The Effect of Comorbidity on Cognitive Efficiency in ASD

Background: Previous studies have found intact working memory in individuals with autism spectrum disorder (ASD), but deficits in processing speed and mixed impairments in executive functioning. Psychiatric comorbidity has negative consequences on cognitive functioning. Understanding whether comorbidity has unique or shared impact on ASD compared to other diagnostic groups has important implications for assessment and treatment.

Objectives: To study the effect of comorbidity on cognitive efficiency in individuals with ASD compared to individuals with diagnoses other than autism.

Methods: Retrospective analyses were conducted on a clinical pediatric sample of 421 individuals (73% Male; Caucasian: 37%, Asian: 2%, African American: <2%, Indian: <1%, Multi-racial: 2%) who presented to a developmental disabilities clinic. Cognitive assessments were conducted by licensed psychologists, and final consensus diagnosis was determined by a multidisciplinary team consisting of psychology, psychiatry, and speech/language specialists following comprehensive evaluation. Analyses consisted of group comparisons of individuals diagnosed with ASD (n=307) and individuals receiving another diagnosis (non-ASD group, n=110; Learning Disability 28%; ADHD 15%; Anxiety Disorder 15%; Language Disorder 14%; Mood Disorder 5%; and behavior disorders, tic disorders, OCD, and thought disorders, each <4%). Those given no diagnosis or those with Intellectual Disability as a primary or comorbid condition were excluded. Cognitive functioning was measured using standardized scores across WISC-III, WISC-IV, and WAIS-III, with the Working Memory Index and Freedom from Distractibility Index combined to create a composite score. The Behavior Rating Inventory of Executive Function (BRIEF) was also analyzed.

Results: Comorbid conditions were diagnosed in 8% (n=26) of individuals with ASD, and 20% (n=22) of individuals in the non-ASD group (χ²=10.57, p<.01). Individuals with ASD were younger than the non-ASD group [ASD: M=9.76±3.75; non-ASD: M=10.82±3.83; t(402)=-2.49, p=.02]. There was a significant difference in Full Scale IQ (FSIQ) across diagnostic groups [ASD: M=94, SD=22; non-ASD: M=101, SD=20; t(397)=-2.93, p<.01). This difference remained true when factoring in comorbidity; only diagnostic groups without comorbidity had significantly different FSIQ [F(3,396)=3.24, p=.02]. FSIQ differences were driven by lower verbal and nonverbal reasoning index scores.

Within the ASD group, working memory index scores did not differ with comorbidity [non-comorbid: M=97, SD=21; comorbid: M=90, SD=18; t(214)=1.33, p=.19), nor did processing speed index scores [non-comorbid: M=88, SD=17; comorbid: M=87, SD=18; t(210)=0.30, p=.77]. The same was found within the non-ASD group for working memory [non-comorbid: M=100, SD=20; comorbid: M=95, SD=19; t(78)=1.05, p=.30] and processing speed [non-comorbid: M=91, SD=17; comorbid: M=88, SD=17; t(76)=0.61, p=.54]. Detailed evaluation of executive functioning from the BRIEF indicated more pronounced deficits in self-monitoring for individuals who received a diagnosis other than ASD [non-ASD: M=68, SD=9; ASD: M=63, SD=11; t(107)=-2.36, p=.02]. There were no other significant differences in executive functioning between diagnostic groups or within groups based on comorbidity.

Conclusions: Overall, individuals with ASD and other psychiatric diagnoses demonstrated similar cognitive efficiency profiles as measured by working memory and processing speed. Executive functioning across ASD and other diagnostic groups was also similar with the exception of self-monitoring which was more impaired in the non-ASD group. Comorbidity did not further impair one’s cognitive efficiency or executive functioning.