Autism could be caused by environmental factors

The largest ever study of twins with autism spectrum disorder (ASD) suggests that environmental influences in the womb or during the birth process and possibly after could play a significant role in autism development.

Similar to smaller twin studies, the California Autism Twins Study of 192 twin pairs found that the chance of 1 identical twin developing autism when the other has the disorder is extremely high-in this case, 70%. What was surprising was that the overlap was 35% among fraternal twins, much higher than found in previous studies.

“Because of the reported high heritability of autism, a major focus of research in autism has been on finding the underlying genetic causes, with less emphasis on potential environmental triggers or causes,” according to the Stanford University researchers. “The finding of significant influence of the shared environment, experiences that are common to both twin individuals, may be important for future research paradigms.”

The autism science and advocacy organization Autism Speaks collaborated with researchers on the study. The group’s vice president of clinical program, Clara Lajonchere, PhD, agreed that “environmental influences are at play.”

Lajonchere noted that the 35% overlap in fraternal twins is higher than the 3% to 14% overlap between different-aged siblings. “This suggests that there are environmental influences uniquely shared by twins-for instance, in the womb and perhaps during birth,” she said.

Another recent study, which examined the use of antidepressant medication in mothers whose children later developed ASD, bolstered evidence of the importance of in vitro exposure in the disease etiology.

That population-based, case-control study reviewed medical records at the Kaiser Permanente Medical Care Program in Northern California and compared 298 children with ASD and their mothers to 1,507 randomly selected control children and their mothers.

Researchers noted that the number of children exposed prenatally to selective serotonin reuptake inhibitors was low in their study (20 case vs 50 control) but said that overall results suggest that exposure increased the risk of ASD 2-fold. The risk was 4-fold when selective serotonin reuptake inhibitors were used in the first trimester, they said.

Also in July, a newly released meta-analysis examined more than 60 perinatal and neonatal factors and found these to be associated with autism risk:

• Abnormal presentation; umbilical-cord complications
• Fetal distress; birth injury or trauma
• Multiple birth; summer birth
• Maternal hemorrhage
• Low birth weight; small for gestational age
• Congenital malformation
• Low 5-minute Apgar score
• Feeding difficulties
• Meconium aspiration
• Neonatal anemia
• ABO or Rh incompatibility
• Hyperbilirubinemia

Anesthesia issues, assisted vaginal delivery, postterm birth, high birth weight, and head circumference were not associated with a higher autism risk. Researchers concluded that there is not enough evidence to single out 1 perinatal or neonatal factor in autism etiology, “although there is some evidence to suggest that exposure to a broad class of conditions reflecting general compromises to perinatal and neonatal health may increase the risk.”