Motor Development Differences in Children With Autism Spectrum Disorders
One of the great difficulties facing practitioners and researchers working with individuals with ASD is the degree to which variability supersedes universality in this population (Pope et al., 2010). That is, two individuals with the same diagnosis of ASD will act and react very differently from each other. These individual differences lead to significant challenges for researchers trying to identify and quantify motor development in this population. For example, individuals with ASD may respond differently to a controlled experimental design due to different sensitivity to sensory stimuli or a lack of understanding of the described procedures. Furthermore, each individual responds differently to different interventions. All in all, typical developmental trajectories are all but impossible to determine. However, researchers have identified some general characteristics related to movement and motor development that suggest that individuals with ASD move differently from their typically developing counterparts (Pope et al., 2010). The importance of determining motor differences, particularly in younger children, cannot be underscored enough: If early developing motor differences exist, then assessment of these deficits may aid in identifying children at risk for ASD at a younger age than is currently possible.
Primarily through retrospective video analysis, researchers have identified early motor differences in children with ASD. Kanner (1943), who first identified ASD, noted that the infants exhibited hypotonia, or low muscle tone (similar to infants with DS). Other researchers have also observed hypotonia (Adrien et al., 1993; Ming, Brimacombe, & Wagner, 2007). For example, Ming et al. (2007) reviewed the clinical records of 154 children diagnosed with ASD in an attempt to determine the types and prevalence of motor impairments in this population, particularly hypotonia, delayed motor milestones, coordination issues, and toe walking. Children ranged in age from 2 to 18 years, with a mean age of 6 years. Reflecting the gender differences within the ASD population, there were 126 males and 28 females within the study. All of the children were on the autism spectrum: 74 were diagnosed with autism, 70 with PDD-NOS, and 10 with AS. The researchers determined that 51% of the group demonstrated hypotonia as infants; the prevalence of hypotonia decreased over time to 38% in the age range of 7 to 18 years. Not all retrospective studies have indicated that infants with autism have hypotonia. Saint-Georges et al. (2010) performed an exhaustive research review using retrospective video analysis and determined that across the 41 studies reviewed, hypotonia was not a consistent finding.
Given that at least a percentage of children with ASD exhibit hypotonia, subsequent issues with the acquisition of motor milestones may exist (as in children with DS). Low muscle tone affects infants’ ability to raise the head while in the prone position, which affects the ability to sit supported, and so on. In fact, another consistent finding among individuals with ASD is a delay in achieving motor milestones and fundamental motor skills as young children. Acknowledging the limitations of looking back at records after diagnosis, Landa and Garrett-Mayer (2006) performed a 2-year longitudinal prospective study in which they followed a group of 84 infants who were either high (n = 60) or low (n = 27) risk for ASD.
Infants were tested at 6, 14, and 24 months using the Mullen Scales of Early Learning (MSEL), a standardized developmental test for children aged 0 to 69 months. The MSEL consists of five subscales: gross motor, fine motor, visual reception, receptive language, and expressive language. At 2 years, all children were given the Autism Diagnostic Observation Schedule (ADOS), which is a semistructured, play-based interview that provides systematic probes for symptoms of autism in social interaction, communication, play, and repetitive behaviors. Of the 60 children deemed at risk, 21 met the ADOS algorithm criteria for ASD as well as received a clinical diagnosis of ASD; 2 children from the low-risk group met the criteria and were included in the high-risk group. Early on, no statistical differences existed between groups. However, by 24 months, the high-risk group showed delays in both gross and fine motor skills, and group members were significantly different from the typically developing group in all domains.
In related research, a group of investigators assessed motor development in children younger than 3 years of age (Provost, Lopez, & Heimerl, 2007). In this study, 19 children with ASD, ranging in age from 21 to 41 months, were examined with the Bayley Scales of Infant Development II (BSID II; Bayley, 1993) and the Peabody Developmental Motor Scales, Second Edition (PDMS-2; Folio & Fewell, 2000). The researchers determined that 84% of the children were significantly delayed on the BSID II and PDMS-2. In a similar study, Ozonoff et al. (2008) examined the early movement behavior of children later identified as ASD and reported a delay in the appearance of motor milestones, including lying in prone and supine position, rolling, sitting, crawling, and walking, with the most significant delay occurring in walking onset. Therefore, it appears that delays in motor milestones exist in many of the children diagnosed with
Acknowledging that delays appear to exist in the ASD population, Ozonoff et al. followed strong inference (see chapter 4) and asked the next logical question: Can motor delays differentiate children with autism from children who have more general developmental delays? If motor delays could be used for differentiation, then practitioners could use motor delays to identify infants at risk for ASD. In the resulting study, Ozonoff et al. (2008) placed 103 children between 24 and 60 months of age in 1 of 3 groups: children with ASD, children with developmental delays (DD), or children with typical development (TD). Home videotapes of the participants were analyzed and coded with the Infant Motor Maturity and Atypicality Coding Scales (IMMACS), which rate six motor milestones (lying in prone and supine position, roll, sit, crawl, walk) as well as protective skills (e.g., righting the body after losing balance). What Ozonoff et al. (2008) found, in contrast to earlier reports and speculation, is that the only group that differed in terms of number of movement abnormalities and protective skills was the DD group; the ASD and TD groups were not significantly different. The ASD and DD groups both showed motor delays, but they were not significantly different from each other. Therefore, the notion that motor delays can be used to identify children at risk for ASD was not supported in this research. Obviously, a prospective study examining both qualitative and quantitative variables is necessary as a next step to confirm this finding.
