© Springer Science+Business Media New York 2014
Jonathan Tarbox, Dennis R. Dixon, Peter Sturmey and Johnny L. Matson (eds.)Handbook of Early Intervention for Autism Spectrum DisordersAutism and Child Psychopathology Series10.1007/978-1-4939-0401-3_33. Related Disorders
(1)
University of Connecticut, Storrs, USA
(2)
University of Connecticut, Storrs, USA
Keywords
ComorbidityRelated disordersIntellectual disability autismADHDLanguage DisordersAutism spectrum disorders (ASDs), also referred to as pervasive developmental disorders (PDDs) in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) (American Psychiatric Association [APA] 2000) encompass a heterogeneous group of neurodevelopmental disorders with varying etiologies that have been behaviorally defined as having impairments in social communication, reciprocal social interaction, and repetitive, restricted behavior and interests (APA 2000). The spectrum includes autistic disorder, characterized by clear deficits in all three domains; Asperger’s disorder, characterized by no significant language delay and average to above average cognitive functioning; and pervasive developmental disorder-not otherwise specified (PDD-NOS), characterized by significant social and communication impairments that do not meet full diagnostic criteria for autistic disorder (American Psychiatric Association 2000). According to current diagnostic criteria, symptoms must be present before 3 years of age. Current literature estimates the prevalence of ASDs to be 1:150, or 0.6–0.7 % (Rapin and Tuchman 2008), and even higher estimates of 1:110 have been published by the Centers for Disease Control and Prevention (CDC 2006) . It should be noted that, at the time this volume is going to press, the American Psychiatric Association is publishing the DSM-V, which changes several aspects of the diagnostic criteria for autism (American Psychiatric Association 2013). However, this chapter discusses the longstanding previous diagnostic criteria, as these criteria have been the subject of the overwhelming majority of research.
ASD symptomatology overlaps with that of a range of disorders, including intellectual disability (ID), attention deficit hyperactivity disorder (ADHD), and language disorders and are frequently comorbid with these conditions as well as with psychiatric disorders such as oppositional defiant disorder (ODD), obsessive compulsive disorder (OCD), anxiety, depression, and schizophrenia (Tager-Flusberg and Dominick 2011). According to current diagnostic criteria, ID and schizophrenia can be diagnosed concurrently with ASD , whereas language impairment and attention symptoms are so prevalent in individuals with ASD that a diagnosis of ASD precludes a diagnosis of expressive language disorder, mixed receptive-expressive language disorder, or ADHD (APA 2000). There has, however, been considerable disagreement in the field as to whether these disorders should be able to be diagnosed concurrently with ASD (e.g., Sinzig et al. 2008).
ASDs have been conceptualized to have social dysfunction as part of their core symptomatology; in contrast, social deficits in other disorders, such as ID, ADHD , and language impairment (LI), are largely viewed as a secondary consequence of other core symptoms. However, there is sometimes so much overlap that it is difficult to differentially diagnose disorders. Furthermore, ID, ADHD, and language disorders are more frequently comorbid with ASD than would be expected by chance. The degree of overlapping symptomatology and frequency of comorbidity among these disorders suggests an examination of the nature of the relationship between ASD and these disorders. Although there are many disorders that are comorbid with ASD, this chapter will focus on three disorders that have been particularly difficult to differentiate from ASD: ID, ADHD, and language disorders. This chapter will discuss differential diagnoses and examine the current theories and research about the relationships of these disorders with ASD.
Autism and Intellectual Disability
Criteria for Intellectual Disability
ID (also referred to as “Mental Retardation” in the DSM-IV) is a developmental disorder characterized by significant deficits in cognitive functioning (usually an IQ<70) and deficits in two or more areas of adaptive functioning, which could include impairment in communication, socialization, or daily living skills such as dressing and feeding (APA 2000). In the DSM-IV, the diagnosis is further delineated by severity: mild mental retardation (IQ 50–55 to 70), moderate mental retardation (IQ 35–40 to 50–55), severe mental retardation (IQ 20–25 to 35–40) and profound mental retardation (IQ below 20 or 25: APA 2000). The prevalence of ID is estimated at 1–3 % of the population (Leonard and Wen 2002). The etiology of ID varies and can include genetic disorders (e.g., Angelman syndrome, Down’s syndrome, fragile X, Prader Willi, Rett syndrome, Williams syndrome, 22q11 deletion), environmental factors (fetal alcohol syndrome, environmental toxins , perinatal problems), or be of unknown origin (idiopathic). ID is frequently comorbid with other conditions, most prominently ASD and ADHD , but also psychiatric conditions such as depression, bipolar disorder, and schizophrenia (Bradley et al. 2004). A lower IQ increases the likelihood of a comorbid disorder (e.g., La Malfa et al. 2004).
