**4. Evidence of symptom overlap – ASD and ADHD**

equivalent ASD symptoms (Russell, Steer, & Golding, 2011). This may be because the disor‐

Because ASD, Dyslexia and ADHD are all behaviorally defined, so 'symptoms' are behaviours. All three conditions are conceived as particular behaviours along a spectrum, where traits have a continuous distribution and extend into the general (non-disordered) population. An arbitra‐ ry cut off point determines who is considered to be within the various categories and who is not. The clinician giving a diagnosis will be responsible for judging where this cut off may come, guided by diagnostic criteria and standards within disciplines as well as perceived im‐ plications: the benfits versus any possible risks of assigning a diagnosis. This is perhaps best es‐ tablished for autism: Constantino and Todd (2003) measured autistic traits in a large community sample, and found no jump in the threshold of autistic behaviours between 'nor‐ mal' individuals and those with an autism spectrum diagnosis, rather they found a continuous distribution. These findings concurred with those in a Scandinavian study (Posserud, Lunder‐ vold, & Gillberg, 2006). One of our own studies has likewise shown that autistic traits do ex‐ tend into the 'subclinical' population (Figure 1). As with dyslexia and ADHD, there is not a sharp line separating severity in those with a diagnosis from less severe traits in those without (London, 2007). In both dyslexia, ADHD and the autism spectrum, some children have more severe difficulties than others, and the symptoms extend into the population of children (and adults) as a whole. For dyslexia, there are many people who may have mild dyslexic difficul‐ ties but perhaps might not qualify as 'dyslexic'. For autism spectrum disorders, many people without an autism diagnosis do have autistic-type behaviours but the severity and frequency of those behavioural symptoms is less severe than in those deemed to qualify for a diagnosis.

**Figure 1.** The distribution of an ASD composite trait in the general population from Russell et al.(2012)

ders tend to be conceptualized as 'male' leading to referral bias.

364 Recent Advances in Autism Spectrum Disorders - Volume I

Various studies have looked for ADHD or ADHD symptoms in samples of children with autism or ASD. Rates of ADHD have ranged from 28% to 78% of these samples (Ronald, Edelson, Asherson, & Saudino, 2010). Studies that look at ADHD symptoms have reported even higher numbers: for example, Sturm, Fernell, & Gillberg, (2004) looked at a sample of around 100 high functioning children with ASD and found 95% had attention problems, 75% had motor difficulties, 86% had problems with regulation of activity level, and 50% had impulsiveness. About three-quarters had symptoms compatible with mild or severe ADHD, or had deficits in attention, motor control, and perception, indicating a considerable overlap between these disorders and high-functioning ASD in children.

In an large analysis of nine hundred forty-six twins, Reierson and colleagues (2008) assigned DSM-IV ADHD diagnoses, and measured autistic traits using the Social Responsiveness Scale. The study showed that there are clinically significant elevations of autistic traits in children meeting diagnostic criteria for ADHD. These findings confirm results in earlier studies (Clark, Feehan, Tinline, & Vostanis, 1999). Santosh and Mijoovic (2004) which found children with ADHD had elevated levels of impairment in all three autistic symptom do‐ mains, namely social deficits, communication and stereotyped behaviors. Clark *et al* found 65-80% of parents of children with ADHD reported difficulties in social interaction (particu‐ larly in empathy and peer relationships) and in communication (particularly in imagination, and maintaining conversation). So the presence of autistic traits in children with ADHD ap‐ pears common (Ronald et al., 2010).

In an analysis conducted with Lauren Rodgers at the Peninsula Medical School in the UK using data from the Millennium Cohort Study, a cohort of around 19,000 children who were all born between 2000 and 2002, we noted 44 children had a dual diagnosis of both ASD and ADHD (proportion of total population 0.3%) by age seven. The prevalence of children with identified ADHD in the ASD sample was 17%. Conversely, the prevalence of children with ASD in the ADHD sample was higher at 27%. Both figures indicate substantial overlap be‐ tween these conditions.

Various European research groups have examined co-morbid disorders in adults with diag‐ nosed ASD. An international team lead by Hofvander studied a group of 122 adults with normal IQ from specialist clinics in three European cities: Gothenburg, Paris and Malmö (Hofvander et al., 2009). Here the overwhelming majority had symptoms of ASD. Nonverbal communication problems were also very common, described in 89% of all their subjects. In this study over half the participants, (52%) were diagnosed with co-morbid ADHD. Interest‐ ingly, participants diagnosed with pervasive developmental disorder. 'Not Otherwise Speci‐ fied' (PDD-NOS) diagnosis had significantly more symptoms of inattention and hyperactivity/impulsivity compared to subjects diagnosed with Asperger's syndrome. How‐ ever, the prevalence of the categorical diagnosis of ADHD did not differ significantly be‐ tween the groups, nor were gender differences apparent. Although the study presents clear evidence of many cases where patients display symptoms of both ADHD and ASD, the clin‐ ical setting may have led to selection bias as patients with complex needs may be more like‐ ly to seek help.

diction. Gillberg has argued that co-existence of disorders is the rule rather than the excep‐ tion in child psychiatry and developmental medicine. He has coined the acronym ESSENCE

