TORONTO – Macroencephaly was not significantly more common in twins with autism spectrum disorders than in their unaffected co-twins, according to data from 202 twin pairs presented at the annual meeting of the American Academy of Child and Adolescent Psychiatry.
Macroencephaly, defined as a head circumference greater than the 97th percentile, is a common physical characteristic of children with autism spectrum disorders, said Dr. Wendy Froehlich of Stanford (Calif.) University and her colleagues.
The researchers recruited 404 children (202 twin pairs) aged 4-18 years. At least one twin in each pair met criteria for an autism spectrum disorder. The children were identified through the California Department of Public Health’s Department of Developmental Services.
"Macrocephaly may represent an endophenotype within autism spectrum disorders."
Overall, there were no significant differences in the rates of macrocephaly between affected and unaffected twins. Macrocephaly was noted in 20% of boys and 27% of girls with autism spectrum disorders and in 19% of boys and 24% of girls who were unaffected co-twins.
The researchers also examined the relationship between head circumference and cognitive measures. After age and gender were controlled for, small but significant negative correlations were found between head circumference and Verbal Intelligence Quotient (VIQ) and Full Scale Intelligence Quotient (FSIQ) in twins with autism spectrum disorders, but there was no correlation between head circumference and other autistic symptoms or functioning.
"Other studies have demonstrated a positive correlation between head circumference and/or brain volume and IQ in typical individuals," the researchers noted.
"Macrocephaly may represent an endophenotype within autism spectrum disorders, and the process leading to macrocephaly is not sufficient for the development of autism," the researchers said. In addition, unaffected co-twins could represent "an intermediate phenotype between affected individuals whose head circumferences correlate negatively and truly unaffected and unrelated individuals whose head circumferences correlate positively with IQ."
The study was funded by grants from Autism Speaks, a nonprofit organization; and the National Institute of Mental Health. Dr. Froehlich had no personal financial conflicts to disclose.