The effects of intellectual functioning on the range of core symptoms of autism spectrum disorders

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Abstract

Autism spectrum disorders (ASD) are a class of conditions categorized by communication problems, ritualistic behaviors, and inappropriate social behaviors. While there is much evidence to support a genetic link for ASD, an identified genetic marker remains elusive. As such, practitioners place considerable emphasis on traditional measures of intelligence and adaptive behavior to aid in diagnosis. Despite the fact that these measures are commonplace, little research has been conducted to shed light on whether deficits in intellectual functioning affect the range of core symptoms for ASD. This study represents a first attempt to determine whether level of IQ has an effect on the expression of ASD symptoms in adults with intellectual disability (ID). Three hundred and six adults, 151 with both ASD and ID and 155 with ID alone, were evaluated with respect to the nature and extent of their ASD symptoms and intellectual functioning. Individuals with autism displayed a higher number of symptoms than those with pervasive developmental disorder-not otherwise specified (PDD-NOS) on all three domains of impairment (social, communication, repetitive behaviors). As expected, persons with ID alone evinced far fewer symptoms than both these groups. IQ level was found to be a moderator for expression of ASD symptoms for the entire sample but not for the autism group.

Introduction

The study of autism spectrum disorders (ASD) has been of paramount interest over the last two decades. The ASD are characterized by quantitative impairments in three major domains: social, communicative, and repetitive and stereotyped behaviors (American Psychiatric Association [APA], 2000; Gillberg & Coleman, 1996; Matson, 2007; Matson, Nebel-Schwalm, & Matson, 2007). Evidence from research supports the notion that these conditions are neurodevelopmental in origin (Lam, Aman, & Arnold, 2006; Matson & Minshawi, 2006). Despite this knowledge, researchers have been unable to isolate any clear genetic markers for these conditions (Klin, Lang, Cicchetti, & Volkmar, 2000). As such, clinicians must rely upon individually administered intelligence tests to aid in the diagnosis of intellectual disability (ID) and ASD. While cognitive deficits are not a defining characteristic of ASD, they are an important differential diagnosis marker (Rapin, 1997). The majority of children diagnosed with autism exhibit mild to moderate cognitive deficits, with a characteristically uneven cognitive profile. Verbal scores are generally lower than performance scores, with rote memory and spatial skills disproportionately high (Rapin, 1996). Furthermore, ASD and ID co-occur at high rates, with severity of ID positively correlated with rate of ASD (Bouras, Holt, Day, & Dosen, 1999; Kraijer, 1997; LaMalfa, Lassi, Bertelli, Salvini, & Placidi, 2004). The difficulty in treating these persons is further compounded by high rates of concomitant disabilities such as comorbid psychopathology challenging behaviors and social deficits (Canella, O’Reilly, & Lancioni, 2006; Chung et al., 2007; Hill & Furniss, 2006; Holden & Gitlesen, 2006; Ingersoll & Gergans, 2007; MacDonald et al., 2007; Matson & Niebel-Schwalm, 2007; Nicholson, Konstantinidi, & Furniss, 2006; Noone, Jones, & Hastings, 2006).

While childhood ASD and ID have been studied to some degree, research is lacking with respect to the degree to which ID effects the expression of ASD symptoms in adults (Schatz & Hamdan-Allen, 1995; Siegel, Minshew, & Goldstein, 1996). This study is an attempt to determine whether intellectual functioning plays a moderating factor in the exhibition of ASD symptoms in adults with ID, and if so, on what specific symptoms. Data of this nature should be of value in better understanding the nature of ASD with respect to ID and should assist in diagnosis and treatment planning. Furthermore, we looked at the effects of higher and lower IQ on autism, pervasive developmental disorder-not otherwise specified (PDD-NOS), and controls (no Axis I diagnosis) as a means of evaluating differences in specific ASD compared to persons with ID only.

Section snippets

Participants

Participants were residents of one of two developmental centers in the Southeastern United States. The sample consisted of 306 individuals with the following diagnoses: autism and ID (n = 56), PDD-NOS and ID (n = 95), and ID only (n = 155). Participants ranged in age from 16 to 88, with an average age of 52 years. A checklist with the combined ASD criteria from DSM-IV-TR and ICD-10 was used to diagnose ASD, following criteria from these diagnostic systems (American Psychiatric Association [APA], 2000

Results

The data were analyzed using a 2 × 3 between-subjects multivariate analysis of variance (MANOVA) with diagnostic group (autism, PDD-NOS, and control) and IQ group (low and high) as the independent variables. The dependent variables consisted of the three subscales of the ASD-DA: social, communication, and repetitive behavior. Mean scores for the ASD-DA subscales are presented in Table 1. Wilks's Lambda revealed a significant main effect for diagnostic group, F(6, 596) = 18.58, p < .05; IQ group, F

Discussion

Based on these results, IQ appears to be an important moderator of symptoms of ASD in intellectually disabled adults when blocked into “high IQ” versus “low IQ”. Looking at ASD symptom patterns of adults are of considerable importance given the distinct nature of the disorders (Lord et al., 2006). The autism group, as expected, had higher rates of ASD symptoms than the PDD-NOS group in all three domains and is consistent with what one would expect across these two diagnostic categories (Matson

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