There has been research in the ASD literature that has examined the prevalence of ID in individuals with ASD; conversely, there has been research in the ID literature about the prevalence of ASD in individuals with ID. The ID literature has found that between 17 and 40 % of individuals with ID meet diagnostic criteria for ASD (Bryson et al. 2008; de Bildt et al. 2004; La Malfa, et al. 2004). This high prevalence of ASD in individuals with ID is significantly more than the 0.6–0.7 % of ASD in the general population. Likewise, the ASD literature has found that many individuals with ASD meet criteria for an ID. Previous estimates of ID (defined as IQ of 70 or lower) in people with ASD used to be as high as 70–90 % (Fombonne 2003). However, with better awareness of autism and the advent of more sensitive diagnostic measures, there have been more ASD diagnoses among individuals with average to above average cognitive abilities. In addition, earlier diagnosis and intensive treatment may lead to higher IQ at follow-up. Nevertheless, recent prevalence estimates of ID in individuals with ASD still range from 26 to 59 % (Chakrabarti and Fombonne 2001; Fombonne 2003).
Overlapping Symptomatology
Intellectual disability in individuals with ASD
ASD encompasses a range of cognitive ability, ranging from profound ID to superior cognitive performance . However, ID is significantly more common in ASD than in the general population. Furthermore, although the prevalence of ID in individuals across all ASDs ranges from 26 to 59 %, the prevalence among individuals with autistic disorder (as opposed to a diagnosis of PDD-NOS or Asperger’s) may be as high as 66–70 % (Chakrabarti and Fombonne 2001; Fombonne 2003). There is also a relationship between specific autism symptomatology and ID: in general, the people with the most severe autism symptomatology have the most severe ID. Furthermore, the impairments that are usually seen in autism, such as delayed or impaired language development (including difficulty in understanding and using language) and common comorbidities, such as attention problems and sensory issues, will make learning more difficult. Likewise, social impairments associated with autism could also delay or prevent learning, including lack of imitating, a foundational skill for learning, and a lack of motivation to please others or demonstrate mastery (Siegel 2010). All of these factors may lead to ID or the appearance of such a disability. However, individuals with ASDs can also have primary cognitive deficits that are independent of these social, language, and attention factors .
Autistic symptomatology with ASD in individuals with intellectual disabilities (ID)
ID has greater symptom overlap than any other disorder (Wilkins and Matson 2009) . Individuals with ID typically have a language and communication delay and socialization difficulties, two of the three main criteria to diagnose ASD (American Psychiatric Association 2000). The distinction between disorders is further complicated because 30–60 % of individuals with ID display repetitive behaviors, especially individuals with severe or profound ID (IQ<35; Bodfish et al. 1995, 2000; Goldman et al. 2009). Lord (1995) found that 8 % of 3-year-olds with ID displayed stereotypies. Bodfish et al. (1995) administered items about stereotyped and self-injurious behavior for a group of 210 adults with severe or profound ID and found that 61 % displayed stereotyped behavior, 40 % had compulsions, and 47 % displayed self-injurious behavior. However, while repetitive behaviors are present in both disorders, the frequency and severity of behaviors are more pronounced in autism than in ID without autism (Bodfish et al. 2000; Lord et al. 1995). Individuals who meet criteria for ID (with an IQ<70) are also more likely than individuals without ID to have echolalia, a common behavior in individuals with ASD. As would be predicted, individuals with more severe ID are more likely to meet criteria for an ASD than those with milder degrees of ID (La Malfa et al. 2004; Vig and Jedrysek 1999). Social and communication deficits are prevalent in this population because people with ID have more difficulty developing communication and socialization skills requiring higher cognitive abilities .
It is sometimes difficult to differentiate a severe ID from ASD . Measures such as the Autism Diagnostic Observation Scale (ADOS) and Autism Diagnostic Interview-Revised (ADI-R) can reliably differentiate children with non-autistic, mild and moderate ID from ASD, but have less reliability differentiating severe ID from ASD (de Bildt et al. 2004): while individuals with mild ID display social competency skills similar to typically developing individuals at about the same developmental level, individuals with severe ID may have severe social deficits (Ingram et al. 2007). Additionally, individuals with severe or profound ID are more likely to be nonverbal, so prototypically autistic features, such as echolalic or stereotyped use of language, and poor use of language for communication, will not be evident .
