Motor impairment has frequently been described in Asperger syndrome (AS), a pervasive developmental disorder included in the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM-IV). Previous research focusing on this motor dysfunction has yielded inconsistent results, and the "clumsiness" observed clinically remains poorly defined. To clarify further the issue of motor impairment, we compared a group of 10 children and young adults who met DSM-IV criteria for AS with a control group with no neurological impairment. Subjects were matched on age, sex, socioeconomic status, and Verbal IQ. A broad battery of motoric tests was administered. Subjects with AS were found to perform more poorly than controls on tests of apraxia, one-leg balance with eyes closed, tandem gait, and repetitive finger-thumb apposition. No significant differences were found on tests of finger tapping, grooved pegboard, trail making, or visual-motor integration. The pattern of impairments suggests that a proprioceptive deficit may underlie the incoordination observed in AS and that these individuals may be overreliant on visual input to maintain balance and position in space.
Asperger syndrome (AS) is a pervasive developmental disorder (PDD) marked by significant impairment in social interaction and restricted or stereotyped interests or behaviors1-3. As a group, the PDDs are characterized by the triad of impairments in reciprocal social interaction, abnormalities in communication, and a restricted range of interests or imaginative activities.1 Autism is the most commonly recognized PDD. AS shares similar symptoms, but individuals with AS have normal propositional language and normal intelligence. Despite normal language skills, these individuals have impaired reciprocal communication and "unusual" use of language.1
The incidence of AS in the general population has been estimated at 0.29% to 0.71%, with a 4- to 15-fold predominance in males.3,4 The syndrome was initially described in 1944 by a Viennese child psychiatrist, Hans Asperger, who labeled the syndrome "autistic psychopathy." Asperger was apparently unaware of Kanner's description of the "autistic syndrome in children" 1 year earlier.5 These initial descriptions highlighted the salient differences between AS and autism. In contrast to the severe language disability described by Kanner, Asperger noted intact and often precocious development of language skills.5 Other differentiating features included poor motor coordination and average- to above-average intelligence observed in AS (see Green, 1990, for review).5 Individuals with AS are generally first diagnosed at a later age than individuals with autism, often showing the first signs of difficulty in the third year of life.6
Much of the research on AS has focused on the potential etiologies of the disorder. For example, a handful of neuroanatomic studies have been completed in an attempt to identify a particular structural abnormality accounting for the deficits seen in AS. Lincoln et al7 studied brain magnetic resonance imaging (MRI) findings of 7 subjects with AS, 49 with autism, and 70 control subjects. The authors found that subjects with AS had a significantly larger area of the anterior portion of the midsagittal corpus callosum when compared with subjects with autism. Neither group differed from control subjects, however, and no differences were found on measures of hippocampus or cerebellar vermis.7 McKelvey et al8 described three subjects with AS who all showed right hemispheric and cerebellar abnormalities on single photon emission computed tomography (SPECT) imaging. As illustrated by the above studies, neuroanatomic research generally suffers from small subject numbers and inconsistencies in findings. To date, no specific pattern of structural abnormalities or lesions appears to predominate.
Neuropsychological testing instruments have also been used in an attempt to localize areas or systems of brain dysfunction, and to better characterize the pattern of neurological or cognitive dysfunction seen in AS.2,9-11 One outcome of these studies is the observation that individuals with AS show a pattern of nonverbal learning disabilities similar to that found in some studies of children and adults with right hemisphere dysfunction11,12. Lesions of the right hemisphere, or clinical evidence of right hemisphere dysfunction such as the presence of a left hemiparesis, have been associated in some studies with social dysfunction, clumsiness, and attention deficit,13-15 all characteristics commonly seen in AS. Individuals with right hemisphere deficits may also achieve lower scores on the performance subtests than the verbal subtests of the Wechsler Scales (i.e., Wechsler Preschool and Primary Scale of Intelligence [WPPSI],16 Wechsler Intelligence Scale for Children-Revised [WISC-R],17 or Wechsler Adult Intelligence Scale-Revised [WAIS-R]18)5,13,19 although not all studies of children with right hemisphere lesions demonstrate this pattern of IQ20 and social problems.21 Individuals with AS have been shown to obtain significantly higher verbal intelligence quotient (VIQ) than performance IQ (PIQ) scores,4,22 suggesting a possible similarity to some patients with right hemisphere deficits. However, it is possible that the results obtained in studies of AS are an artifact of the diagnostic criteria applied, which require preservation of verbal skills in individuals with AS.23
In addition to the focus on right hemisphere pathology, dysfunctions in other brain areas have been postulated to contribute to the AS profile. Evidence of frontal lobe dysfunction in AS is suggested by repetitive, aimless movements and speech, lack of insight, social isolation, shallow or flat affect, and lack of appreciation for social rules, as well as communication difficulties.9 In addition, Ozonoff et al2 found that subjects with AS performed significantly more poorly on tests of executive function than those with high-functioning autism (HFA). Abnormalities in the limbic system and related structures have been theorized to contribute to AS as well and could account for the impairment in social functioning and imaginative play.24
It has been estimated that 80% of subjects with AS display "motor dyspraxia," or clumsiness.3 Asperger's original description of the syndrome included a discussion of motor abnormalities. Wing25 considered poor motor coordination a central feature of the syndrome. Tantam6 reported that the concept of AS should be reserved as a descriptive term for autistic children who display a number of specific abnormalities based on Asperger's description. One of these criteria is that the children are "conspicuously clumsy."6 In a study of 23 AS subjects in which motor abnormalities were not an inclusion criterion, motor problems were observed in 83% of the sample.26 The authors qualitatively described the motor incoordination, observing that children with AS "appeared to be generally clumsy and had a stiff or awkward way of walking (often without arm-swing), or were uncoordinated in posture and gestures."26 Proposed criteria described by Szatmari et al27 do not require motor incoordination for diagnosis. In these criteria, "clumsiness" appears through other diagnostic criteria, including "a clumsy social approach" and "gestures [which] are large and clumsy." According to these authors, these factors are not required for diagnosis but may be present.5 The research criteria published in 1993 by the World Health Organization in the International Classification of Diseases and Disorders (ICD-10) state that "motor clumsiness is usual (although not a necessary diagnostic feature)."28 The DSM-IV (1994) states that "motor clumsiness is often observed."1
Case studies and general descriptions of individuals with AS report that clumsiness is a characteristic of the disorder.8,22,29,30 Despite these frequent clinical observations of motor abnormalities, controlled studies comparing individuals with AS to those with autism or to other control subjects have often shown no significant differences.30-33 Because it has been noted that individuals with AS generally have higher IQ scores than individuals with autism, many studies have used subjects with high-functioning autism (HFA) as a control group to remove intelligence as a confounding factor. When comparing AS and those with HFA, motor clumsiness is often suggested as a distinguishing factor between the two groups.10,26,29
The consistency of the clinical observations of motor dysfunction in AS and the inconsistency of objective data suggest that these abnormalities have been insufficiently examined. The present study was exploratory in nature and aimed to broaden the scope of investigation into the motor abnormality in AS. To better characterize the nature of the motor dysfunction, we chose to compare subjects with AS with those with no neurological impairment. Tasks were selected to assess motor speed, fine motor control, motor planning, balance, visuomotor function, and praxis.