Elsevier

Research in Autism Spectrum Disorders

Volume 6, Issue 1, January–March 2012, Pages 277-285
Research in Autism Spectrum Disorders

Autism and ADHD: Overlapping and discriminating symptoms

https://doi.org/10.1016/j.rasd.2011.05.009Get rights and content

Abstract

Children with ADHD and autism have some similar features, complicating a differential diagnosis. The purpose of our study was to determine the degree to which core ADHD and autistic symptoms overlap in and discriminate between children 2–16 years of age with autism and ADHD. Our study demonstrated that 847 children with autism were easily distinguished from 158 children with ADHD. All children with autism had 15 or more of the 30 Checklist for Autism Spectrum Disorder symptoms (mean 22), and none of the children with ADHD did (mean 4). Three of the symptoms were present only in children with autism. Almost all 30 symptoms were found in over half of the children with autism, whereas none were present in the majority of children with ADHD-Inattentive type (ADHD-I) or in children with ADHD-Combined type (ADHD-C) without comorbid oppositional-defiant disorder. In contrast, ADHD symptoms were common in autism. Children with low and high functioning autism and ADHD-C did not differ on maternal ratings of attention deficit, impulsivity, and hyperactivity. For children with normal intelligence, nonsignificant differences were found between children with autism, ADHD-C, and ADHD-I on neuropsychological tests including measures of attention, working memory, processing speed, and graphomotor skills.

Highlights

► Our study demonstrates that children with autism can easily be distinguished from children with ADHD based on their symptom profiles. ► The core symptoms of ADHD (attention deficit, impulsivity, and hyperactivity) are part of autism, and autism and ADHD have similar underlying neuropsychological deficits. ► Autistic symptoms are not common in ADHD. ► Because of the high frequency of ADHD symptoms in autism, children with autism may initially be misdiagnosed with ADHD.

Introduction

Children with attention deficit hyperactivity disorder (ADHD) and children with autism have many overlapping features, including attention deficit and overactivity (American Psychiatric Association, 2000, Mayes and Calhoun, 2004a, Mayes and Calhoun, 2006a, Mayes and Calhoun, 2007, Mayes and Calhoun, 2008, Mayes and Calhoun, 2011, Sturm et al., 2004), behavior problems (American Psychiatric Association, 2000, Mayes and Calhoun, 2011), and difficulty with social skills (Clark et al., 1999, de Boo and Prins, 2007). These shared symptoms complicate a differential diagnosis. Because of the presence of ADHD symptoms in both disorders, a diagnosis of autism is often delayed and some children with autism are initially misdiagnosed with ADHD (Hartley & Sikora, 2009). ADHD symptoms are considered part of autism by the DSM-IV, which states that “attention-deficit/hyperactivity disorder is not diagnosed if the symptoms of inattention and hyperactivity occur exclusively during the course of a pervasive developmental disorder” (p. 91). Researchers substantiated that most children with autism have ADHD symptoms (Frazier et al., 2001, Gadow et al., 2005) and that the symptoms are similar in both groups (Frazier et al., 2001, Hattori et al., 2006, Luteijn et al., 2000).

In addition to ADHD symptoms, children with autism and children with ADHD (particularly ADHD-Combined type) have many similar comorbid problems. In comparison to the norm, both groups have more irritability and anger (Brereton et al., 2006, Green et al., 2000, Mayes et al., 2011a) and behavior problems (American Psychiatric Association, 2000, Barkley, 2006, Brereton et al., 2006, Clark et al., 1999, Connor et al., 2010, Efron and Sciberras, 2010, Faraone et al., 1998, Gadow et al., 2005, Greene et al., 2002, Mayes and Calhoun, 2011, MTA Cooperative Group, 1999). Furthermore, children with autism and children with ADHD have similar neurocognitive weaknesses, including executive function deficits (Corbett et al., 2009, Happe et al., 2006), slow processing speed (Calhoun & Mayes, 2005), dysgraphia (Mayes and Calhoun, 2004a, Mayes and Calhoun, 2007), learning disability in written expression (Mayes and Calhoun, 2006b, Mayes and Calhoun, 2007), and deficits in attention, motor control, and perception (DAMP, Sturm et al., 2004). Children with autism and children with ADHD often have early language delay (Mayes and Calhoun, 1999, Mayes and Calhoun, 2011, Miniscalco et al., 2007) and sleep problems (Mayes, Calhoun, Bixler, & Vgontzas, 2009).