Differential Diagnosis
When diagnosing an individual with ASD and ID, the clinician must decide whether the behaviors indicate : ASD only, ID only (where the social and communication deficits are seen as resulting from low cognitive developmental levels rather than an underlying primary social deficit), or comorbid ASD and ID. In more straight forward cases, individuals with ASD and ID will be able to be differentiated by several main factors. First, individuals with ASD are more likely to have idiosyncratic language (e.g., stereotyped speech, scripted speech, neologisms, pronoun reversal) and atypical intense interests, such as an interest in vacuum cleaners or weather patterns. Furthermore, individuals with ASD have socialization scores on the Vineland Adaptive Behavior Scales (VABS; Sparrow et al. 1984) that are usually significantly below their cognitive scores, whereas individuals with ID will have socialization scores that are more consistent with their mental age (Liss et al. 2001; Hauck et al. 1995). This pattern will also be observed in other areas of adaptive behavior functioning, with the ID group displaying scores that may be low, but are generally consistent with their cognitive functioning, whereas the ASD group will be displaying adaptive functioning lower than would be predicted given their cognitive functioning. For example, a pattern that may be observed is that a child has greater language abilities (vocabulary, etc.) apparent in cognitive testing than is utilized day to day (i.e., Vineland Communication scores) because of a lack of motivation to utilize his or her language skills in daily life. Since Vineland Receptive Language items draw heavily on interest in and attention to language, this score can be particularly low in children with ASD .
ASD only versus comorbid ASD and ID
In “straightforward” cases, an individual would meet criteria for a comorbid ID if he or she has scores lower than 70 on standardized cognitive tests such as the Wechsler Preschool and Primary Scale of Intelligence (WPPSI; Wechsler 2002) , Wechsler Intelligence Scale for Children (WISC; Wechsler 2003), Wechsler Adult Intelligence Scale (WAIS; Wechsler 2008), or Stanford Binet (Roid 2003). This ASD + ID subgroup will have socialization skills and cognitive scores that are both low and significantly impact adaptive functioning. In contrast, individuals with only an ASD diagnosis will have cognitive scores above 70 and their adaptive functioning scores will likely be more impacted due to social factors rather than cognitive disability. However, many cases are not this straightforward. For example, many individuals with ASD have receptive language difficulties that will impact their ability to understand the directions for even nonverbal cognitive tests. Furthermore, they may have comorbid attention problems and have difficulty focusing on the test, or have behavioral issues and refuse to complete the test. Likewise, there will be many whose IQ scores will border 70, and in these cases the clinician will have to evaluate the degree to which the cognitive limitations are impacting adaptive functioning .
ID only versus ASD (either with or without comorbid ID)
Many individuals with ID will have difficulty communicating and socializing, whether due to lack of understanding language or not developing socialization skills commensurate with their chronological age. Hauck et al. (1995) studied 33 children ages 7–14 years old who were attending special education programs: 18 had an autism diagnosis (mental age from 2.8 to 11.6 years); and 13 had an ID diagnosis (mental age from 2.7 to 8 years). The children were matched according to their nonverbal ability. Hauck and colleagues found that although the children did not differ in Vineland Communication scores, vocabulary, or nonverbal skills, the Vineland Socialization scores were significantly lower for children with autism. Furthermore, the authors coded behavior in two contexts: during meals and during free play. They found that while children with autism initiated interaction with adults at the same frequency as children with ID, the children with autism displayed “lower level” interactions, such as asking for help, ritualized greetings, or touching. In contrast, children with ID participated in “higher level” behaviors, such as giving information to the teacher, imitating, and engaging in more reciprocally interactive behaviors. Additionally, children with autism were 1/3 less likely to initiate interaction with peers than children with ID: the authors postulated that interaction with peers may have more of a social quality, whereas interaction with adults may be more need based, resulting in children with ASDs engaging more in the latter activity than the former. Hauck and colleagues also found that children with autism initiated more in the structured setting (lunch) than in the nonstructured setting (free play), indicating that children with autism may display more social behaviour in structured settings.
Jackson et al. (2003) conducted another study on the same group of 33 children. The ASD and ID groups were matched on their verbal ability and chronological and mental age. Jackson and colleagues evaluated the children’s response to initiation of social contact by adults and peers as well as their ability to sustain play interactions. The authors found that while neither the frequency of conversation nor frequency of response to adults differed between groups, children with autism were less likely to sustain interaction with peers (such as sustaining play activity). Furthermore, children with autism had more “no responses” (ignored social bids) and fewer overall positive responses (such as compliant, cooperative, or helping behavior) than their ID peers.
