Autism spectrum disorder is a neurodevelopmental disorder characterized by sometimes profound impairments in social interaction and communication skills. People with autism have difficulty reading social cues such as tone of voice and body language. They have difficulty making eye contact and often engage in repetitive movements such as rocking or twirling. The symptoms of autism comprise a spectrum of behaviors and individuals may be more or less impaired by their condition.

People with autism have difficulty reading social cues such as tone of voice and body language. They have difficulty making eye contact and often engage in repetitive movements such as rocking or twirling.

Autism is typically diagnosed in the first few years of life and infants and toddlers may be referred for evaluation if they have delayed language development, poor social interactiveness, or unusual and repetitive behaviors and preoccupations. Older children may merit assessment if they have difficulty making friends, sustaining conversations, engaging in imaginative interactive play or have, restricted interests or preoccupations, or rituals and routines to which they adhere strongly. A definitive diagnosis is most often made by a multidisciplinary team that may include physicians, psychologists, speech therapists and others.

The number of people diagnosed with autism has increased dramatically over the last decade, and it is not clear whether this is because of improved screening and diagnosis or whether there are actually more people with autism than there used to be. The current prevalence of autism is 1-2% and there are ten times as many people currently diagnosed with autism as there were 50 years ago. Recent statistics show that the prevalence of autism has increased from 4-5/10,000 in 1996 to 100 cases per 10,000 today.

A Controversial Shift Back to Environmental Influences

The causes of autism remain unknown despite abundant research People with autism have been found to have abnormalities in their brains including size, structure, and neurochemical function. This suggests that the changes leading to autism occur during the earliest times of brain formation, the prenatal and earliest neonatal periods. Studies on twins and siblings with autism have pointed to an underlying genetic abnormality as the cardinal cause of autism. But a recent study questioned these findings and concluded that the role of genetics has been overemphasized.

This is different from previous research which has come out much more strongly in favor of genetic influences and it is a controversial finding.

The new study may help shift research away from pure genetics to the prenatal, perinatal (delivery)and neonatal (newborn) periods and the potential environmental influences during these times. Many of these possible influences have already been under investigation. Possible influences have ranged from exposures to such things such as medications and environmental toxins, to birth order and the age of parents, to the time of year of conception and birth.

In the recent study of genetics vs. environmental causes of autism, the researchers looked at 54 pairs of identical and 130 pairs of non-identical twins in which at least one had been diagnosed with autism. They evaluated the identified autistic twin to rigorously confirm the diagnosis of autism and the second twin to explore the possibility of an autism diagnosis.

The diagnostic criteria for autism include social impairments, communication impairments, repetitive ritualistic and stereotyped behaviors and age of onset. For children to be diagnosed as autistic they had to meet five criteria within the first three areas. Children who had most, but not all of the characteristics were defined as having autism spectrum disorder.

A definitive diagnosis is most often made by a multidisciplinary team that may include physicians, psychologists, speech therapists and others.

The twins’ chromosomes were examined to see if they came from one egg (identical) or from two (fraternal). Other potential influences on the development of autism were considered including the ages of the parents, the years of maternal and paternal education, ethnicity of parents, difference in birth weight between twins, and gestational age.

Studying Twins In Order to Determine Genes' Influence

Identical twins share all genetic material but non identical twins do not. If both twins in an identical twin pair did not both have autism, the researchers felt that something other than genetics must be responsible for autism. Since they shared 100% of their genetic material, they should both be identical in every single way.

The study found that environment explained 58% of cases of autism while genetics explained 37%.

If the second twin of a non identical pair also had autism, the researchers felt this showed that their shared environment, not shared genetic material, was the important influence. The shared environment could include prenatal factors such as maternal age, birth size, and duration of pregnancy, maternal infection, maternal use of medications or exposure to environmental toxins. It could also include delivery and neonatal complications, and early environmental exposures.

Their results showed that although there was a strong genetic component to the likelihood of autism occurring in twins, the contribution of the environment was much more significant in the study population. The study found that environment explained 58% of cases of autism while genetics explained 37%.

This is different from previous research which has come out much more strongly in favor of genetic influences and it is a controversial finding. The authors of this new study propose that their difference in results stems from their more rigorous approach to the diagnoses of autism/autism spectrum disorder. Instead of just accepting the diagnoses available from medical records of their study population, the authors reassessed each patient, using the currently accepted diagnostic tools to confirm the diagnosis before including the children in the study. These tools include a rigorous and validated interview of parents and a similarly rigorous set of behavioral observations of the children. They felt this allowed them to have a more valid pool of patients and to exclude other developmental causes of communication/social skills delays.

The authors are concerned that research that is primarily focused on the genetics of autism is missing the opportunity and need to further investigate environmental factors. Because the prenatal environmental and early postnatal environment are shared between twin individuals, they hypothesize that at least some of the environmental factors impacting susceptibility to autism exert their effect during this critical period of life and deserve thoughtful investigative attention.
Nutrition, Antidepressants, Viruses and Prenatal Influences – A Huge List of Possible Environmental Factors
Part of the attraction of a possible genetic cause of autism is it potential simplicity. In contrast, the literature on possible environmental factors is enormous and diverse and reflects the complexity of the question of the etiology, the cause or causes, of autism. Because autism presents a broad range of symptoms with varying degrees of severity and can limit a child and an adult's life, research to find its cause or cause has been broad-ranging and ongoing.

It is important to distinguish between conditions that occur together and conditions that occur because of each other.

