Elsevier

Human Movement Science

Volume 27, Issue 1, February 2008, Pages 154-169
Human Movement Science

Co-occurring disorders: A possible key to visual perceptual deficits in children with developmental coordination disorder?

https://doi.org/10.1016/j.humov.2007.09.002Get rights and content

Abstract

A study was conducted to examine how visual perceptual functioning in children with DCD may be influenced by co-occurring learning problems such as reading disabilities (RD) and/or attention deficit hyperactivity disorder (ADHD). Participants included seven groups of children: 27 children with DCD only, 11 with ADHD only, 14 with RD only, 63 with DCD and at least one other disorder (i.e., DCD + ADHD, DCD + RD, DCD + ADHD + RD), and 73 typically developing controls. Visual perceptual skills were assessed using the Test of Visual Perceptual Skills (TVPS) and the Rey Osterreith Complex Figure (ROCF; copy and delayed recall). Children with DCD and at least one other disorder were found to have impairments on the TVPS compared to children with DCD only, ADHD only, and typically developing controls, particularly on subtests assessing visual memory. On the ROCF, children with DCD and at least one other disorder scored significantly lower than children with ADHD only or RD only. Children with DCD plus one other disorder were then subdivided into three groups: DCD + ADHD, DCD + RD, and DCD + ADHD + RD and compared to children with DCD only, ADHD only, and RD only. Results indicated that children with DCD + ADHD + RD had significant impairments on the TVPS compared to children with DCD only and children with ADHD only. On the ROCF, children with DCD + ADHD + RD scored significantly lower than all of the groups, except the DCD + RD group. These findings suggest that DCD on its own is not associated with visual perceptual problems; rather, it is the presence of co-occurring disorders that is a possible key to visual perceptual deficits in children with DCD. The number of co-occurring disorders present with DCD is associated with the severity of the visual perceptual dysfunction. Deficits in visual memory skills appear to be a specific area of difficulty for children with DCD and co-occurring RD and/or ADHD.

Introduction

Children with specific motor skills deficits are referred to as displaying developmental coordination disorder (DCD) (American Psychiatric Association., 1994). These children demonstrate motor deficits in virtually every motor domain. They tend to work more slowly than their typically developing peers (Missiuna and Pollock, 1995, Schoemaker et al., 2001) and their performance on tasks that require rapid or accurate goal-directed movements is often impaired (Huh et al., 1998, Johnston et al., 2002, Schoemaker et al., 2001). Visual perceptual problems have also been reported to be common among children with DCD (Wilson & McKenzie, 1998).

Studies that have examined visual perceptual skills such as discrimination of shape, area, slope, pattern, line length, and size constancy have reported that children with DCD were less proficient than typically developing children (Henderson et al., 1994, Hulme and Lord, 1986, Hulme et al., 1982, Hulme et al., 1984, Hulme and Snowling, 1992). Even when the motor component was removed from tests of visual perception, children with DCD were found to be less proficient (Parush, Yochman, Cohen, & Gershon, 1998). Based on their review of the existing literature, Wilson and McKenzie (1998) reported that children with DCD were inferior to comparison children on almost all measures of information processing and the greatest deficits were in the area of visual spatial processing. These deficits were found to be more pronounced for visual perceptual tasks that demanded a motor response; however, impairments were still found on visual perceptual tasks without motor responses. They concluded that visual perceptual abilities appeared to be a specific area of deficiency in children with DCD (Wilson & McKenzie, 1998).

In addition to visual perceptual difficulties, children with DCD frequently present with other co-occurring neurodevelopmental disorders (Dewey and Wilson, 2001, Kaplan et al., 1998). It has been estimated that the overlap between DCD and attention deficit hyperactivity disorder (ADHD) is about 50% (Kadesjo and Gillberg, 1998, Pitcher et al., 2003). Similar levels of co-occurrence have been reported between DCD and reading disability (RD) (Kaplan et al., 1998). Despite the relatively high rates of co-occurrence among these neurodevelopmental disorders, few studies have examined whether children with only DCD, ADHD or reading/learning disabilities differ from children with more than one of these co-occurring developmental disorders in terms of visual perceptual functioning. Studies by Pitcher and colleagues reported children with both DCD and ADHD showed significant deficits in perceptual organization, timing, force control, and processing speed compared to children with only ADHD (Piek and Pitcher, 2004, Pitcher et al., 2003). Jongmans, Smits-Engelsman, and Schoemaker (2003) found that Dutch children with co-occurring DCD and learning disabilities (LD) displayed more severe perceptual motor dysfunction than children with only DCD, particularly for tasks demanding balance or unimanual dexterity. It should be noted, however, that in this study, children were classified as LD based on their enrollment in special education classes. No formal individual assessments were conducted to confirm whether they met criteria for a learning disability.