Therefore, while children with intellectual disabilities display communication and social behavior below what would be expected given their chronological age, their social behavior tends to be more consistent with their other abilities, whereas children with autism display more social difficulties than children with ID even when matched on mental ability. Furthermore, qualitative differences in social initiation and response are displayed in more social-emotional reciprocity, social engagement, and engagement with peers in the ID group, whereas social interactions in the ASD group is more adult oriented and interactions are more need based.
ID only versus comorbid ID and ASD
Hepburn et al. (2008) studied 20 two-year-old children with Down’s syndrome with a formal diagnosis of ID and evaluated autism symptoms by administering the Autism Diagnostic Observation Schedule-Generic (ADOS-G) and ADI-R and following up 2 years later to evaluate the stability of the diagnoses. They noted that children with Down’s syndrome who did not meet criteria for an autism diagnosis nevertheless displayed behaviors associated with autism, including poor eye contact, restricted and idiosyncratic interests (such as an interest in parts of objects), and repetitive behaviors (such as rocking, hand-flapping). Hepburn and colleagues found that 3 out of 20 children (15 %) met the criteria for an ASD according to the ADOS. They also noted that while many children had difficulty with communication and play as well as restricted interests, they continued to display behaviors indicating social reciprocity. This included reciprocal social smiling, directing facial expression, joint attention, and attempts to imitate adults during the ADOS. However, none of the children met criteria for autism on the ADI-R, although the same three toddlers who met criteria for ASD on the ADOS met on the communication domain on the ADI-R. Clinically, two out of the three toddlers who met criteria on the ADOS were given an ASD diagnosis, whereas the third did not because she showed appropriate reciprocal social behaviors (e.g., effective modulation of eye contact, sharing affect, nonverbal reciprocity); rather her communication and socialization scores suffered because of her low mental age (< 12 months). Therefore, communication and restricted and repetitive interests did not sufficiently differentiate the groups, although the clinical judgment about the presence or absence of reciprocal social interaction did. These findings are consistent with those of Jackson et al. (2003) and Hauck et al. (1995), described above, suggesting that quality of social engagement, rather than communication difficulty or repetitive behavior, is the best differentiator of children with ID who have and do not have an ASD.
Therefore, while individuals with ID may have difficulty communicating due to low level of receptive and expressive language, they are more likely to display reciprocal social behaviors and compensate through other social avenues of communication, such as gesture. However, if all levels of functioning are low, it will be difficult to differentiate whether socialization scores are meaningfully below other cognitive abilities.
Genetic Studies
ASD and ID occur more often together than they would by chance. Is there a genetic relationship between the two sets of disorders? Before commenting on this topic, it is important to note that ASD and ID are both heterogeneous disorders. Multiple genetic factors can contribute to both clinical phenomena; furthermore, the etiology for some individuals may not have a genetic component but result from environmental factors, such as perinatal insult, or a more general environmental factor disturbing neuronal and brain development.
There are some genetic disorders associated with ID where a comorbid ASD diagnosis is more likely than would occur in the general ID population. For example, 5–10 % of individuals with Down’s syndrome, 25–47 % of individuals with fragile X syndrome, and 16–48 % of individuals with tuberous sclerosis have a comorbid ASD diagnosis (review by Kaufman et al. 2010). Sikora et al. (2006) tested 14 children 3–16 years old with Smith-Lemil-Opitz Syndrome, a genetic form of ID, and found that approximately 75 % met criteria for an ASD, with 50 % of those diagnoses meeting criteria for autistic disorder. Furthermore, certain behaviors associated with ASD are more likely to be observed in certain syndromes. For instance, individuals with fragile X are more likely to avoid one’s gaze or turn their head away in response to a greeting (Gillberg 2006). It is unclear, however why the ID and ASD diagnoses occur more often together in these conditions, whether it is due to overlapping symptomatology, genetic or neurologic vulnerability, or other factors. In their study of autism symptoms in toddlers with Down’s syndrome, Hepburn et al. (2008) speculated whether individuals with Down’ syndrome who meet criteria for ASD had “classic” autism, or whether there was a pattern of cognitive and communication difficulties in Down’s syndrome that could negatively impact the development of communication skills and social reciprocity that would lead some children to meet criteria for an ASD as a secondary consequence of their Down’s syndrome deficits.
Nevertheless research has found similar genetic regions or mechanisms impacted in ASD and ID. These include rare copy number variants (CNVs; deletions and duplications in the genome) that are present in both ASD and ID (Kaufman 2010). Areas that have been associated with both ASD and ID include SYNGAP1, SHANK3, and IL1RAPL1 (Pinto et al. 2010). Pinto et al. (2010) speculated that with more research more genes associated with ID would be linked to ASD .
