Children who have an older sibling with autism are seven
times more likely than other kids to be diagnosed with the disorder themselves,
according to a new study from Denmark.
That extra risk is smaller than had been suggested in
earlier studies. Researchers also found a higher than average risk among
children whose older half-sibling had an autism spectrum disorder (ASD) especially
if the two kids shared the same mother.
"I think a lot of autism researchers agree that the
causes of autism are many and it's very complex," Therese Gronborg, who
led the study at Aarhus University, said. "If it was only genetics we
would see a much higher recurrence rate" among siblings, she told Reuters
About one in every 88 US children has an ASD, according to
the Centers for Disease Control and Prevention. For their new study, Gronborg
and her colleagues used birth, civil and psychiatric registries to track 1.5
million children born in Denmark between 1980 and 2004.
Through 2010, just over 13 000 of them had been diagnosed
with an ASD. That included 276 children with an older sibling with autism who
were also diagnosed with the disorder.
The researchers found the likelihood of a younger sibling
being diagnosed with autism when an older sibling had an ASD varied between 4.5
and 10.5%, with an average of about 7%. There was no clear increasing or
decreasing trend during the study period.
For half-siblings, the extra risk of autism was smaller:
younger half-siblings who shared a father with an older sibling had a 1.5 times
greater risk of ASDs if their sibling also had one, a finding that could have
been due to chance.
Children who shared a
mother with a half-sibling had a 2.4 times greater risk if their older brother
or sister had autism, according to findings published in JAMA Pediatrics.
Gronborg said a
woman's lifestyle during pregnancy or something about the intrauterine
environment could affect her children's risk of autism. "But it could also
be something in the upbringing," she added. The researchers noted that it's possible siblings of a child
with an ASD are more likely to be diagnosed themselves in part because parents
are more aware of the condition and its symptoms. But they said the extra risk
could also be an underestimate if parents who have one child with very severe
autism are less likely to try to have more kids.
'Reassuring' for parents
Gronborg and her colleagues said an autism risk of
approximately 7% among younger siblings of children with the disorder should be
"reassuring" for parents.
Zachary Warren, from the Treatment and Research Institute
for Autism Spectrum Disorders at the Vanderbilt Kennedy Center in Nashville,
Tennessee, said that rate is significantly lower than what's been found in
studies of other groups.
Those have estimated autism recurrence rates in families are
as high as 18%. Warren, who was not involved in the new research, said the
Danish findings might represent a "slightly conservative
estimate.""But it's a good study, and it's good information to
have," he told Reuters Health.
Parents "really want to have more definitive
information about recurrence risk. I would still be, at the end of the day,
more comfortable talking to families about the range of known
risks." "No doubt there is an elevated risk and whether it (seems
like) a high or low risk for families, they have to decide," Gronborg
said. "It's also an average based on many families, so for some families
the risk will be higher."