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o morbidity of attention deficit hyperactivity disorder (ADHD) and reading disabilities (RD) is greater than what would occur by chance. Considering the well-documented adverse impact of both ADHD and RD on development, the presence of both conditions may lead to particularly poor outcomes for affected people (Miranda et al., 2011).

Research estimates the comorbidity of RD in children with ADHD between approximately 20–40% (Del’Homme, et al., 2007; Semrud-Clikeman, et al., 1992). The comorbidity of ADHD in the RD population is estimated between 26–50% (De Jong, et al., 2009).

Attention-deficit/hyperactivity disorder (ADHD) is currently defined as a cognitive developmental disorder where all clinical criteria are behavioral. Overactivity, impulsiveness, and inattentiveness are presently regarded as the main clinical symptoms (Sagvolden, 2011).

Developmental dyslexia is linked with deficits in a number of functions including phonology, perception (visual and auditory), attention and memory (Pavlidou and Williams, 2010)

Although both disorders are diagnosed in different ways (ADHD by parent reports; RD by reading tests), they share some behavioral symptoms like inattentive behavior and academic difficulties (Hinshaw, 1992).

This makes differential diagnosis difficult and urges research into cognitive and neurobiological variables that might better distinguish between both disorders (Rashid et al., 2001).

There are a number of competing explanations of the comorbidity between dyslexia and ADHD. In one of the first papers to address this issue, Pennington et al. (1993) proposed that the symptoms of ADHD associated with dyslexia are a secondary consequence of reading problems (‘phenocopy’ hypothesis), other studies reported that comorbid dyslexia and ADHD is associated with a combination of the cognitive impairments seen in dyslexia and ADHD alone (Adams and Snowling, 2001; Raberger and Wimmer, 2003; Willcutt et al., 2005). A different view of the aetiology of comorbid dyslexia and ADHD is that the condition arises from shared genetic risk factors that contribute to the development of separate cognitive impairments that underlie the two disorders. In this view, those same genetic risk factors (acting in concert with other genetic and environmental risk factors) may lead to the development of both underlying cognitive impairments, in turn producing the comorbid condition (‘shared aetiology’ hypothesis; de Jong, et al., 2006).

 

Finally, it has occasionally been argued that comorbid dyslexia and ADHD may reflect different causal mechanisms from those operating in either condition alone (‘cognitive subtype’ hypothesis, de Jong et al., 2006).

Based on what is mentioned above, the main aims of our study were to investigate the neuropsychological functions in ADHD only, SLD only and comorbid ADHD and SLD, we tried to search for any neuropsychological dysfunctions that could be characterized to each group and the main effect of each group on the neuropsychological functions.

Accordingly 90 child aged from 7-13 years old altogether with their both parents were included if having a diagnosis of ADHD, SLD or both in the outpatient clinic, IQ above 90 and without any neurological problems, both sex were included.

The tools used in the study fell into several categories

  • Tools to assess intelligence:
  • Wechsler Intelligence Scale for Children (WISC) Arabic version (Ismail and Melekia, 1999).
  • Tools for diagnosis
  1. The Kiddie Schedule for Affective Disorders and Schizophrenia, Present and Lifetime versions (K-SADS-PL) (Ibrahim et al., 2000), was used for diagnosing ADHD.
  2. El Ziat Scale for diagnosis of specific learning disability and assess severity (El Ziat, 2007).
  • Tools to assess severity of ADHD symptoms
  • Conner's Parent Rating Scale –revised –long version (CPRS-L) (El Seikh et al., 2003).
  • Fahmy and El-Sherbini 's Social Classification Scale, to classify the social class of the subjects (Fahmy and El-Sherbini, 1986).
  • Executive functions battery:
  • We choose our battery from The Psychology Experiment Building Language (PEBL). The software is freely downloadable (http://pebl.sourceforge.net) (Mueller, 2010b).
  • From these tests we chose the following tests:
    1. PEBL Wisconsin (Berg) Card Sort Test:
    2. PEBL Trail making test:
    3. PEBL Stroop Color and Word test:
    4. PEBL Tower of London:
    5. PEBL The Conners’ Continuous Performance Test:
    6. PEBL Match to sample task:
  • Verbal working memory – Digit span test (Wechsler, 1991)

The procedural protocol for conducting this study involved referral of ADHD children or SLD children or both fulfilling the inclusion criteria for psychiatric interview. An informed consent was taken then followed by assessment of IQ using Wechsler Intelligence Scale to make sure IQ above 90. Then Conner's parent rating scale was applied to assess symptom severity, these two scales were done by the clinical psychologist. This was followed by applying the KSADS-PL to confirm diagnosis of ADHD, determine which type, El Ziat Scale for diagnosis of specific learning disability and assess severity, Fahmy and El-Sherbini 's Social Classification Scale to determine social class, then Executive functions battery was applied to children.

For the parents full family history of ADHD or SLD or both was verified and also, history of delayed language development was verified.

The results of the study were obtained using the statistical package of the SPSS version 16. The statistical process performed were the Pearson chi square test, the student’s t-test, the paired t- test, multiple logistic regression analysis and estimation of the Effect size test using Cohen's d.