Hoekstra et al. (2009) found that there is little genetic overlap, at least for genes associated with “extreme” autism traits and genes for ID, but speculated that there may be genetic overlap with genes associated with communication abilities. Hoekstra et al. (2009) explored whether individuals with more severe autism symptoms were more likely to have a low IQ. They used a community sample and studied the 5 % of individuals with the most severe autism traits and the 5 % with the lowest IQs and found that the top 5 % of the “most extreme” autism cases were 4.3 times more likely to perform in the bottom 5 % on an IQ test. However, they found that the severity of autism was only modestly related to ID. Instead, most of the association between ID and ASD were due to communication items that were associated with ASD, suggesting that areas of genetic overlap between ASD and ID may be genes associated with communication abilities. Hoekstra and colleagues cited previous studies that showed that relatives of individuals with autism were more likely to have social and communication deficits in addition to lower IQs.
Theories About the Relationship Between ASD and Intellectual Disability
There are many ways to conceptualize the relationship between ASD and ID. On one hand, the “core” deficits from each disorder are likely to contribute and exacerbate the symptoms that are associated with the other disorder (i.e., lack of social motivation in ASD contributing to decreased motivation to learn, low cognitive ability in ID contributing to poor social skills) . However, many individuals have ASD alone or ID alone; therefore, it is clear that symptoms of autism in many cases are not attributable to cognitive limitations, and vice versa. There are other factors at work. However, it is true that individuals with more severe ASD symptoms are more likely to have low cognitive functioning, and individuals with severe ID are more likely to have more ASD symptoms. What, then, is the relationship between ASD and ID?
Some attribute the high incidence of comorbidity to common genes that impact brain development interacting with the environment to produce a range of phenotypes, from ID to ASD, to a mix of other symptomatology (e.g., Morrow et al. 2008). Minshew and Meyer (2006) postulated that ID in ASD is a more severe expression of cognitive deficits that are seen in individuals with ASD who do not have ID. Siegel (2010) proposed that clinicians and researchers reconceptualize ASD as a type of learning disability, with different “autistic learning disabilities” (ALDs) and specific “autistic learning styles” (ALS). Other researchers have speculated that some individuals with a primary diagnosis of ID have autistic symptoms that have a different quality and etiology than “prototypical autism” (Hepburn et al. 2008).
Is there a difference between “ASD with comorbid ID” and “ID with comorbid ASD?” There has also been the question of how to diagnose individuals with such severe ID that it is difficult to determine whether ID or ASD is the primary impairment; however, although these questions may be important for genetic or other biological research, they are unlikely to dictate clinical decisions about assessment or treatment .
Conclusion
It is difficult to specify the relationship between ASD and ID, as both are behaviorally defined disorders with a range of etiologies. ID could result from various genetic disorders or environmental insults to the brain. Likewise, the behaviors attributed to ASD could arise from genetic or environmental factors as well as interactions between the two.
ASD encompass a wide range of ability and disability, from superior cognitive abilities to severe ID. Likewise, there is a range of social motivation and social behaviors seen in those with ID. There is behavioral overlap and frequent comorbidity between the two sets of conditions, and it is important to assess the cognitive functioning of people with ASD as well as the social and adaptive functioning of people with ID.
In many cases, it is important to differentiate whether an individual has a sole diagnosis of either ASD or ID, or comorbid ASD and ID, as this could inform treatment. For example, children with low IQ have poorer prognosis (Itzchak et al. 2008; Sutera et al. 2007), and individuals with comorbid ASD and ID are more likely to display stereotyped, repetitive, and self-injurious behavior than those with a sole ASD or ID diagnosis (Munson et al. 2008). Some studies suggest that a comorbid diagnosis increases the likelihood for additional psychopathology, including mood, anxiety, and sleep problems (Bradley et al. 2004), although other studies have found no differences in the number of psychiatric diagnoses in individuals with ID and those with comorbid autism and ID (Tsakanikos et al. 2006). However, as ASD and ID belong in a spectrum of ability and disability, there will be ambiguous cases, and in these cases it is most important to fully assess and be aware of the presenting behaviors, skills, and deficits, and treat them accordingly.
Autism and Attention Deficit Hyperactivity Disorder (ADHD)
Criteria for Attention Deficit Hyperactivity Disorder (ADHD)
ADHD is a developmental disorder characterized by inattention, hyperactivity, and impulsivity . There are three subtypes specified in the DSM-IV: Predominantly Inattentive Type characterized by difficulty sustaining attention, organizing tasks, and paying attention; Predominantly Hyperactive-Impulsive Type, characterized by fidgeting, excessive physical activity, and difficulty inhibiting impulses; and Combined Type, characterized by inattention and hyperactivity and impulsivity symptoms (APA 2000). According to current diagnostic criteria, the onset of attention symptoms must be present before 7 years of age (APA 2000). The prevalence of ADHD ranges from 2 to 18 % of the population (Rowland et al. 2002). ADHD is frequently comorbid with other disorders, including ODD, Conduct Disorder (CD), ID, depression, anxiety, and ASD (Duric and Elgen 2011; Elia et al. 2009) .