What do we mean by "environment" and what is already known about these factors? Here are some of the possible factors that have been studied and the results to date:

  • A study published in 2002 in Sweden evaluated the association of maternal, pregnancy, delivery, and infant characteristics and risk of infantile autism. Their results suggested that the risk of autism was increased by daily smoking in early pregnancy, cesarean delivery, being small for gestational age and having certain congenital malformations or a low apgar score. They generalized the medical results of these diverse conditions and concluded that pregnancy and neonatal factors that negatively impacted intrauterine growth or caused fetal distress were important in the causation of autism.

  • Another study which looked at prenatal, perinatal and neonatal risk factors for autism spectrum disorder found that. autism spectrum disorder occurred more frequently when mothers were older and/or had had more children.

  • The time interval between pregnancies and whether it impacted the risk of autism in subsequent births was the subject of other prenatal research.. The study found that children born after shorter intervals between pregnancies were at increased risk of developing autism with the highest risk associated with pregnancies spaced less than one year apart.

  • Children who were born less than one year after their first born siblings were three times more likely to have autism than those who were born three years or more later. The authors of the study thought that this may be related to the fact that the mother is nutritionally depleted after a pregnancy and hasn’t had time to fully recover when a second pregnancy follows closely. Low folate, iron, and polyunsaturated fatty acids levels as well as maternal stress may be a factor here

  • Nutrition may be an environmental factor. A study examined the use of prenatal vitamins by mothers of children with autism and found that they were less likely than those of normally-developing children to have taken prenatal vitamins during the first three months before or during the first month of pregnancy. They concluded that use of prenatal vitamins around the time of conception might reduce the risk of having children with autism especially for genetically susceptible mothers and children. This study importantly highlights the potential for environmental factors, such as nutrition and vitamins, to influence the underlying genetic tendencies toward autism.

  • Mothers' or neonatal exposure to viruses, medications, or seasonal environmental toxins was the subject of other research. Yet another study looked at whether the season of birth or conception influenced the risk of autism. They found that children who were conceived in December, January or February had a higher risk of developing autism as compared with those conceived in July. One possible explanation for this observation is that certain infections and exposures are more common at different times of the year and may cause changes in the developing infant which result in autism.

  • The influence of prescription medication, particularly antidepressants has also been studied. Research investigating the impact of selective serotonin reuptake inhibitors (such as Prozac, Zoloft, Paxil, Lexapro ) on the likelihood of autism found that women who were prescribed an SSRI were more than twice as likely to have a child diagnosed with ASD as women who did not take an SSRI. Although this raises concerns, more study is necessary and pregnant women should not stop medications without consulting their physicians for safe alternatives. Untreated depression during pregnancy carries its own risks to mother and fetus.

  • Finally, a recent Meta-Analysis which analyzed 40 studies looking at perinatal and neonatal factors that might cause autism concluded that there wasn’t one specific factor but a variety of conditions which might increase the chances of a child developing autism. These included intrauterine growth retardation, having a cord around the neck, a low apgar score, or low birth weight. These all share the common denominator of causing low oxygen during the delivery and neonatal periods and may explain the increased risk.

The authors of this last study proposed an interesting possible interpretation of the data that takes the blame for causing autism away from problems during the delivery and newborn period. Perhaps, they speculate, these obstetric and neonatal complications occur as a result of the autism condition itself, already present in the fetus that lead to a more complicated delivery and neonatal period. Or, taking it one step further, perhaps an underlying abnormality causes both the development of autism and the occurrence of high risk obstetric and neonatal situations, that is, they both arise from a common problem.

What We Can Conclude from All The Findings

It is important to distinguish between conditions that occur together and conditions that occur because of each other. This is what makes figuring out what factor or group of factors cause(s) autism so difficult.

Perhaps autism occurs more commonly when mothers are older because older mothers are more experienced and more likely to bring a differently developing child for evaluation. Or perhaps there is a change in the genetic material that occurs in older mothers that predisposes to autism spectrum disorder. It is not clear yet. Perhaps serotonin use during pregnancy raises the risk of autism, or perhaps it is the depressive or anxiety condition in the mother for which the serotonin is prescribed that is the culprit. It’s not clear yet.

In some cases autism may be purely inherited and expressed fully. In others a genetic tendency toward autism may be influenced by environmental factors which either enhance or diminish the expression of the trait.

Autism is a complex neurodevelopmental syndrome with an unknown etiology despite a huge body of research. It is likely that both genetics and environment play a role, both individually and in concert. In some cases autism may be purely inherited and expressed fully. In others a genetic tendency toward autism may be influenced by environmental factors which either enhance or diminish the expression of the trait. And in others, new changes in the developing brain may occur spontaneously or from toxic exposures or incidents during early brain development which result in autism.

The Takeaway

Research which targets genetic risk as well as environmental mediators will be the most helpful in developing targeting interventions and potential preventative measures. Given the wealth of possibilities, investigators must remain open -minded in their studies.

Pregnant women and new parents should follow their doctors’ advice for best and safest prenatal and neonatal practices. Avoiding a medically complex birth is a goal common to all patients and health care providers and will help decrease a myriad of acute and chronic health issues in infants, young children and adults.

Pregnant women as well as women who are planning pregnancies are urged to consult with their physicians on how to best avoid risky exposures and maintain safe nutrition and prenatal health. Appropriate nutrition, exercise, and regular prenatal and neonatal care are the mainstays for optimizing both mother’s and baby’s health. When prescription and over-the-counter medications and nutritional supplements are needed, it is best to consult a health care provider. But keep in mind that appropriately treating maternal illness with the safest possible therapies is preferable to avoiding treatment and having poorly controlled health issues during a pregnancy.