A description is provided of the way in which Autistic Spectum Disorder and Attention Deficit (Hyperactivity) Disorder may share certain symptoms, or co-exist, with implications for the gathering of accurate epidemiological data as well as for the planning of interventions.  

(Reference is also made to the domain of language usage and understanding as an area where symptoms converge, with Central Auditory Processing Disorder as a condition that may be confounded to some extent with ASD or ADHD.)


M.J.Connor                                                                                              January 2008



ASD and ADHD : Presenting Symptoms


A recent overview of the relationship between ASD and ADHD is provided by Kolevzon (2007) who begins by highlighting the core symptoms of autism and ASD as a matter of social impairment, language impairment, and stereotyped or repetitive behaviours. 


He goes on to note that symptoms of attention deficit, impulsiveness, and hyperactivity are relatively common among individuals diagnosed with ASD, and were acknowledged by Kanner in his original description of the symptomatology associated with autism.

Anomalies in attention that may be observed among individuals with ASD involve either too narrow a focus, or too wide a focus with a vulnerability to all and any distracting stimuli such that it may prove difficult either to engage their attention, to shift attention from some specific object or activity, or rapidly to (re-)orient attention.


It is further argued that attention deficits associated with ASD may contribute to the way in which ASD presents such that, for example, a selective attention to specific stimuli (or component parts of stimuli) can be linked to a repetitive or unusual handling of the objects in question; and a failure to attune to the global stimulus rather than to discrete elements can underlie a failure to recognise the feelings or moods of other people and to pick up all the available social and non-verbal cues that guide inter-personal interactions. 

A fragmented and disorganised perception of the surrounding environment, and the complex range of stimuli from other people and events in, say, a busy classroom, may underlie the growth of anxiety and a tendency to withdraw from group settings into an individual and more controllable world. 


The current edition of the Diagnostic and Statistical Manual (DSM-IV) holds that a diagnosis of ASD precludes a comorbid diagnosis of ADHD; nevertheless, the advice from Kolevzon is that one should assess for symptoms of ADHD in all individuals meeting the diagnostic criteria for ASD, and a full diagnosis (as a basis for planning intervention) requires a comprehensive developmental and familial history. 


Thus, despite the existence of ASD as an exclusionary criterion for identifying ADHD, it is increasingly common for professionals to perceive inattention or impulsiveness or hyperactivity as a likely set of comorbid features; and clinical trials have demonstrated that medication can be effective in managing the ADHD symptoms among children who have a diagnosis of ASD (with the corollary that applying this exclusionary criterion will deny some children the potential benefits of medication as part of the intervention package). 


On the other hand, it is recognised that ASD and ADHD have different trajectories, outcomes, and treatment “protocols”.  For example, there is well-established and converging evidence (going back to Campbell and Cohen 1978) that the effects of psycho-stimulants among children with ASD are less reliable than among children with ADHD alone.  It is also suggested that too great an emphasis upon ADHD could lead to unreasonable expectations for an overall improvement in the presenting symptoms and behaviour of children validly identified with ASD, while any reliance upon medication may inhibit the establishment of behavioural and educational programmes within the overall intervention.


Meanwhile, and perhaps inevitably, a combination of ASD and ADHD will bring about increased impairment in day to day activities; but, because DSM-IV treats the two conditions as separate, there is limited information about the extent or nature of overlap. 

However, according to the survey of Lecavalier (2006), more than half of a sample of children and adolescents with ASD showed moderate to severe symptoms of inattention and hyperactivity; and a large scale survey in the UK (Fombonne et al 2001) had indicated that individuals with ASD have a significantly greater probability of showing high levels of hyperactivity than individuals not so diagnosed. 


The mechanisms involved are not yet clear, but it is speculated that a sub-sample of those people who are diagnosed with ASD have inherent problems of inattention or impulsiveness or hyperactivity; or that anther sub-sample have comorbid ASD and ADHD.  Either view is reinforced by findings from neuro-imaging studies that both conditions involve some anomalies in the functioning of the frontostriatal or cerebellar regions of the brain. 


