WISC-IV performance of children with Chronic Tic Disorder, Obsessive–Compulsive Disorder and Attention-Deficit/Hyperactivity Disorder: results from a German clinical study

Chronic Tic Disorder (CTD) (incl. Tourette Syndrome, TS), Obsessive–Compulsive Disorder (OCD) and Attention-Deficit/Hyperactivity Disorder (ADHD) are common neurodevelopmental disorders in childhood which tend to occur frequently as comorbid conditions [1‐4].

Tics are sudden, involuntary movements or vocalizations that wax and wane in severity, frequency and complexity [5]. The mean onset of CTD is at the age of 6 years. In about 0.5% to 3% of all children tics become chronic and last longer than 1 year. Boys are about three to four times more often affected than girls [6, 7]. The most common comorbidity of CTD is ADHD (up to 60%), followed by OCD (about 20 to 30%) [8, 9].

OCD is characterized by recurrent, distressing thoughts and/or compulsive acts [5] with a prevalence of 1 to 3% in childhood and adolescence [10]. Compared to the onset of CTD the average onset of OCD is later in childhood and the gender ratio is nearly balanced [3, 11]. In children with OCD about 12% fulfil the diagnostic criteria of CTD and approximately 8% suffer also from ADHD [3, 12]. ADHD is characterized by the core symptoms inattention, hyperactivity and impulsivity and at least some of these symptoms have to be present before the age of 7 years for a period of at least 6 months [5]. About 5–7% of children suffer from ADHD [13, 14] with considerably more boys than girls being affected (sex ratio: 3–4:1) [15]. Children with a primary diagnosis of ADHD often fulfill also the diagnostic criteria of CTD (about 20%) or OCD (notable variability in ADHD-OCD co-occurrence in pediatric samples) [2, 3].

The high rate of comorbidity suggests a shared neurobiological basis [4, 16, 17] and may be associated with stronger impairment and more performance difficulties [18‐22]. Underlying neurobiological mechanisms have been investigated in recent decades, though much remains to be uncovered (e.g. [3]). Cognitive and neuropsychological deficiencies include inhibited executive functioning (i.e. planning, working memory, impulse control), verbal and non-verbal memory impairment, and other intellectual deficiencies. These accompanying symptoms might affect school performance, learning, as well as social and emotional abilities. For example, ADHD is thought to result mainly from prefrontal-striatal dysfunction, leading to impairments in executive functioning [23], which in turn leads to poor performance in processing speed, inhibition, working memory, verbal fluency, and shifting [24]. The relationship between etiological models of CTD and OCD and intelligence or rather the role of intelligence deficits in CTD and OCD is rarely investigated (e.g. [25]). Thus it is not clear whether they may be seen as a part of a common etiology, as a consequence or both.

The examination of strengths and difficulties in intellectual ability in children with CTD, OCD and/or ADHD is crucial, because this is important for academic development, social functioning and well-being [26‐28]. Different models and intelligence tests are available for the assessment of intellectual ability (e.g. Ravens Progressive Matrices, Wilde Intelligence Test) with the Wechsler Intelligence Scale for Children (WISC, according to the Carrol-Horn-Cattell-Model) being one of the most frequently used instruments in both clinical practice and research [29‐31].

Previous studies on intelligence profiles in children with CTD and/or OCD are rare and studies on ADHD are characterized by large methodological differences with regard to the use of different WISC versions (e.g. WISC-R, WISC-III, WISC-IV) [29‐31], different study designs and the examination of different diagnostic and control groups.

As illustrated in Table 1, previous studies investigated intelligence profiles with or without consideration of comorbid disorders or explicitly focused on comorbidity [4, 32‐39]. Overall, there is ample evidence that children with ADHD show impairments in intellectual ability as compared to control groups (for meta-analysis see [40]). Regards of the WISC edition [29‐31] the most studies showed significantly lower values in the WISC scores currently called Processing Speed Index (PSI) and Working Memory Index (WMI) in children with ADHD [32, 34, 36, 38, 40, 41] and it is even discussed to consider the WISC profiles in the clinical diagnostic process of ADHD [38, 40, 42].

