ReviewAutism spectrum disorders in Down syndrome: A review
Introduction
While some disorders have a well established association with autism spectrum disorders (ASDs), such as fragile X syndrome (Bailey et al., 1999) and tuberous sclerosis (Gillberg et al., 1994, Hunt and Shepard, 1993), it has been claimed that the association of ASDs and Down syndrome is uncommon (Rutter & Schopler, 1988). The typical personality profile of Down syndrome (i.e., that of an individual who is affectionate and outgoing) goes against the behavioral phenotype of ASDs (Ghaziuddin et al., 1992, Howlin et al., 1995). While people with Down syndrome have been traditionally described as friendly, affectionate, and extroverted (Gibbs & Thorpe, 1983), there is evidence that not all individuals with Down syndrome possess these personality characteristics. Some 10–20% of persons with Down syndrome display significant behavioral problems (Flynt and Yule, 1994, Gibson, 1978). It is possible that the 10–20% of persons with Down syndrome manifesting behavioral difficulties contain a substantial number of individuals with co-morbid ASDs, although the exact prevalence rate of ASDs in individuals with Down syndrome has been difficult to ascertain (Ghaziuddin et al., 1992, Howlin et al., 1995). However, it is apparent that individuals with Down syndrome and an ASD, have been under-reported and inadequately studied with regard to their neurobehavioral phenotype, and how it compares to either ‘typical’ Down syndrome individuals or individuals with ASDs without Down syndrome (Capone, Grados, Kaufmann, Bernad-Ripoll, & Jewell, 2005).
Section snippets
Why diagnose ASDs in those with Down syndrome?
The early identification of ASDs may increase the success rate of interventions (Greenspan, 1992, Lovaas, 1987), and may also allow time for genetic counselling and to initiate parental support (Baird et al., 2001). Over the last decade, it has been increasingly demonstrated that the diagnosis of Autistic Disorder can be made accurately in children as young as 2 years (Cox et al., 1999, Lord, 1995, Stone et al., 1999), though there is more variability with children with early diagnoses of
Prevalence of ASDs in the Down syndrome population
While a number of studies have attempted to ascertain the prevalence of ASDs in children with Down syndrome, prevalence figures have been influenced by the sample size, age range of sample, and the instruments, informants, and diagnostic criteria employed (Kent et al., 1998). Many of the studies have not employed ‘gold standard’ diagnostic instruments and are based on outdated diagnostic criteria. Wing and Gould (1979) carried out an epidemiological study of 35,000 children under the age of 15
Manifestation of ASDs in Down syndrome: standardised instruments
Much of the published research on ASDs in Down syndrome has not utilised comprehensive assessments or ‘gold standard’ diagnostic tools. However, the published research that has utilised standardised instruments gives an insight into the manifestation of ASDs in Down syndrome. The study of Kent et al. (1999) found that the ASSQ items which correlated significantly with an eventual ICD-10 diagnosis of an ASD in their Down syndrome population were; ‘Clumsy, ill-coordinated movements and gestures’,
Manifestation of ASDs in Down syndrome: case reports
Much of the early reports of ASDs in individuals with Down syndrome consisted of case reports. One of the earliest reports was that by Wakabayashi (1979) who reported a 7-year-old boy with Down syndrome who also met criteria for autism. The boy showed evidence of physical and motor regression in the third year of life and appeared to be functioning at a very low level, being dependent on adults for all his needs. Bregman and Volkmar (1988) described a 12-year-old girl with Down syndrome and
Medical factors associated with a diagnosis of an ASD in an individual with Down syndrome
A number of case reports make reference to medical factors likely to be of importance in contributing to the development of ASDs in individuals with Down syndrome. Although there does not appear to be a single medical factor that contributed to presence of ASDs in Down syndrome, an increased prevalence of medical difficulties may be associated with an increased risk of ASDs in this population (Rasmussen, Börjesson, Wentz, & Gillberg, 2001). Eleven of the 25 participants in Rasmussen et al.’s
Cognitive functioning in individuals with Down syndrome and ASDs
Most individuals with Down syndrome tend to function in the moderate range of mental retardation (Connolly, 1978), with declines occurring as the children get older, and reach adulthood (Hodapp, Evans, & Gray, 1999). The reported levels of cognitive functioning of children with Down syndrome and co-morbid ASDs have varied, although there appears to be more children operating in the severe and profound range than in the typical Down syndrome population. Two of four individuals reported to have
Gender ratio of individuals with Down syndrome and an ASD
Studies based on both clinical and epidemiological samples have found a higher incidence of ASDs in males than females, with a reported ratio averaging around 3.5 or 4.0 to 1 for Autistic Disorder (Fombonne, 2003). In terms of the ratio among Down syndrome population, Capone et al. (2005) reported that of 73 persons with Down syndrome and co-morbid ASDs in the literature, 50 (73%) were male, 18 (27%) were female, and the gender of five individuals was not reported. Lund (1988) reported five
Family history of individuals with Down syndrome and ASDs
One of the documented risk factors among children with Down syndrome for ASDs has been family history of the broader autism phenotype and/or a family history of psychiatric difficulties. Ghaziuddin (2000) examined 11 subjects with Down syndrome who were diagnosed with Autistic Disorder based on DSM-III-R criteria (APA, 1987). Family history was obtained from biological parents based on a shorter version of the Family History Schedule (Bolton et al., 1994), an investigator-based interview
Diagnosing ASDs in children/adults with Down syndrome
One of the issues regarding the identification of ASDs in Down syndrome is what instruments/protocols should be adopted when undertaking a diagnostic evaluation. The presentation of children with Down syndrome and ASDs may be different from that which professionals expect to find, based on their previous experience of diagnosing ASDs in children without Down syndrome (Kent et al., 1999). For example, some of the obsessional traits which may be seen in individuals with Down syndrome may be
Conclusion
The finding of the co-occurrence of ASDs contrasts with the usual view of people with Down syndrome as being sociable and outgoing. There are now a substantial number of studies to demonstrate that a subgroup of individuals with Down syndrome meet the diagnostic criteria for an ASD, and as a result are likely to need qualitatively different approaches in terms of supports and interventions to maximise their educational potential and quality of life. Whether those with Down syndrome and ASDs
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