Reference is made to the study of Roeyers et al (1998) which used retrospective parental reports to attempt to highlight differences between pervasive developmental disorder (autistic spectrum)  and ADHD in early childhood. 

The former group more frequently showed symptoms matching the core ASD characteristics, such as a lack of response to social approaches, poor peer interaction, and a lack of symbolic play. Motor tics, behaviour problems, and anxiety were also more frequently observed.

The latter group were more likely to have shown signs of distress, such as loud crying in the early days, hyperactive behaviour, and reckless behaviour.

Concerns about the developmental progress of the PDD children were evoked significantly earlier than concerns about the children with ADHD; although marked differences in behaviour (greater hyperactivity among the ADHD group) during the period between 7 and 12 months of age tended to lessen as the PDD children gradually increased their activity level.


A further diagnostic overlap may arise in that many children with ADHD have difficulty in relating to others such that problems of peer interaction and reciprocity, considered a significant element of ASD, may develop in the ADHD children because of these social difficulties combined with the lack of sustained attention. 


With regard to the usefulness of medication, Kolevzon’s review of relevant data suggests that the use of a psycho-stimulant (methylphenidate) can be effective in reducing the ADHD-related symptoms in children with ASD, although the benefits in this particular group may be less consistent, and more associated with side effects, than the gains observed among children with ADHD but not ASD. 

(Evidence is cited that atomoxetine is better tolerated than methylphenidate.)


Risperidone has been found to be effective in managing hyperactivity among children with ASD, and is approved for the management of irritability reflected in aggressive behaviour, temper tantrums, or self harm among children and adolescents. 


The likely benefits of serotonergic medication are described as less clear or convincing in that it can bring about some reduction in repetitive behaviours or anxiety and aggressiveness in some cases, but increased hyperactivity appears to be a consistent risk. 


The implication set out by Kolevzon is that, when medication is considered as a potential means of managing specifically-targeted symptoms, the dosage should be initiated at a very low level and gradually increased, and that short or long term benefits must be balanced against risks and side effects.


In his conclusion concerning medications, the author acknowledges that the review has some weaknesses in that the emphasis is upon children and adolescents, but that less is known about the effects of psychotropic prescriptions among these younger people than among adults, and the level of efficacy and tolerability may differ markedly between the two groups. 

Further, while the studies reviewed have largely concerned randomized and controlled trials, the actual methodologies have been little discussed, and it is recognised that controlled trials may still have weaknesses (and that helpful information may emerge from alternative methodologies including single case studies).


ASD Symptoms in Twins with ADHD


The paper by Reiersen et al (2007) also emphasises the DSM-IV diagnostic criteria in not allowing a diagnosis of ADHD if the symptoms are associated with a pervasive developmental disorder. 

Nevertheless, they argue, converging findings from clinic-based studies (such as that of Goldstein and Schwebach 2004) indicate that symptoms consistent with ADHD are present in a significant proportion of children with a PDD ... anything between 50+ and 75%.  (Similarly, symptoms characteristic of ASD have been observed in children who meet the criteria for ADHD.)


Further, genetic studies provide evidence for an overlap between autism and ADHD suggesting that some genes may exert an influence in the aetiology of both conditions ... although it is recognised that some of the putative overlap may actually be reflective of inappropriate diagnostic classification.


An epidemiological study of 219 pairs of male twins (Constantino et al 2003) identified an association between scores on the attention scale of the Child Behaviour Checklist and total scores on the Social Responsiveness Scale  (a rating scale completed by parents and teachers to assess autistic traits).  While it was reported that the genetic factors influencing the attention problems and the social responsiveness could be separated, it was still the case that reciprocal interactions could underlie the relationship between the two measures. 


One problem is that most of the studies reporting the co-occurrence of ADHD and ASD have been based upon small and clinic-based samples which could involve some sampling bias; and the corresponding need is to examine samples of children drawn from the population as a whole to determine if there is a general trend towards the clustering of ADHD and ASD symptoms. 

If this trend were to be observed, it would have implications for the generalisability of findings of studies based upon the exclusionary criterion, and for the practical issues of how to organise assessment and to plan intervention.


