مهارت تصویرسازی حرکتی کودکان مبتلا به اختلال نقص و اختلال رشد هماهنگ کننده

Up to 50% of children with ADHD experience motor impairment consistent with DCD. Debate continues as to whether this impairment is linked to inattention or is a genuine motor deficit. This study aimed to determine whether (1) inattention was greater in ADHD + DCD than in ADHD alone and (2) motor imagery deficits observed in DCD were present in ADHD + DCD. Four groups aged 7-12 years–ADHD, combined type, with motor impairment (ADHD + DCD; N = 16) and alone (ADHD; N = 14), DCD (N = 10) and typically developing comparison children (N = 18) participated. Levels of inattention did not differ between ADHD groups. On an imagined pointing task, children with DCD did not conform to speed accuracy trade-offs during imagined movements, but all other groups did. However, on a hand rotation task, both the ADHD + DCD and DCD groups were less accurate than the non-motor impaired groups, a finding not explained by differences in IQ, age, or working memory capacity. Overall, there was evidence that children with ADHD + DCD experience genuine motor control impairments indicating the impact of motor impairment in ADHD and its causal risk factors require more study. Motor impairment in ADHD should not be dismissed as a by-product of inattention.

Attention-Deficit Hyperactivity Disorder (ADHD) occurs in up to 12% of children, has a negative impact on social, behavioral and educational domains, and many of its symptoms are now believed to persist into adulthood (Biederman & Faraone, 2005). As a result, ADHD has been studied extensively and we know much about the presentation of the disorder. One interesting finding regarding ADHD is its high comorbidity or co-occurrence with other developmental disorders in childhood, including Developmental Coordination Disorder (DCD) (Dewey, Kaplan, Crawford, & Wilson, 2002). DCD is defined by the American Psychiatric Association (APA) as an impairment in motor skills, not attributable to a known neurological or physical medical condition, that significantly interferes with a child’s activities of daily living and/or academic achievement (APA, 1994). DCD, or motor impairment, is commonly observed in a large proportion of children with ADHD. Pitcher, Piek, and Hay (2003) found that approximately 50% of a sample of children with ADHD, regardless of subtype, had definite or borderline motor impairment. In an earlier study, the same authors had found approximately two-thirds of their ADHD sample was experiencing motor problems (Piek, Pitcher, & Hay, 1999). Despite research showing that children with both ADHD and DCD are likely to experience more long-term negative outcomes in a range of domains than children with either disorder alone (Rasmussen and Gillberg, 2000 and Tervo et al., 2002), DCD in children with ADHD is often overlooked in the clinical context as more prominent behavioral issues involving impulsivity and hyperactivity overshadow the motor issues (Gillberg, 2003). Although the motor impairment observed in both ADHD and DCD appears to be similar, it is not clear if they stem from the same underlying aetiological risk factors (Sergeant, Piek, & Oosterlaan, 2006). Some researchers argue that the motor impairment present in ADHD is a result of the child’s increased inattentiveness and working memory deficits rather than being a genuine motor deficit (Barnett et al., 2005 and Ferrin and Vance, 2011). Indeed, the American Psychiatric Association, in the 4th edition of their Diagnostic and Statistical Manual (DSM-IV, APA, 1994), suggests that the motor difficulties of children with ADHD are “usually due to distractibility and impulsiveness, rather than to motor impairment” (p. 54) in the differential diagnosis section for DCD. Support for this comes from studies that have demonstrated a link between the severity of inattentiveness and motor impairment (Piek et al., 1999 and Tseng et al., 2004); research showing that children with ADHD on stimulant medication do not display the same response time slowing that is apparent in those not on medication (Klimkeit, Mattingley, Sheppard, Lee, & Bradshaw, 2005), and recent research showing that neurological soft signs, including those involving fine motor movements, are related to spatial working memory deficits (Ferrin & Vance, 2011). In contrast, other studies support the presence of a genuine motor deficit in children with ADHD + DCD, unrelated to inattentive symptomatology. Pitcher et al. (2003) found that an ADHD + DCD group was significantly more impaired on a manual dexterity task than both ADHD only and control groups, who did not differ. Interestingly, there were no significant differences in the inattentive symptomatology of the two ADHD groups, indicating that the poor manual dexterity of the ADHD + DCD group could not be attributed to increased inattentiveness. Miyahara, Piek, and Barrett (2006) used distractor tasks to determine whether an increased attentional load would result in poorer motor performance in children with ADHD, but found that there was no decrease in performance as attentional demands increased. Due to these conflicting findings, it remains unclear whether the motor impairment observed in ADHD stems from the same underlying causal risk factors as that in DCD – this is confounded by the fact that we do not yet know exactly what is causing DCD itself, with researchers continuing to search for underlying deficits that are likely to play a causal role in the disorder (Wilson, 2005). One line of research has demonstrated consistently that children with DCD have a reduced ability to accurately represent movements in the brain via motor imagery (Deconinck et al., 2009, Maruff et al., 1999, Williams et al., 2006, Williams et al., 2008, Wilson et al., 2001 and Wilson et al., 2004). Motor imagery (MI) refers to the imagination of a motor task without actual movement execution (Decety & Grèzes, 2006) and is believed to represent one’s ability to accurately utilize forward internal models of motor control (Sirigu et al., 1996, Williams et al., 2006, Wilson et al., 2004 and Wolpert et al., 1998). Forward internal models provide stability to motor systems, by predicting the outcome of movements before slow, sensorimotor feedback becomes available (Wolpert, 1997). They are important for smooth, accurate movement, reducing the reliance on feedback and allowing corrections to the movement to be made as it unfolds when necessary. A deficit in the ability to utilize such models results in slow, poorly coordinated movements and as such, has been hypothesized to be one of the underlying causes of motor impairment in DCD (Wilson et al., 2001 and Wilson et al., 2004). A recent study examined the motor imagery ability of children with ADHD, combined type (ADHD-C) parsed into those with and without comorbid DCD, and compared them to a sample of children with DCD only and a sample of healthy control participants (Lewis, Vance, Maruff, Wilson, & Cairney, 2008). The study used a test of motor imagery that requires participants to complete a series of real and imagined pointing movements between targets that vary in size – typically, a speed-accuracy trade-off is found in both real and imagined movements, indicating that motor imagery adheres to the same motor control laws as actual movement (Sirigu et al., 1996). It was found that, as in previous studies, the DCD group did not conform to the speed-accuracy trade-off for the task during their imagined movements, indicating atypical motor imagery performance. In contrast, both the healthy controls and ADHD only group performed typically. Interestingly, the performance of the ADHD + DCD group also conformed to the typically observed pattern, with the group displaying no apparent deficits in motor imagery ability. This is the first study to look at underlying motor control processes in children with ADHD and suggests that the motor impairment present in many children with ADHD might not have the same underlying aetiology as that of children with DCD alone. The authors concluded that the motor impairment often observed in ADHD may stem from executive and attentional control problems, but did not include a measure of either in their study. The aim of the current study is to extend the work of Lewis et al. (2008) by including (1) measures of attention, and for the ADHD groups, working memory and (2) extending the motor imagery analysis by using another task commonly used with children with DCD – the hand rotation task (Deconinck et al., 2009, Williams et al., 2006, Williams et al., 2008, Williams et al., 2011 and Wilson et al., 2004). Although correlations have been identified between attention and motor skills in children with ADHD (Tseng et al., 2004), direct comparisons of attention in ADHD alone versus ADHD + DCD groups have failed to identify group differences (Pitcher et al., 2003). Based on this, we did not expect to find a significant difference between our two ADHD groups on measures of attention, but expected both groups to be significantly more inattentive than the DCD and comparison groups. Based on the recent findings regarding a relationship between working memory and neurological soft signs in children with ADHD (Ferrin & Vance, 2011), we did expect to find that children with ADHD + DCD would score more poorly than children with ADHD alone on measures of working memory. Interestingly, motor imagery has long been recognized as having a working memory component (Decety, 1996), though some tasks have a greater working memory load than others – for example, the hand rotation task requires images of the hand (either the stimulus or the participant’s own) to be held in working memory during imagined rotation. Thus, we expected that, in line with Lewis et al. (2008), children with ADHD + DCD would perform similarly to children with ADHD alone and typically developing comparison children on the visually guided pointing task. In the hand rotation task however, with its greater working memory load, we would expect children with ADHD + DCD, like those with DCD alone, to be slower and/or less accurate than children with ADHD alone and typically developing comparison children.