Co-Occurrence of Developmental Disorders: Children Who Share Symptoms of Autism, Dyslexia and Attention Deficit

Hyperactivity Disorder

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http://dx.doi.org/10.5772/54159

Neurodevelopmental Clinical Examinations). This describes cases where a combination of symptoms including inattention, hyperactivity, social and reading difficulties are observed. Major problems in at least one ESSENCE domain before age 5 years often signal major prob‐

To summarize, although ADHD and ASD are separate and recognizable, there is good evi‐

There is only a small literature on the overlap in symptomology between autism spectrum disorders with those of dyslexia. Officially, as for ADHD, ASD is an exclusionary criterion for diagnosis of dyslexia and vice versa, but ASD also shows overlap with dyslexia in both cognitive and behavioural features (Reiersen & Todd, 2008, Simonoff et al., 2008). A propor‐

The number of children that do share symptoms of ASD and dyslexia is likely to be small (Wright, Conlon, Wright, & Dyck, 2011). The frequency of reading disorder in combination with disorder of written expression (i.e. dyslexia) was around 14% in a sample of adults with Asperger's Syndrome (AS) so according to this result around one in seven individuals with AS will have co-occurring dyslexia (Hofvander et al., 2009). However the proportion of individuals with dyslexia who have co-occuring AS is likely to be low as Asperger's Syn‐

A common problem for children with dyslexia is misinterpretation of spoken language, which can also manifest itself in comprehension. This produces further overlap with prag‐ matic language impairment (PLI) which itself is virtually indistinguishable from communi‐ cation difficulties associated with high functioning autism. Pragmatic language difficulties may involve literal interpretation so 'run on the spot' would have a child looking for a big black spot to run on, for example. Children with PLI will often fail to interpret the core meaning or saliency of events. This causes a penchant for routine and 'sameness' (also seen in autism and Asperger's Syndrome) as PLI children struggle to generalize and take hold of the meaning of novel situations. Obvious and concrete instructions are clearly understood and carried out, whereas simple but non-literal expressions such as jokes, sarcasm and gen‐ eral social chatting are difficult and may be misinterpretated. PLI may therefore impact on the social abilities of the child who has difficulty interpreting jokes. Current thinking is that PLI is not a problem rooted in language skills but one of social communication and informa‐ tion processing. Griffiths (2007) identified difficulties of this type in dyslexic students, showed they were impaired in making inferences from a story and choosing the right punch-line for a joke. This of course can have implications for written language and exami‐

dence that these conditions co-occur, constituting an amalgam of problems.

tion of children share symptoms between dyslexia, ADHD *and* ASD.

nations under stress, as well as for a range of social interactions.

(referring to Early Symptomatic Syndromes Eliciting

lems in the same or overlapping domains years later.

**5. Comorbidity between dyslexia and ASD**

drome is much a rarer condition than dyslexia.

Because behaviours associated with both conditions lie on a spectrum extending into the normal range, some studies have found a range of frequency and severity of symptoms. In Mulligan et al.'s (2009) study, for example, 75 of children with ADHD had severe autism traits, and over half showed sub-clinical autism symptoms. Kadesjö and colleagues (Kades‐ jö, Gillberg, & Hagberg, 1999), looked at comorbidity of ADHD in Swedish school-age chil‐ dren and found only 1% of children meeting the threshold for ADHD had comorbid Aspergers Syndrome (AS). The estimates of co-morbidity of ADHD symptoms with ASD symptoms vary widely because of differing methods of case ascertainment. An additional problem is that the estimate of the prevalence of ASD itself has increased so much in west‐ ern countries, making ASD itself a 'moving target' (Figure 2).

Patricia Howlin (2000) reviewed the estimated rates of co-existing psychiatric disorders in subjects with high functioning ASD and found these estimates varied from 9% to 89% - very substantial differences. However it is possible to generalise; thirty years of research have confirmed that attention deficits and hyperactivity are relatively common in children and adults with ASD even if the exact extent of overlap is dependent on methodology and ascer‐ tainment (Hofvander et al., 2009, Sturm, Fernell, & Gillberg, 2004).

**Figure 2.** The rising prevalence of autism spectrum disorders over 50 years. (Data from 'Autism Speaks' and CDC, USA)

Recent trends have made categorical diagnosis an integral part of everyday clinical and re‐ search practice (Sonuga-Barke & Halperin, 2010). Christopher Gillberg (2010) points out that clinicians have become focused on dichotomous categories of disorder and that clinics have become increasingly specialized and overlook difficulties not within their immediate juris‐ diction. Gillberg has argued that co-existence of disorders is the rule rather than the excep‐ tion in child psychiatry and developmental medicine. He has coined the acronym ESSENCE (referring to Early Symptomatic Syndromes Eliciting

Neurodevelopmental Clinical Examinations). This describes cases where a combination of symptoms including inattention, hyperactivity, social and reading difficulties are observed. Major problems in at least one ESSENCE domain before age 5 years often signal major prob‐ lems in the same or overlapping domains years later.

To summarize, although ADHD and ASD are separate and recognizable, there is good evi‐ dence that these conditions co-occur, constituting an amalgam of problems.