Accordingly, the present authors set out to examine for overlap of ASD and ADHD in a population-based twin sample, with the predictions that children with ADHD would have an enhanced level of ASD traits (as measured by the SRS) and that SRS scores would differ among children showing different ADHD subtypes.


Twin pairs (N=473) were selected for the study if at least one of the children had been rated by parents as showing at least 3 indicators of attentional problems either currently or in the past. 

Assessment measures included the CBCL and the SRS, plus a semi-structured interview with parents to gather information about ADHD symptoms thus to determine ADHD subtype. .

(As controls, the authors randomly selected 183 twin pairs, and included 104 twin pairs selected for a high score on the CBCL anxiety and withdrawal scale.)


The results indicated that the mean social responsiveness scores for children with the predominantly inattentive and combined subtypes of ADHD were significantly higher than for children who did not meet the criteria for ADHD. 

The authors held that such a finding added to existing data in demonstrating that autistic traits (quantitatively assessed via the SRS) are more evident in children with ADHD drawn from a non-clinical population, and that different ADHD subtypes are associated with markedly different levels of social impairment.


It is possible to cite converging findings that children with attention deficits and or hyperactivity show increased rates of impairment corresponding to the triad of core characteristics of autism and ASD (social and interactional problems, communication impairments, and repetitive/stereotyped behaviours); but the current findings indicate that children with the combined form of ADHD have the highest levels of symptoms in these three autistic domains.


On the other hand, the authors accept that it is still not clear whether poor scores in the social responsive measure indicate true autistic traits or a more general social weakness.  Nevertheless, after introducing controls into the analysis for the CBCL scales of “social problems” and “social competency” which are largely independent of autistic symptoms, the basic results were not altered, suggesting that the low SRS scores cannot be explained simply by non-autistic social deficits. 


In respect of weaknesses in this current study, it was noted that rating scales were only completed by a sub-sample of the originally identified parents, with the possibility of some response bias according to socioeconomic status or IQ or age. 

It was further acknowledged that this twin-sample may not be representative of children who are not part of twin pairs in terms of the rate of attention problems or of autistic traits or of perinatal factors which could have some aetiological influence.


(It has, for example, been noted in a study by Ho et al (2005) that among boys, but not girls, social responsiveness scores are poorer in twins than in non-twins.)


In their summary and conclusion, Reiersen et al highlighted the evidence for an association between ADHD and autistic symptoms in their population-based twin sample. The strongest evidence for this association was gained among children showing the combined subtype of ADHD; and, among girls with ADHD, there appeared to be a more enhanced probability of symptoms of social impairment. 


(It may be speculated – MJC – that this latter finding about girls could be a function of the more uncommon findings of marked and observable [combined form] ADHD symptoms among girls than among boys with possible implications for some differences in they way they are perceived by peers and in peer reactions, with these differences underlying at least some of the observed social anomalies ??)


In any event, the authors argue that the finding that around 30% of boys and around 70% of girls showing the combined form of ADHD at a severe level also met clinical criteria for some autistic symptomatology suggests a gene association and genetic linkage. 

Alternative explanations may include environmental influences upon both conditions, or measurement overlap because of inaccurate diagnostic tools; but their conclusion holds that it is important to consider the existence of both ADHD symptoms and autistic features when studying either condition ... especially given their experience that managing a combination of ADHD and ASD symptoms is more challenging than managing ADHD alone.


Central Auditory Processing Disorder – A Further Area of Overlap    


Deficits in communication, notably in fluent and meaningful two-way exchanges, are part of the core characteristics of autism. 

However, symptoms involving some language and communication impairment may be indicative of other conditions, including a central auditory processing disorder (CAPD).


The guidelines produced by Kutscher (2007) describe central auditory processing as a matter of interpreting and organising “unrefined” auditory material.  It is more than hearing, and involves making sense of what is heard.

CAPD is not some single entity but a collective name for various problems that can occur in any permutation, and which may be experienced in around 2 or 3%  of the population (with boys affected more commonly than girls). 