With regard to CTD there is a considerable lower number of studies that revealed heterogeneous results and high variations in WISC profiles [20, 33]. Several studies have identified impaired intellectual functioning unique to the comorbidity of CTD TS and ADHD/OCD (e.g. [4, 37, 43‐45]). For example, Debes et al. (2011) investigated the intellectual ability with the WISC-III in a clinical sample of N = 266 children with TS and comorbid ADHD and/or OCD. They found WISC-III index scores to be below average in children with TS with and without comorbidities. The only exception were children with TS+OCD who scored higher in Full Scale IQ (FSIQ) as compared to the children of the other clinical groups (TS-only, TS+ADHD, TS+ADHD+OCD) [33]. Another study by de Groot et al. found significantly lower Verbal IQ and Performance IQ scores in children with TS+OCD and TS+OCD+ADHD and lower FSIQ scores in children with TS+OCD as compared to TS (without OCD and ADHD) [45]. Overall, studies on cognitive performance in children with CTD with and without comorbid disorders revealed heterogeneous results and a large variation in WISC scores [4, 20, 33, 36, 39].

Finally, only a few studies investigated WISC profiles in children with OCD. For instance, in a study by Beers et al. children with OCD did not show cognitive impairment in WISC-III scores [46] whereas Shin et al. reported performance deficits in the WISC-R subtests assessing perceptual organization ability [36].

To summarize, there is ample evidence on WISC performance of children with ADHD, but WISC profiles in children with CTD and OCD were rarely examined. Comparison of studies is hampered by different study designs, substantial methodological differences (e.g. different WISC versions, age groups, sampling procedures) and different consideration of comorbidity (see Table 1). The present study aims to investigate intellectual ability in children with CTD, OCD and/or ADHD, that commonly co-occur (especially) in clinical settings [1‐3]. Here, the examination of intelligence profiles in children with CTD and/or OCD is of particular interest because of the low number of studies [4, 20, 33, 36, 39, 46] that revealed heterogeneous results and a large variation in WISC scores.

The Department of Child and Adolescent Psychiatry (CAP) Dresden offers special consultation hours for these disorders and we were therefore able to analyze clinical data of a substantial number of affected children. Due to our comprehensive and standardized assessment process, we were furthermore able to identify comorbid and non-comorbid disorders. As we used field data, no healthy control group was assessed. Using the clinical data from the CAP Dresden electronic health records, this study was conducted in order to evaluate whether or not children with only one (namely CTD-only, OCD-only or ADHD-only) differ from those with comorbid disorders (e.g. CTD+ADHD, CTD+OCD+ADHD) in intellectual functioning. For this, WISC-IV profiles of different groups will be compared to the German WISC-IV norm and among each other. The study was also undertaken as previous studies on intelligence profiles in CTD and OCD produced conflicting results and few have been published from Europe. Based on a large body of literature, we hypothesized that ADHD is associated with deficits in WISC-IV WMI and PSI scores [35, 38, 41, 47]. Due to the heterogeneous studies on intelligence profiles in children with CTD and/or OCD, no specific hypotheses were formulated.

The Department of Child and Adolescent Psychiatry (CAP) of the University Hospital Carl Gustav Carus at the Technische Universität Dresden (Germany) offers special consultation hours for children with CTD, OCD and ADHD. Families mainly come from the area of Dresden, but also from all over Germany. All children pass an extensive diagnostic procedure including a physical examination, a comprehensive anamnesis, several clinical assessments (Child Behavior Checklist [48], Strengths and Difficulties Questionnaire [49]; and for specific diagnoses, e.g. Yale Global Tic Severity Scale [50]; Yale Brown Obsessive Compulsive Scale (CY-BOCS) [51, 52], Conners’ Scales [53, 54]), and if necessary, neuropsychological tasks (incl. WISC-IV) and behavioral observation at home and in school. The diagnostic procedure at CAP can comprise several appointments (usually 5–7 appointments). All children receive an ICD-10 diagnosis [5] based on the assessments and the consensus of a multi-professional team (incl. physicians, psychologists, social worker).

This study investigated WISC-IV profiles in a German clinical sample of children with comorbid and non-comorbid CTD, OCD and/or ADHD who consulted the CAP Dresden (Germany) during the study period from 01.01.2010 until 31.03.2016. In accordance with the earlier onset-age (1) children with ADHD-only and CTD+ADHD were significantly younger than children with CTD-only, OCD-only and CTD+OCD [6, 11]. However, no age or gender related bias was assumed for this study, since the WISC-IV index and FSIQ scores were standardized. (2) When comparing WISC-IV profiles of the considered groups to the German WISC-IV norm, unpaired t-tests revealed that children with ADHD-only showed significant lower PSI scores, whereas children with CTD-only and OCD-only had significant higher VCI scores. (3) ANOVA revealed that children with CTD+OCD scored significantly higher in WMI than children with ADHD-only.

For the discussion of our results, it is important to consider that previous studies followed different methodological approaches (e.g. different WISC-versions, age groups, sampling procedures, consideration of comorbidities).

