known, and there's no cure. Mothers used to be blamed for being "too cold" towards their children when autism first was identified in the first part of the 20th century, but towards the end of that century, genetics were believed to be a major culprit, and so far several specific genes have been linked to ASDs.
But in recent years, some scientists have been saying that there has to be something else triggering autism in genetically susceptible children. Hallmayer, who is the principal investigator for A California Population-Based Twin Study of Autism, says, "We're finally moving to a little bit of a middle position and we have to really study both factors." Hallmayer says his study adds more weight to the argument that both aspects are key to understanding autism.
He says this study was not designed to determine which specific environmental factors are at play, therefore the researchers cannot pinpoint which issues add to the autism risk. Hallmayer and his colleagues suggest in their paper that some of the non-genetic environmental factors that could influence a child developing autism might be parental age, low birth rates, multiple births or maternal infections during pregnancy. They also could not pinpoint when a fetus is at greatest risk for environmental factors interfering with normal brain development (i.e. early in pregnancy, later in the pregnancy or at birth). So a lot more research is needed.
This study did find that fraternal twins also are at risk for autism, not just identical twins as some previous, smaller studies have suggested. It found the other research had seriously underestimated the concordance rate or rate of both children having an autism diagnosis in fraternal twins. Hallmayer adds that his study provides evidence that the environmental factors have been underestimated because fraternal twins share only about half of their genes with their sibling and if both have the same or at least some form of ASD, then there has to be more than just genes affecting the twins' brain development. Fraternal twins do share the same environment in their mother's womb, are exposed to the same bacteria or viruses right after birth and while in the hospital and presumably are drinking the same breast milk all possible sources of environmental triggers for autism.
Dr.Max Wiznitzer, a pediatric neurologist at Rainbow Babies & Children's Hospital in Cleveland, Ohio, who was not involved with the study, thinks one of the two factors may carry more weight. "I would argue it's more of environmental influence in genetically susceptible people," he says. It is when a baby is in the womb, Wizniter says. "During fetal life is the time for the greatest brain growth." In those nine months is when the "greatest rate and changes in brain growth occur," he says, so it makes sense that there is a risk that outside factors can have an influence on brain development.
Wiznitzer suggests perhaps births too close together and nutritional deficiencies could provide some of these environmental factors. "Multiple-birth pregnancies are themselves associated with increased risk of developmental disorders such as cerebral palsy and autism," says study co-author Clara Lajonchere, Ph.D., in a statement. She is vice president of clinical programs for Autism Speaks, a science and advocacy organization, which partially funded this study, along with the National Institutes of Mental Health.
Hallmayer and Wiznitzer both say that in addition to researching genetic risk factors for autism more research on environmental influences is needed. That's something other experts have been calling for during the past few years.