There is an agreement that ADHD symptoms are common in autism and that children with autism and children with ADHD have many similar comorbid problems. However, consensus is lacking as to whether core diagnostic criteria for autism are common in ADHD. Three studies suggest a nonsignificant difference in some autistic symptoms between children with autism and children with ADHD, such as communication impairment and restricted and repetitive behavior (Hattori et al., 2006) and difficulty adapting to change and over- or under-reactivity to sounds (Koyama, Tachimori, Osada, & Kurita, 2006). Hartley and Sikora (2009) reported nonsignificant differences between autism and ADHD in lack of social and emotional reciprocity, impaired conversation skills, restricted interests, nonfunctional rituals, stereotypies, and preoccupation with parts of objects. However, sample sizes in these three studies were small (less than 28 children per group), thus power may have been inadequate to detect significant differences.

In a study by Clark et al. (1999), questionnaires were mailed to and completed by 49 parents of children diagnosed with ADHD. These parents rated their children on a list of DSM-IV autism symptoms. A high rate (sometimes to frequently a problem) was noted for many autistic symptoms, including lack of awareness for the feelings of others (86%), difficulty making friends (82%), difficulty beginning or sustaining a conversation (78%), odd forms of speech which included “frequent use of irrelevant remarks” (76%), stereotyped hand or body movements (71%), and problems with nonverbal communication which included “problems in observing others’ personal space” (71%). The authors stated that many of these symptoms could be explained by the symptoms of ADHD. However, the high frequencies led the authors to speculate that some children diagnosed with ADHD actually had autistic disorder or pervasive developmental disorder not otherwise specified (PDD-NOS).

Several studies have shown that children with autism and children with ADHD differ significantly in their symptoms. Studies found that children with autism had more problems with language and communication (Geurts et al., 2004, Luteijn et al., 2000); social interaction (Hattori et al., 2006, Luteijn et al., 2000); peer relationships, stereotyped and idiosyncratic language, and imaginative play (Hartley & Sikora, 2009); relating to people, emotional responsiveness, stereotypies, odd and repetitive object use, eye contact, too much or too little fear, and verbal and nonverbal communication (Koyama et al., 2006). In the latter study, children with ADHD had higher activity levels and were more likely to touch, taste, or smell objects than children with autism (Koyama et al., 2006). Interestingly, some of these findings are at odds with the aforementioned nonsignificant differences between children with autism and children with ADHD.

The purpose of our study was to determine the degree to which ADHD and autistic symptoms overlap in and discriminate between children with autism and children with ADHD. Our study has several advantages in comparison to previous research, including large sample sizes, a comprehensive assessment of both ADHD and autistic symptoms (using rating scales, neuropsychological tests, and an autism instrument with demonstrated validity in distinguishing autism from ADHD), separate analyses for children with ADHD-Combined type (ADHD-C) and ADHD-Inattentive type (ADHD-I), and rigorous diagnoses using DSM-IV criteria and standardized diagnostic instruments. Our study was designed to clarify the shared and distinguishing features of autism and ADHD, which will help to facilitate an accurate differential diagnosis between two disorders that are sometimes mistakenly confused and about which there is controversy regarding overlapping symptoms. This is critical to insure appropriate and timely treatment, particularly because the intensive behavioral intervention empirically proven to be effective for children with autism is significantly more effective if implemented during the preschool years than later (Fenske et al., 1985, Harris and Handleman, 2000).

Section snippets

Sample

All clinical children were consecutive referrals to our psychiatry diagnostic clinic who met inclusion criteria. Primary analyses were conducted on 1,005 children 2–16 years of age including 523 children with high functioning autism (HFA, IQ  80), 324 children with low functioning autism (LFA, IQ < 80), 112 children with ADHD-C and IQ  80, and 46 children with ADHD-I and IQ  80. Demographic data are reported in Table 1. IQ tests administered were the WISC-IV, WPPSI-III, and Bayley Mental Scale,

ADHD symptom scores

In comparison to children with HFA, LFA, and ADHD-C, children with ADHD-I had significantly lower raw scores on all PBS ADHD subscales, including attention deficit (F = 5.9, p = .001, Bonferroni p < .02), impulsivity (F = 22.9, p < .0001, Bonferroni p < .0001), and hyperactivity (F = 12.7, p < .0001, Bonferroni p < .0001). Children with LFA, HFA, and ADHD-C did not differ significantly from each other on any of these subscales (Bonferroni p > .07), and effect sizes between the autism and ADHD-C groups were small (d

Discussion

Our study demonstrates that children with autism can easily be distinguished from children with ADHD based on their symptom profiles. All children with autism had 15 or more CASD symptoms, and none of the children with ADHD did. The mean CASD score for children with autism was 22 and for children with ADHD, 4. Three of the CASD symptoms were present only in children with autism (i.e., unusual fascination with repetitive movements, language regression, and special abilities). Almost all CASD

Acknowledgement

We wish to thank Autism Speaks (Grant #2529) and the Children's Miracle Network for their support of this study.

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