Autism Spectrum Disorder and Additional Areas of Difficulty: Patterns of Difficulties in a Population-Based Twin Sample
There is increasing interest and concern regarding additional psychiatric problems that co-occur with Autism Spectrum Disorder (ASD), as reflected in recent changes to diagnostic schemes. Since additional disorders may add to the burden for individuals with ASD and their carers, and may be amenable to treatment, recognition and diagnosis are important. Previous studies have reported raised rates of additional psychiatric difficulties in ASD, however, the majority of these have included clinic samples, which may be biased towards more complex or severely impaired cases.
The study aims to establish, for the first time, the rates and patterns of co-occurring difficulties in a large population-based twin sample covering the full range of the autism spectrum, and to examine possible risk factors.
Three groups of participants were included: 135 twins who met criteria for a research diagnosis of ASD (87% male; mean age 13 years 5 months); 55 non-ASD co-twins (36% male; mean age 13 years 6 months); 144 comparison twins (69% male; mean age 12 years 9 months). Participants and their parents completed the Strengths and Difficulties Questionnaire (SDQ) as part of a wider research assessment. The SDQ includes five domains of difficulties: emotional, conduct, hyperactivity, peer and pro-social; and produces mean and cut-off scores. Measures of possible risk factors were also included: age, full scale IQ, SES, sex, zygosity and autism severity (ADI-R, ADOS-G, Childhood Autism Spectrum Test).
For both mean scores and cut-offs on the SDQ the ASD twin group showed significantly higher levels of difficulties than both the non-ASD co-twins and the comparison twins. No differences were seen between the co-twins and the comparison group.
The frequency of multiple problems was examined (limited to the emotional, conduct and hyperactivity domains), showing 46% of the ASD twin group had difficulties in more than one domain. In contrast, 11% of the co-twins and 7% of the comparison group showed multiple areas of difficulty.
Analyses of risk factors found that for all three sample groups autism severity was the biggest contributor in terms of the mean SDQ scores. The higher participants scored in terms of autism severity the higher their SDQ scores. This link occurred for cut-off scores, albeit reduced and confined to the ASD group only.
The results of the study support previous research findings of high levels of comorbid difficulties for those with ASD. The population-based design and the use of standardised measures allow a clearer understanding of the rates and the nature of those difficulties for the group across the full autism spectrum. The severity of ASD symptoms was the risk factor most strongly associated with SDQ ratings and rates of multiple problems, for all three sample groups, with other risk factors showing fewer connections. The very high rates of difficulties shown by the ASD group indicate that comorbidity of difficulties is a serious issue and more evidence concerning the nature of comorbid difficulties is needed to help develop and target interventions for those with ASD.