NYTimes- Environment Poses a Knotty Challenge in Autism
Parents of children with autism often ask pediatricians like me about the cause of the condition, and parents-to-be often ask what they can do to reduce the risk. But although there is more research in this area than ever before, it sometimes feels as if it’s getting harder, not easier, to provide answers that do justice to the evidence and also offer practical guidance.
Recent research has taught us more about the complexity of the genetics of autism, but the evidence also has suggested an important role for environmental exposures. It has become a very complicated picture: Genes matter, but we usually can’t tell how. Environmental exposures matter, but we usually don’t know which.
In July, a study of autism in twins was published online in Archives of General Psychiatry. Researchers looked at almost 200 sets of twins in California. In each pair, one twin was autistic.
The study sought to determine how likely the second twin was to have some form of autism. If autism was highly heritable, identical twins should have been far more likely to both have autism than fraternal twins. But the researchers found that fraternal twins were unexpectedly likely to both have autism.
The implication is that something in their common gestational or early childhood experience may have contributed to this similarity.
The data definitely did surprise me, said Dr. Joachim Hallmayer, the lead author of the study and an associate professor of psychiatry and behavioral sciences at Stanford University. I expected the fraternal twin rates to be lower than what we found.
This new twin study supported the importance of genetics, but also the role of environmental exposures and that is part of a general shift in how autism is being discussed.
In the 1950s, autism famously was blamed on bad parenting and emotionally remote refrigerator mothers.As the research advanced, including early important twin studies, the inherited basis of the disorder became clear. In a 2010 article published in an American Psychiatric Association journal, autism spectrum disorder is described as among the most heritable of psychiatric disorders.
Yet in recent years, researchers have implicated a variety of possible environmental associations as well. Today many scientists believe autism results both from genetic predisposition and from environmental influence.
But environment is a tricky word. To many scientists studying autism, it means everything that’s not the inherited DNA, said Irva Hertz-Picciotto, a professor of epidemiology at the University of California, Davis, MIND Institute. An environmental influence might be a chemical the fetus is exposed to via the placenta, or it might refer to aspects of nutrition, maternal health, stress or perhaps exposure to a microbe.
The causal links most strongly supported by research include rubella infection during pregnancy and prenatal exposure to medications like thalidomide and valproic acid, an anti-seizure drug. Other environmental factors, like air pollution and exposure to certain pesticides and other chemicals, have been found to be associated with autism, but without evidence of causality.
In a 2010 paper in the journal NeuroToxicology, Dr. Amir Miodovnik, a pediatrician at the Mount Sinai School of Medicine, and his colleagues showed that children who had been exposed to high levels of phthalates prenatally were more likely to show social impairments at 7 to 9 years of age.
Phthalates, chemicals found in many consumer products, are so-called endocrine disruptors, hormonally active substances that can interfere with a variety of developmental processes, including brain development. Yet these data don’t demonstrate cause and effect, Dr. Miodovnik said, only that these substances are associated with symptoms found in autism. Conversely, taking prenatal vitamins around the time of conception has been associated with a lower risk of autism in a recent study.
These epidemiologic associations may point us in the direction of still other factors involved in the making of autism. Every case is probably a result of the confluence of many factors, Dr. Hertz-Picciotto said. No case probably has one cause.
So it’s hard and frustrating to offer prospective parents advice about avoiding risks we still can’t clearly identify and factors that may differ from family to family. But some advice seems well grounded, if unsurprising: Take prenatal vitamins before trying to conceive. Make sure your immunizations are up to date. Get good prenatal care. Talk to your doctor about the risks and benefits of any medications you take.
Dr. Miodovnik points out that potentially toxic substances are ubiquitous and cannot be completely avoided, but suggests that prospective parents try to avoid pesticides, don’t microwave plastics that may contain endocrine disruptors, and consider choosing fragrance-free personal products (phthalates are used in many fragrances).
Still, it’s hard to talk about this without terrifying parents. And I wonder if in giving advice about prevention, we risk repeating the errors of the past, making parents feel they’re to blame for a child’s autism because they failed to micromanage an environment full of complex agents with potential to interact with fetal genes in a range of damaging but poorly understood disruptions.
Learning more and understanding more ought to make us more helpful to parents, and of course to children. We respect them by acknowledging that there is nothing simple in the development of autism: The causes are genetic, and not simple, and environmental, and not simple. And if the studies tell us anything clearly, it is that we have much to learn about the interplay of genes and environmental exposure and the individuality of this complex disorder.