In conclusion, this study demonstrated that children with ADHD + DCD experience genuine motor control impairments (manifest by a reduced ability to accurately represent movement at a neural level) that do not appear to be linked to increased levels of inattention or decreased working memory capacity (relative to children with ADHD alone). On the VGPT, this group’s imagined movements conformed to the same laws as their actual movements, unlike those with DCD alone, but the correlation between their real and imagined movements was very low, indicating performance may not have been completely typical. Clear deficits in motor imagery ability were noted for the ADHD + DCD group on the hand rotation task, with performance as poor as that for children with DCD alone. Though it is unclear whether motor imagery deficits play a causal role in motor impairment in ADHD or are in fact a symptom of such impairment, an inability to accurately represent movements internally is likely to result in problems with motor planning and the efficient use of feedforward models of motor control (Blakemore et al., 2002 and Flanagan et al., 2003). In DCD, this is reflected not only in motor imagery as it is here, but also in motor planning (van Swieten et al., 2010) and online movement control (Hyde and Wilson, 2011a and Hyde and Wilson, 2011b). It is critical therefore that clinically, motor skill assessments are included in assessments of children with ADHD and impairments are considered seriously, with interventions provided. It is also vital that researchers continue to explore motor control in ADHD + DCD to further delineate the underlying aetiological risk factors of motor impairment, which will enhance not only interventions provided, but also improve clinical recognition.