Head Circumference in the Autistic Spectrum: a Comparative Study

Background: A significant proportion of autistic children have a larger-than-typical head circumference (or macrocephaly) after 14 months of age. Most studies on autistic children show a macrocephaly prevalence of 16% - 20% against 3% in typically developing children. In adulthood, Lainhart and colleagues (1997) report an increased macrocephaly prevalence rate of up to 19.4% in autistic adults. Despite numerous head circumference studies involving autistic and non-autistic children, studies in well-defined groups of autistic adults remain limited. Moreover, few studies have controlled for height and IQ despite the fact that head circumference is usually proportional to the height of the individual (Bushby et al., 1992), and that IQ could be associated with brain size (Aylward et al., 2002).

Objectives: 1) To construct an adult head measurement reference chart for typically developing adults that takes height into consideration, and that could be used for clinical purposes. 2) To compare the proportion of macrocephaly among a group of autistic adults and a group of typically developing adults.

Methods: 1) 100 male adults will be recruited among personnel at Rivières-des-Prairies Hospital and students at the University of Montreal in order to construct the reference scale. Height, weight and head size will be measured. 2) Two groups of 40 male subjects, autistic and non-autistic (aged 18-40; IQ between 80 and 130), will be recruited within the hospital’s database. Non-autistics who experienced past head trauma and who have a family history of psychological/neurological disorders, and autistics who have evident neurological/medical anomalies or other axis 1 disorders (except ADHD and dysphasia), are excluded from the study. Between-group differences will be tested using the scores obtained from the newly-made reference scale. Macrocephaly is defined as head circumference above the 97^th percentile when controlling for height.

Results: 1) For the reference scale, so far, we have obtained measurements from 49 participants. Preliminary results show a positive relationship between head circumference and height (r=0.36). 2) So far, complete data have been obtained for 12 autistic and 20 typically developing participants matched on Wechsler full-scale IQ (F=0.378, p=.54). Preliminary results show a 16.7% prevalence of macrocephaly in the autistic group compared to no macrocephaly cases in the typically developing control group (according to Bushby’s 1992 chart, as a temporary reference).

Conclusions: Preliminary findings in a reference group indicate the necessity of taking height into account when measuring head circumference. Preliminary findings in the head circumference comparison study indicate that macrocephaly in autism persists into adulthood.