The specific components of auditory processing include discrimination (such as differentiating between words that sound quite similar); localisation (identifying the precise source of the sounds and determining where to direct attention); auditory attention (sustaining a focus upon the stimuli); figure-ground discrimination

(separating the significant input – such as the teacher’s voice – against background noise in the classroom); closure (the capacity to understand the whole of a word or a message even if part of it is missed); synthesis (combining sounds to form whole words); association (attaching a meaning to the sounds/speech); and auditory memory (short or long term storage of what is heard as a basis for immediate or subsequent response).


What matters is that some delay in responding or in offering a response that is unrelated to the stimulus question or remark (or not to respond at all) can impair or prematurely end an exchange, with implications for social interaction and acceptance, and for the development of anxiety.  Thus, the impacts of CAPD may be similar to those of symptoms of ASD or of ADHD, and the conditions may be confounded (or overlap).


Kutscher describes how CAPD and ADHD can be differentiated.

For example, CAPD effects are specific to listening and language demands, with background noise having a “scrambling” impact upon the information, and with comprehension problems still possible after the establishment of attention, albeit with no impact upon executive functioning once understanding is achieved. Acting-out behaviours are unusual.  

ADHD effects may arise across a range of types of stimuli, with background noise increasing the difficulty of attending; but comprehension is usually adequate once attention has been established even if this is not necessarily translated into appropriate executive functioning. Hyperactive and over-reactive, even apparently aggressive, behaviour may be noted.


CAPD and ASD share the characteristics of sensitivity to noise, a difficulty in focusing attention upon the appropriate person, problems in following lengthy and multi-step instructions or in sharing a conversation (pragmatics), and difficulty in sustaining attention (to imposed tasks at least). 


One implication that the present writer - MJC - would draw from this is for some danger in the use of labels since they tend to be unitary and may deflect attention away from the possibility of “multiple aetiology” (the co-existence of a number of areas of difficulty).  Meanwhile, any label can provide only a general indicator of the area(s) of concern which, albeit a helpful starting point, still requires following up with an exposition of the individual profile of strengths and weaknesses, and of the precise circumstances where the problems with be most evident, thus to plan how, specifically, to prioritise and deal with observed difficulties.



                                    *          *          *          *          *          *


M.J.Connor                                                                                              January 2008





Campbell M. and Cohen I.  1978   Treatment of infantile autism.  Comprehensive Therapy  4  33-37


Constantino J., Hudziak J., and Todd R.   2003   Deficits in reciprocal social behaviour in male twins; evidence for a genetically independent domain of psychopathology.   Journal of the American Academy of Child and Adolescent Psychiatry  42  458-467


Fombonne E., Simmons H., Ford T., Meltzer H., and Goodman R.  2001   Prevalence of pervasive developmental disorders in the British nationwide survey of child mental health.   Journal of the American Academy of Child and Adolescent Psychiatry  40  820-827


Goldstein S. and Schwebach A.  2004   The comorbidity of pervasive developmental disorder and attention deficit hyperactivity disorder.   Journal of Autism and Developmental Disorders  34  329-339


Ho A., Todd R., and Constantino J.  2005   Brief report – autistic traits in twins vs non-twins.   Journal of Autism and Developmental Disorders  35  129-133    


Kanner L.  1943   Autistic disturbances of affective contact.   Nervous Child  2  217-250  


Kolevson A.  2007   Helping the hyperactive child  : when autism looks like ADHD.  Medscape Learning Activity.  May 29th  2007


Kutscher M.  2007   Central Auditory Processing Disorders.  SEN Issues  29  46-50


Lecavalier L.  2006   Behavioural and emotional problems in young people with pervasive developmental disorders.   Journal of Autism and Developmental Disorders  36  1101-1114


Reiersen A., Constantino J., Volk H., and Todd R.  2007   Autistic traits in a population-based ADHD twin sample.   Journal of Child Psychology and Psychiatry 48(5)  464-472


Roeyers H., Keymeulen H., and Buysse A.  1998   Differentiating ADHD from pervasive developmental disorder not otherwise specified.   Journal of Learning Disabilities  31  565-571



Further Reading in this Series


Central Auditory Processing Disorder  (July 2000)



This article is reproduced by kind permission of the author.

© Mike Connor 2008.

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