Anxiety Disorders in Adults with Autism: A Population-Based Study.

Background:  Anxiety is common in children with autism, but studies of the prevalence of specific anxiety disorders in adults with autism are limited. It has been suggested that anxiety disorders may be more prevalent in autistic individuals without intellectual disability, who may have more insights into their problems. Furthermore, autism may be genetically linked to anxiety disorders, but few studies have examined the rates of anxiety disorders in non-autistic full and half siblings of individuals with autism.

Objectives:  1) To assess the associations of autism with adult anxiety disorders compared with the general population. 2) To study whether the associations are different for people with autism with and without intellectual disability. 3) To examine the risk of anxiety disorders in non-autistic full and half siblings of probands with autism, compared to the general population.

Methods:  The Stockholm Youth Cohort comprises persons aged 17 years or younger who had ever lived in Stockholm County, Sweden, from January 1, 2001, until December 31, 2011. We included cohort members aged 18 or older on December 31, 2011 who had ever received a diagnosis of autism (n = 3544) and their full and half-siblings never diagnosed as having autism. Of the 3544 individuals with autism we distinguished the 1046 with intellectual disability from those without intellectual disability (n= 2498). We calculated risk ratios (RRs) for all anxiety disorders ascertained using registers from outpatient and inpatient public service use, adjusted for parental age; highest parental education; household income; whether the child or either of their parents were born abroad; parental mental illness; sex and age at end of follow up.

Results:  Around a fifth (20.8%) of people with autism had an anxiety disorder diagnosed in adulthood, compared to 8.6% of the general population (RR= 2.76, 95% CI 2.58-2.94). Individuals with autism without intellectual disability were much more frequently diagnosed with anxiety disorders in adulthood (24.38%), than those who had autism with intellectual disability (12.24%). The adjusted RRs for all anxiety disorders for people with autism without intellectual disability was 3.16 (95% CI, 2.95- 3.38); whereas those for people with autism and intellectual disability were 1.72 (95% CI, 1.47- 2.02). Non-autistic full siblings of people with autism also had a greater risk of anxiety disorders than the general population [RR 1.35 (95% CI, 1.22- 1.48)] but there was no evidence for an increased risk of anxiety disorders for non-autistic half-siblings (RR = 1.03; 95% CI, 0.83-1.27) as compared to the general population.

Conclusions:  People with autism without intellectual disability have notably higher rates of anxiety disorders diagnosed in adulthood than the general population, and than individuals with autism and intellectual disability. The associations with anxiety disorders were highest in individuals with autism, followed by non-autistic full siblings and lowest in non-autistic half siblings suggesting that greater genetic load for autism may increase the risk for anxiety disorders.