Executive functions in children with Tourette Syndrome : the confounding effects of comorbid attention deficit hyperactivity disorder

Abstract

Tourette syndrome (TS) is characterised by semi-involuntary motor and vocal tics. It is also accompanied by a spectrum of behavioural, emotional and cognitive features. However, pure TS(TS-only) is relatively rare as it more frequently co-occurs with Attention Deficit Hyperactivity Disorder (AD/HD).The proposed core psychological impairment in AD/HD is that of behavioural inhibition, a component of executive functions. underlying psychological impairments in TS, however, are less well understood. Both disorders have been proposed to result from dysfunction of the fronto-stratal circuitry and the dysfunction of the dopaminergic system which mediates it. Adequate executive functioning requires that this circuitry functions appropriately. In this thesis the executive function impairments in children with TS and confounding effects of comorbid AD/HD were examined. Clinical groups were compared to a matched normal control group. The effects of medication were also examined. The study was based on the theoretical framework of Baddeley's model of working memory. Deficits in executive functions were hypothesised to result from non-optimal functioning of the central executive of working memory. Several executive function task were used ( The Tail Making Test, Tower of London, Stroop to Response Test). Although each task assessed multiple aspects of executive functioning, one common feature of each of the tasks was that is required to ability to efficiently allocate attentional resources to the predominant activity and inhibit a competing impulse to carry out a more dominant action The profiles of behavioural, emotional and tic problems were also obtrained by means of parent, teacher and child rating scales (the Achenback Child Behaviour Checklist, Conners' Parent Rating Scales - Revised, Yale Tic Severity Scales, and Movement, Obsessions, Vocal Evaluation Scales). This was intended to allow a confirmation of the diagnoses of TS and AD/HD, and also to obtain an indication of functional impairments. It was hypothesised that compared to controls, children with TS and/or AD/HD would obtain significalty higher rating on measures of behavioural problems and demonstrate impairments on these aspects of executive function tasks for which cognitive demans would exceed the processing capacity of the central executive of working memeory. The participants wer 132 children aged 7 to 16-years (M=10.64, SD=2.3 years), 47 diagnosed with TS+(33 males, 7 female), 40 with AD/HD(31 males, 9 female) and 45 normal control childrean (35 male, 10 female). Of the 47 Children diagnosed with TS, 11 children were diagnosed with pure TS(8 male, 3 female) while 36 had multiple comorbid disorder (28 AD/HD, 10 ODD, 18 OCD, 1 CD). Groups were statistically matched for age, IQ and sex distribution Multivariate analyses were conducted, with age being covaried. Although there was a clear delineation in the behavioural presentation between TS and/or AD/HD children (children with AD/HD were rated as having more difficulties with attention, and those with TS were identified as experiencing more emotional problems) there performance on a wide range of executive function measure revealed only subtle difficulties. These difficulties related mainly to response timing. Unlike the behavioural rating scales, the executive function performance measure offered little additional information to enable distinctions between the clinical groups. Moreover, the relationship between the rating scales and the executive function performance measure was found to be poor. It was argued that the absence of evidence of executive dysfunction in our clincial groups was attributed to the limitation of the neuropsychological measure used. This was discussed in the context of ecological validity of neuropsychological tests, that is their ability to reliably reflect the functioning in everyday life, the constraints posed by the controlled clinical setting, and the different nature of cognitive requirements needed in the realm of the real world. Although the evaluation of Buddeley's model of central executive of working memory was confounded by these limitations, it was concluded that the model would also need to be revised to incorporate the temporal (sustainability), motivational and emotional factors which influence children's processing capacity of the system. Finally, suggestions for further research were proposed.