 

The main findings in the Study Were

  • As regards the demographic data,
  • The present study revealed that 70% of the sample was males and 29.17 % were females which go with abundant research showing ADHD to have male predominance. With the mean age of the sample in the study was (10.18± 1.46) years
  • Prevalence of ADHD subtypes:
  • Our study revealed the prevalence of ADHD combined type was 63.3%, ADHD inattentive type was 31.7%, ADHD hyperactive – impulsive type was 5.0% in both ADHD group and ADHD +SLD group.
  • Within ADHD group, the prevalence of ADHD combined type was 53.0%, ADHD inattentive type was 37.0%, ADHD hyperactive - impulsive type was 10.0%.
  • Within ADHD+SLD group, the prevalence of ADHD combined type was73.0%, ADHD inattentive type was 27.0, ADHD hyperactive - impulsive type was 0%.
  • Which is in line with many studies that revealed that ADHD combined type is the most common type.
  • Prevalence of SLD types:
  • Our study revealed that prevalence of dyslexia only was 15.0%, dyscalculia only was 3.3%, Dysgraphia 0.0 %, and the combined Dyslexia, Dyscalculia, and Dysgraphia were 55.0%, combined Dyslexia and Dysgraphia were 21.7%, combined Dyslexia and Dyscalculia were 1.7%, combined Dyscalculia and Dysgraphia were3.3% in both SLD group & ADHD + SLD group
  • Within SLD group, the prevalence of Dyslexia only was 27.0%, dyscalculia only was 0.0%, Dysgraphia only was 0.0 %, and the combined Dyslexia, Dyscalculia, and Dysgraphia were 27.0 %, combined Dyslexia and Dysgraphia were 40.0 %, combined Dyslexia and Dyscalculia were 3.3 %, combined Dyscalculia and Dysgraphia were 3.3 %.
  • Within comorbid ADHD and SLD group, the prevalence of Dyslexia only was 3.3%, dyscalculia only was 6.7%,Dysgraphia 0.0 %, and the combined Dyslexia, Dyscalculia, and Dysgraphia were 77.0 %, combined Dyslexia and Dysgraphia were 10.0 %, combined Dyslexia and Dyscalculia were 0.0 %, combined Dyscalculia and Dysgraphia were 3.3%.
  • Which in line with many studies that revealed that combined dyslexia is more frequent than pure dyslexia, and high co morbidity between dyslexia and ADHD.
  • Risk factors for ADHD, SLD, and ADHD+SLD:
  • Our study revealed that history of delayed language development is a risk factor for presence of SLD in children.
  • Wechsler Intelligence Scale for Children (WISC): Arabic version (Wechsler, 1991):
  • Although total IQ, verbal IQ, and performance IQ were all in the average and above average range, but there was statistically significant differences between cases and control as regards verbal IQ, performance IQ, and Total IQ. This could explain the bad academic performance in children with ADHD, SLD or both.
  • In VIQ; the least one was SLD group followed by comorbid ADHD and SLD group followed by ADHD group but in PIQ and TIQ, the least one was comorbid ADHD and SLD group, followed by SLD group, followed by ADHD group.
  • As regards the executive functions tests:
  • Our study revealed that all three groups (ADHD, SLD, and comorbid ADHD and SLD) had significant impairment in all executive functions tests; (working memory (verbal and visuospatial), Cognitive flexibility and set shifting (WCST, TMT, and Stroop color and word test), Planning (TOL), and response inhibition (CPT).
  • The least one was comorbid ADHD and SLD group, followed by ADHD group, followed by SLD.

 

  • As regard to weighted effect size:
  • The mean weighted effect size for ADHD group versus SLD group was small weighted effect size(0.36),
  • The mean weighted effect size for ADHD group versus comorbid ADHD and SLD group was moderate weighted effect size(0.66),
  • The mean weighted effect size for ADHD versus control was large weighted effect size (0.9).
  • Our study revealed that there is no difference between ADHD subtypes as regards neuropsychological functions.
  • Our study revealed that gender differences in executive functions were found in three tests: verbal short term memory, females were better than males, and males were more impulsive than females and this was apparent in response time in visuospatial working memory and in commission errors in CPT.
  • On correlation between age and executive functions;
  • Our study revealed that, the age is significantly negatively correlated with the mean response time in visuospatial working memory, with unique errors in WCST, and Trial A mean total time in Trail making test,, and mean time in And is positively correlated with the mean accuracy rate Trail A& B in Trail making test. We concluded from this that executive functions improve with increasing age.
  • On correlation between Verbal, Performance, Total IQ and executive functions:
  • Our study revealed few significant correlations between Wechsler IQ (Verbal, Performance, and Total) and executive functions. Verbal IQ and Total IQ are significantly correlated with Verbal working memory (Digit backward). Performance IQ and Total IQ are significantly correlated with Visuospatial working memory (No of correct trails). Performance IQ only is positively correlated with Trail making test (Trail B accuracy rate), and negatively correlated with CPT (Foil accuracy rate).
  • We concluded from that traditional intelligence tests do not appropriately evaluate executive functions and we can't depend upon them only on assessment of executive function.
  • From the above results, we concluded that ADHD and SLD had both shared etiological hypothesis and both had underlying cognitive impairment that lead to executive dysfunctions, and when ADHD comorbid with SLD, they carry poor prognosis and more cognitive impairment than each one alone.

 

  • At the end, we suggest some points,
  • Any ADHD case or SLD case could be well investigated to exclude the presence of other disorder.
  • History of delayed language development is a predictive for presence of SLD and early management of delayed language disorder in children can help us in avoiding SLD in the future.
  • IQ alone isn't predictive of child's intelligence or academic achievement, examination of executive functions is very important in children with ADHD, SLD, or both.
  • Assessment of executive functions in children with ADHD or SLD is important and could help us in assessment of severity of the case, in follow up of the case.