A study by Reiersen et al. (2007) found that 36 % of people diagnosed with ADHD (Combined Type) met the threshold for clinically significant autism symptoms as reported on the Social Responsiveness Scale (SRS). Moreover, 65–80 % of children with ADHD had significant difficulties with social interaction, conversation, and communication, core deficits seen in individuals with autism (e.g., Clark et al. 1999). Furthermore, attention problems are prevalent in individuals with ASD: an estimated 53–78 % of people diagnosed with ASD meet diagnostic criteria for ADHD, and attention problems are frequently present in individuals with ASD even though they may not meet diagnostic criteria for ADHD (Goldstein and Schwebach 2004; Lee and Ousley 2006; Sinzig et al. 2009). However, according to current diagnostic criteria in the DSM-IV, attention symptoms are subsumed under the ASD diagnosis, and a diagnosis of ASD precludes a diagnosis of ADHD (APA 2000), although it is likely that this prohibition will be eliminated in the next DSM. There continues to be debate about the nature of the relationship between ASD and ADHD, whether they should be regarded as comorbid disorders, whether they are discrete disorders with overlapping symptomatology, or disorders that lie on a continuum of symptomatology .
Overlapping Symptomatology and Differential Diagnosis
ASD Symptoms In Individuals with ADHD
Problems with Social Interaction
Shared symptomatology
Mayes et al. (2012) administered the Checklist for Autism Spectrum Disorder (CASD) to informants concerning 847 children with autism and 158 children with ADHD ages 2–16 years old. The autism groups were differentiated by level of functioning (low-functioning autism (LFA) versus high-functioning autism (HFA)) and the ADHD groups were differentiated by ADHD subtype (ADHD combined (ADHD-C) and ADHD inattentive (ADHD-I)). Although the autism and ADHD groups were significantly different on all the items, the items with the most overlap involved problems with social interaction, which included difficulty making friends (100 and 96 %, for the LFA and HFA groups, respectively versus 26 and 13 % for the ADHD-C and ADHD-I groups, respectively), socially indiscriminate or insensitive behavior (75 and 74 % for the LFA and HFA groups, respectively; 39 and 24 % for the ADHD-C and ADHD-I groups, respectively) and problems with social skills (100 and 98 % for the LFA and HFA groups, respectively and 39 and 24 % for the ADHD-C and ADHD-I groups, respectively) .
Differential symptomatology
Questions that differentiated the most strongly ASD and ADHD groups included: limited reciprocal social interaction (88 and 79 % for the LFA and HFA groups versus 5 and 2 % for the ADHD-C and ADHD-I groups) and the descriptor of seeming self-absorbed, or “in [his or her] own world” (90 and 83 % for the LFA and HFA groups versus 1 and 0 % for the ADHD-C and ADHD-I groups). A study by Hartley and Sikora (2009) corroborates some of these findings, but contradicts others. Autism symptomatology was evaluated in 55 children with HFA and 23 children diagnosed with ADHD by interviewing parents using DSM-IV-TR criteria for ASD. They found that the social relatedness domain differentiated the groups; however, while children with ASD had greater impairment in utilizing nonverbal social behaviors (directing facial expressions, use of gesture) and more difficulty developing friendships, parent endorsement of the quality of social and emotional reciprocity did not differentiate the groups . The former finding is consistent with Koyama et al. (2006), while the latter finding is consistent with Geurts et al. (2004). In addition, according to current diagnostic criteria, the onset of social deficits must have been evident before 3 years of age for a diagnosis of ASD, whereas the onset of symptomatology for ADHD must be before 7 years of age (APA 2000). Therefore, in general, individuals with ASDs are more likely to have difficulty in the area of social relatedness, utilize fewer nonverbal communication techniques, and tend to be described as more aloof than their ADHD peers; however, there is conflicting evidence on the ability to display social reciprocity, with some studies suggesting that individuals with ASD struggle more with displaying reciprocal social behaviors than their ADHD peers (Mayes et al. 2012; Koyoma et al. 2006), whereas others suggest that individuals with ASD and ADHD do not differ in the quality of their social and emotional reciprocity (Hartley and Sikora 2009; Geurts et al. 2004) .