The result that children with ADHD-only showed significantly lower performances in PSI as compared to the WISC-IV norm (t-tests) and as compared to children with CTD+OCD (ANOVA) is in accordance with previous studies [35, 38, 41, 47] and can be interpreted in a similar direction. The results were in line with another German study of Schmidtendorf et al. who reported significant deficits in PSI and WMI in children with ADHD (N = 433) and furthermore significant deficits in PSI in the subsample of ADHD cleared of comorbidities [35].

As already mentioned, some clinicians even discuss to consider the WISC-IV profiles in the clinical diagnostic process of ADHD [38, 40, 42, 59]. However, it is important to consider that these empirical findings were found at a group level. WISC profiles should be used for individual diagnostic.

When comparing WISC-IV profiles of the considered groups to the German WISC-IV norm, unpaired t-tests revealed that children with CTD-only had significant higher VCI scores. As other studies did not find above-average VCI scores in children with CTD, we have to discuss this result with caution. Other studies that examined cognitive performance in children with CTD (with and without comorbid disorders) revealed heterogeneous results and a large variation in WISC scores [4, 20, 33, 36, 39]. Rizzo et al. found no significant differences in FSIQ between children with TS-only and controls and significant lower FSIQ in subjects with TS+ADHD [4] whereas Debes et al. found lower IQ scores in subjects with TS-only compared to controls and the general population, except for children with TS+OCD who scored higher in FSIQ [33]. Overall, we cannot rule out that our result might be partially due to a selection bias (e.g. fewer WISC scores of children with severe and impairing CTD available from our CAP electronic health records). However, selection bias would likely also pertain WISC performance on the other WISC scores. Moreover, large standard deviations indicate that at least a meaningful proportion of children with CTD performed well during the WISC-IV examination. Further studies are needed to examine intelligence profiles in children with CTD, including TS.

As already mentioned, only a few previous studies investigated WISC-profiles in children with OCD [36, 46]. The present study revealed that children with OCD presented higher VCI scores as compared to the WISC-IV norm. Although an overestimation of the mean WISC scores due to selection bias (see below) cannot be ruled out verbal intelligence performance seems to be rather good within the overall intelligence profiles of children with OCD. Our results were in line with the study by Shin et al. who examined subjects with OCD, TD, ADHD, depression and healthy controls and found that subjects with OCD tended to have higher verbal ability (Verbal Intelligence Quotient) as compared to children of the other clinical groups and similar verbal IQ as compared to the healthy control group [36]. Also, children in a study of Beers et al. performed as good as healthy children in WISC-III and other neuropsychological tests. The authors investigated a pediatric sample of OCD children (age: M = 12.03, SD = 2.9) and concluded that although OCD is associated with central nervous dysfunction, this does not interfere with cognitive ability at an early stage of illness [46]. Given the association between intelligence performance and the development of OCD symptoms [60, 61], more research is needed to investigate the prospective relations between intelligence profiles and OCD.

Interestingly, children with comorbid disorders (CTD+ADHD, CTD+OCD, CTD+OCD+ADHD) showed no significant deviations from the WISC-IV norm in our sample. This result contradicts the common assumption that children suffering from comorbid disorders may have more severe impairments (e.g. cognitive abilities, social functioning [18, 21, 22]), but must be discussed in light with the limitations of our study (see below).

Interestingly, the result that children with CTD+OCD showed significantly higher scores in WMI as compared to ADHD-only (ANOVA: F = 2.335, p = 0.044) was in line with the study of Debes et al. who reported that children with TS+OCD scored higher in FSIQ than the other groups (TS, TS+ADHD and TS+ADHD+OCD) [33]. Regarding ADHD the literature is inconsistent pertaining the relation of comorbidity and cognitive impairments and our study did not show lower WISC performance in children with comorbid ADHD plus OCD and/ or CTD. This is in line with the results of Moura et al. who did not find additional neurocognitive impairments in children with ADHD and comorbid developmental dyslexia as compared to children with ADHD-only [24]. Shanahan et al. argued that processing speed might be a shared cognitive risk factor for both disorders that may help explain the comorbidity of reading disability and ADHD and examined a range of speeded tasks in reading disorder, ADHD and reading disorder+ADHD. They found that the comorbid group did not differ significantly in processing speed from the other groups [62].

On the other hand, Roessner et al. [8] investigated different domains of executive functions (using different tasks e.g. Wisconsin Card Sorting Test, Matching Familiar Figures Test and Stroop Test) in children with non-comorbid and comorbid ADHD and CTD and found that ADHD was associated with neuropsychological performance deficits especially in case of comorbid CTD+ADHD. Thus, further research is necessary to investigate how comorbid conditions are related to cognitive dysfunctions and in particular, working strategies that may be used by the affected children to deal or rather compensate their difficulties [8].