Are individuals with ASD merely delayed in motor skill acquisition, or do they differ in the ways in which they move? Several research studies (see table 9.5) have found at least one consistent difference between children with ASD and children without ASD, and that is their walking patterns. Over three decades ago, researchers found that children with ASD demonstrate abnormal limb movements, shortened steps, and persistent toe walking (Damasio & Maurer, 1978; Vilensky, Damasio, & Maurer, 1981). More recently, Ming et al. (2007), in their retrospective study, determined that nearly 20% of children with ASD walked on their toes rather than on their entire foot. Esposito and Venturi (2008) used retrospective video analysis as well as an observational scale to examine 42 children with ASD who had been walking at least 6 months and also found differences in early walking patterns. Vernazza-Martin et al. (2005) found significant differences in gait when comparing 15 children aged 4 to 6 years with and without ASD, as did Woodward (2001) in a study of children with ASD aged 3 and 10 years. One consistent finding is that the gait cycle is slower and less consistent in children with ASD.
Dewey, Cantell, and Crawford (2007) compared the performance of children with different disabilities on the Bruininks-Oseretsky Test of Motor Proficiency-Short Form. Participants included 49 children with ASD, 46 children with developmental coordination disorder (DCD) and attention-deficit/hyperactivity disorder (ADHD), 38 children with DCD, 27 children with ADHD, and 78 children with typical development. The results indicated that although all the atypical groups displayed significant impairment of motor skills, children with ASD were significantly more impaired compared with their cohorts with specific motor skill deficits. They were also the only group to show impairment on gestural skills. Several researchers have found deficiencies in fine motor skills in children with ASD. These range from delays in manual dexterity (Miyahara et al., 1997) and graphomotor skills (Mayes & Calhoun, 2003) during early and middle childhood to motor control issues in prehension (Mari, Castiello, Marks, Marraffa, & Prior, 2003). Such fine motor skill deficits influence handwriting as well as many functional activities involving the hands and arms. Across these studies, it appears that individuals with autism do move differently when compared with their typically developing counterparts and that deficits range from fine to gross motor skills.
In sum, although motor deficits are not listed as part of the DSM-V diagnostic criteria for ASD, individuals with these disorders tend to exhibit motor skill deficiencies as well as differences in motor development. Infants with ASD may exhibit hypotonia (although this finding is not as robust as it is in infants with DS). In addition, delays may exist in the acquisition of motor milestones and fundamental motor skills. Differences in skills such as gait and manual dexterity persist into childhood.
Developmental Coordination Disorder
Another disorder with an unknown etiology that has garnered considerable empirical attention as of late is DCD (Wilson & Larkin, 2008). DCD, also known as developmental dyspraxia, is characterized by extreme lack of motor coordination and by other movement deficits in the absence of neurological defects. DCD has been calculated to affect as much as 6% of the elementary school population (American Psychiatric Association, 1994; Barnhart, Davenport, Epps, & Nordquist, 2003; Barnett, Kooistra, & Henderson, 1998). Although the disorder was described as early as 1937, it took until 1994 for a group of 43 experts in various movement disorder fields to come to a consensus on a name and description for DCD in order to facilitate both research and diagnosis (see the sidebar for the DSM-V description of DCD). Individuals characterized as having DCD fall into the 0 to 10th percentile of performance on the Movement Assessment Battery for Children (M-ABC), a standardized motor skills test used to assess motor proficiency in children with disabilities (Geuze, Jongmans, Schoemaker, & Smits-Engelsman, 2001; Henderson & Sugden, 1992; Miyahara & Möbs, 1995). Because DCD is categorized as a learning disability, it also may influence a child’s ability to perform academically. Motor difficulties can range from gross to fine motor to balance skills and can include motor planning deficits and visual or spatial difficulties (Cermak & Larkin, 2002; de Castelnau, Albaret, Chaix, & Zanone, 2007; Kaplan, Wilson, Dewey, & Crawford, 1998; Przysucha & Taylor, 2004; Whitall et al., 2006). Table 9.6 provides examples of functional motor issues that children with DCD may have.