Problems with Communication
Shared symptomatology
Like individuals with ASD, individuals with ADHD are more likely to have deficits in pragmatic language, including difficulty beginning and sustaining a conversation and nonverbal communication (e.g., gestures, direction of facial expression; Clark et al. 1999). Furthermore, according to the study by Mayes et al. (2012), delayed speech, while significantly more common in children with ASD, is also more likely to be present in children with ADHD than in the general population (83 and 63 % for the LFA and HFA groups, respectively versus 20 and 9 % for the ADHD-C and ADHD-I groups, respectively).
Differential symptomatology
However, there were factors that differentiated children with ASD and ADHD; children with autism were significantly more likely to display atypical speech or repetitive vocalizations (92 and 86 % for the LFA and HFA groups versus 4 and 0 % for the ADHD-C and ADHD-I groups). This is corroborated by the study by Hartley and Sikora (2009); they found that individuals with ASD were significantly more likely to display stereotyped and idiosyncratic language than their peers with ADHD. Furthermore, Mayes et al. (2012) found that no children with ADHD or typically developing controls were reported to have language regression, whereas 52 % of children with LFA and 20 % of children with HFA were reported to have lost some language after 1 year. Children with HFA were also significantly more limited in their play and imagination activities than their peers with ADHD (Hartley and Sikora 2009; Koyama et al. 2006) .
Repetitive, Restricted Behaviors and Interests
Shared symptomatology, conflicting evidence
There is conflicting evidence as to the degree that restricted, repetitive behaviors and interests are present in individuals with ADHD. Clark et al. (1999) found that 71 % of 49 children with ADHD displayed stereotyped hand or body movements, and some studies have found no significant difference in repetitive movements, nonfunctional routines and rituals , restricted interests, and preoccupation with parts of objects between individuals with ASD and individuals with ADHD (Hartley and Sikora 2009; Koyama et al. 2006). In contrast, in the study by Mayes et al. (2012), significantly more individuals with ASD were reported to have stereotypies (such as hand flapping and walking on their toes) than their ADHD peers (90 and 72 % for the LFA and HFA groups versus 6 and 4 % for the ADHD-C and ADHD-I groups) .
Differential symptomatology
Mayes et al. (2012) concluded that the presence or absence of restricted and repetitive behaviors was useful in differentiating ASD from ADHD, with higher levels of restricted and repetitive behavior and interests consistent with ASD. This included intense restricted interests (94 and 94 % for the LFA and HFA groups versus 1 and 0 % for the ADHD-C, and ADHD-I groups) and repetitive play such as lining up toys (87 and 65 % for the LFA and HFA groups versus 2 and 0 % for the ADHD-C and ADHD-I groups). Furthermore, no children with ADHD were characterized as showing an interest in repetitive movement such as spinning wheels and revolving fans and staring at the end credits of a television show or movie, whereas 51 % of children with LFA and 28 % of children with HFA displayed the behavior . In addition, no children with ADHD were characterized as having “special” skills that were incommensurate with their other abilities (sometimes called “savant” skills) which included characteristics that are associated with restricted repetitive interests, such as memorizing lines from movies and books displaying a comprehensive knowledge of a narrow subject, having an unusually good memory, reading early (before 3 years old) or having exceptional visual-spatial skills (such as a 2-year-old completing a 100-piece puzzle). However, it should be noted that 2 % of typically developing children were also reported to have an interest in objects with repetitive movement and 11 % were characterized as having “special abilities.” In contrast, Hartley and Sikora (2009) concluded that the communication and social relatedness domains were better able to differentiate ASD and ADHD than restricted repetitive behaviors and interests. Future studies should examine whether there are differences between groups when severity and frequency of these behaviors, as well as qualitative features, are examined, rather than whether or not the behavior is present.
Problem Behavior
Children with ASD and children with ADHD are more likely to display problem behaviors than typically developing children. Both groups were more likely to be overreactive, get distressed by change, have meltdowns, and display aggressive behavior than typically developing controls (Mayes et al. 2012). There are also elevated rates of comorbid ODD diagnoses in both groups (Mayes et al. 2012) .