Whether children with only DCD, LD, or ADHD differ from children with any combination of these developmental disorders in terms of the severity and pattern of perceptual-motor dysfunction is an area of study that has received little attention. We have reported, however, that for children with ADHD, the presence of co-occurring disorders had a significant impact on cognition and behavior (Crawford, Kaplan, & Dewey, 2006). Specifically, we found that among children with ADHD, poorer performance on tests of memory and visual motor skills, more prevalent behavioral problems, and more impairment in everyday functioning were associated with a higher number of co-occurring disorders.

Different theoretical approaches have attempted to explain the co-occurrence of neurodevelopmental disorders. These approaches have ranged from those that assume that each of the disorders has an independent etiology (Cruddance & Riddell, 2006) to the other extreme that suggests that the frequent co-occurrence of DCD, ADHD, and RD is the result of generalized atypical brain development (ABD) (Gilger and Kaplan, 2001, Kaplan et al., 1998). A significant body of research has examined the co-occurrence of ADHD and RD. Willcutt, Pennington, Olsen, Chhabildas, and Hulslander (2005) tested four competing hypotheses that have been proposed to explain the co-occurrence of ADHD and RD: the cross-assortment hypothesis (i.e., co-occurring ADHD and RD result from genetic deficits in both parents, resulting in an additive combination of symptoms from each individual disorder) (Faraone et al., 1993), the phenocopy hypothesis (i.e., having one disorder creates an increased risk for the other) (Pennington, Groisser, & Welsh, 1993), the cognitive subtype hypothesis (i.e., children with co-occurring disorders will have a different pattern of disabilities from the additive deficits in each separate disorder) (Rucklidge & Tannock, 2002) and the common etiology hypothesis (i.e., a common genetic basis for individual disorders results in co-occurring disorders in more severe cases, so that children with co-occurring disorders will have weaknesses in common with children with the individual disorders; therefore, if deficits are underadditive in the co-occurring group, this would suggest that the deficits in each disorder are partially shared) (Willcutt et al., 2003). Willcutt et al. (2005) found that the children with ADHD only, RD only, and ADHD + RD all exhibited deficits on measures of response inhibition, processing speed, and a subset of measures of working memory and reading and language skills, and concluded that these results supported the common etiology hypothesis. Few studies have addressed the co-occurrence of DCD, ADHD, and RD, but preliminary evidence suggests that the co-occurrence of these disorders is significantly more frequent than would be expected if they had independent etiologies (Cruddance and Riddell, 2006, Kaplan et al., 1998).

The main purpose of the present study was to investigate visual perceptual deficits in children with DCD and to determine whether these deficits are associated with the presence of other neurodevelopmental disorders, specifically RD and/or ADHD. Secondary aims of the study were: (1) to investigate whether the severity of the visual perceptual deficits was associated with the number of disorders co-occurring with DCD; (2) to investigate whether children with DCD and different types of co-occurring disorders displayed different patterns of motor difficulties; and (3) to use deficits in visual perceptual skills to examine possible hypotheses regarding the co-occurrence of DCD, ADHD, and RD.

Section snippets

Participants

A fixed ratio sampling method was employed in several special schools and clinics (e.g., every second or fifth record, depending on the size of the facility) that served children with developmental disorders. Families were telephoned and invited to participate in a study on learning and attention disorders. Typically developing controls were recruited from public schools in communities that the children with neurodevelopmental disorders resided. The normal control children were selected by

Sample characteristics

Significant group differences emerged for the short form FSIQ estimate, F(4, 183) = 7.90, p < .001, partial eta2 = .147. Post-hoc group comparisons using the Scheffé test showed that the DCD + at least one other disorder group had significantly lower estimated FSIQ scores compared to the DCD only group and the typically developing controls. It is important to note, however, that the mean estimated FSIQ for the DCD + at least one other disorder group was within the average range (see Table 1). Significant

Discussion

Wilson and McKenzie (1998) concluded that visual perceptual abilities were a specific area of deficiency in children with DCD. The findings of the present study suggest, however, that DCD on its own is not associated with visual perceptual problems. Rather, the presence of visual perceptual difficulties in children with DCD appears to be associated with co-occurring ADHD or RD. Consistent with the findings of Jongmans et al. (2003), the number of co-occurring disorders was a key factor in the

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