ADHD Symptoms in Individuals with ASD
Studies have shown that it is difficult to differentiate ASD and ADHD by evaluating ADHD symptoms. Mayes et al. (2012) found that, according to parent report on the Pediatric Behavior Scale (PBS), the ASD groups displayed attention symptoms that were indistinguishable from the ADHD-C group. Likewise, other researchers have found similar attention profiles endorsed by individuals or families of children with ADHD and ASD, with significant attention symptoms endorsed by both groups (e.g., Frazier et al. 2001; Sinzig et al. 2009). Furthermore, the mean age of onset for ADHD symptoms (around 3–3.5 years old) was similar in the ASD and ADHD groups, suggesting that the course of ADHD symptom presentation is indistinguishable (Frazier et al. 2001). Sinzig et al. (2009) found that children with ASD fit the diagnostic criteria for the three ADHD subtypes. Parents of 83 children with ASD (ages 5–17) filled out the Diagnostic Checklist for ADHD, a checklist that covers the DSM-IV criteria for ADHD, and found that 53 % of the children met criteria for ADHD. More specifically, 46 % met criteria for the inattentive subtype, 22 % for the hyperactive/impulsive subtype, and 32 % for the combined subtype. The authors suggested that ASD and ADHD should be diagnosable as comorbid disorders .
Attentional processes in autism
Studies have found very abnormal attentional processes in individuals with ASD. The ability to orient appropriately to novel stimuli or to selectively attend to particular sensory stimuli in the face of distraction may be deficient in individuals with autism: this inability to filter out incoming stimuli could lead to behaviors associated with inattention (Belmonte 2000; Posner and Rothbart 2007). Other studies suggest that individuals with autism are more likely to have difficulty shifting attention, such as disengaging from one target onto another target (Courchesne et al. 1994). Pascualvaca et al. (1998) administered the Wisconsin Card Sorting Task (WCST) and an alternative task called the Same-Different Computerized Task, and found that the children did not differ from controls on the latter task, although they performed worse than controls on the WCST. The authors concluded that children with autism have the ability to shift their attention, but have more difficulty doing so if they are already engaged in another activity. However, Goldstein et al. (2001) argue that the deficits in shifting attention may be explained by the cognitive flexibility required for some of the tasks (e.g., WCST), as these skills are deficient in some individuals with ASDs. Goldstein et al. (2001) also found that other differences in attentional processes were diminished when motor speed was used as a covariate. Nevertheless, Corbett and Constantine (2006) found that children with autism have deficits in visual and auditory attention. They administered the Integrated Visual and Auditory (IVA) Continuous Performance Test (CPT) to 15 children with ASD, 15 with ADHD , and 15 typically developing children ages 7–12 and found that children with ASD and ADHD displayed deficits in auditory and visual attention. However, the ADHD group showed significantly greater impairment with auditory stimuli than the ASD group, whereas the ASD group showed significantly greater impairment with visual stimuli than the ADHD group. Furthermore, the ASD group displayed significantly greater difficulty with response control than the ADHD group, although both groups showed impairment relative to the control group. This suggests that children with ASD may have more difficulty with impulse control than children with ADHD. However, this study should be interpreted with caution due to the small sample size.
Individuals with ASD and individuals with ADHD both have difficulty sustaining attention on non-preferred tasks, but do well in sustaining attention on preferred activities. Garretsonm et al. (1990) investigated sustained attention to a non-language continuous performance test in children with ASD and controls matched for nonverbal mental age. They found that under certain circumstances (tangible reinforcement, first few minutes of the task, and slow rate of stimulus presentation), the children with ASD performed as well as their matched controls, but as time went on, their attention waned, and it was particularly impaired when social rather than tangible reinforcers were used and when rapid rate of presentation required more mental effort. In addition, they noted that while in the control group, repetitive (fidgety) behaviors were associated with poorer detection of targets, in ASD group, these behaviors were associated with better performance, suggesting that at least in some children, repetitive behaviors might be automatic and not detract from attention to stimuli .
There is also a tendency in individuals with autism to hyperfocus (Kinsbourne 2011). Some researchers suggest that this hyperfocusing may result from sensory overarousal or instability leading to a narrowing of attentional focus, which in turn leads to the restricted interests and behaviors that are observed in individuals with ASD (such as spinning the wheels of a toy car, staring at dust particles in the light, or an intense interest in toilets, vacuums, or other narrow subject matter; Baron-Cohen et al. 2009; Kinsbourne 1991; Kinsbourne 2011; Liss et al. 2006). while individuals with ASDs have an abnormally narrow focus and tend to concentrate on minute details rather then looking at the bigger picture, they also have difficulty inhibiting attention to irrelevant aspects of stimuli (review by Travers et al. 2011). Individuals with ASD are also less likely to attend to social stimuli and social cues, which may be due to a lack of social motivation or “social inattention” (Dawson et al. 2004; Garretson et al. 1990). Travers et al. (2011) suggested that this “social inattention” may result from general deficits in the ability to input attention and process attentional information: While individuals with ASDs also display a deficit in processing nonsocial information, the dynamic aspects of social interaction make this process even more difficult (for a review of attention in autism, consult Travers et al. 2011) .
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