The validity of the autism spectrum disorders-diagnosis for intellectually disabled adults (ASD-DA)
Introduction
Researchers have argued that autism spectrum disorders (ASD) should no longer be viewed as rare (Filipek et al., 1999). This hypothesis may be driven at least in part by the rapidly evolving development of methods to assess ASD (Matson, 2007; Matson & Boisjoli, 2007; Matson & Wilkins, 2008). Additionally, methods of identifying a number of ASD have been emphasized in recent publications (Tidmarsh & Volkmar, 2003). The fact that these developments are rather new is exemplified by the fact that only autism, of the five ASD, was included in the DSM-III. More recently, a broader range of conditions have been included in the DSM-IV under the rubric of Pervasive Developmental Disorders (Volkmar et al., 1994), a term sometimes used interchangeably with ASD. Five conditions are included with PDD-NOS, autism, and Asperger's syndrome being the most common. Some have argued that these three disorders occur on a continuum of severity of autistic symptoms and/or level of intellectual ability (Towbin, 1997). Rett's Disorder and Childhood Disintegrative Disorder are included in the ASD grouping, but they tend to be quite rare, and at present, have clearer biological markers. Thus, in practice, most diagnostic research has focused on the three former disorders, with particular emphasis on autism and PDD-NOS since Asperger's syndrome does not occur in those with intellectual disability (ID).
Researchers have argued that common to all five ASD conditions are deficits in socialization and communication in addition to the presence of restricted or repetitive behaviors and interests (Bolton et al., 1994; Castelloe & Dawson, 1993; Chung et al., 2007; Hilton, Graver, & LaVesser, 2007; Ingersoll & Gergans, 2007; Wing, 1997). Methodologically, autism and scales that are used exclusively to diagnose autism have dominated the literature (Matson, Nebel-Schwalm, & Matson, 2007). Furthermore, most of the diagnostic research consists of correlating a scale for autism with a pre-existing group established via a DSM diagnosis (Matson et al., 2007a, Matson et al., 2007b, Matson et al., 2008). One of three conditions usually comprises the second group—PDD-NOS, ID, or autism. The people studied are invariably children, typically 3–7 years of age. This approach is very understandable given the view that early recognition and thus treatment can result in better long-term outcome (Honda & Shimizu, 2002; Matson & Minshawi, 2006). However, researchers have pointed out the difficulty in categorizing ASD, both across specific forms of the spectrum and when compared to nonspectrum individuals. Criteria for making such distinctions, at present, have been described as arbitrary (Williams & Brayne, 2006). Additionally, these conditions tend to persist over time. While some controversy exists, most researchers state that prognosis is poor for the condition (Howlin, Goode, Hutton, & Rutter, 2004).
Researchers have consistently reported on the persistence of ASD symptoms (Nordin & Gillberg, 1998). For example, in the Camberwell study, fully 3/4 of the children diagnosed with ASD who were followed up 12 years later still evinced this disability (Beadle-Brown et al., 2002). However, studies aimed at developing methods to differentially diagnose autism and ASD in adults with ID have been practically nonexistent. Whether these disorders are still present and the extent and nature of the symptoms displayed in adulthood would appear to be a worthy area for investigation.
The general view is that about 70% of persons with autism are also intellectually disabled (Ben-Itzchak & Zachor, 2007; Croen, Greher, Hoostrate, & Selvin, 2002; Edelson, 2006; Magnusson & Saemundsen, 2001). Additionally, other forms of ASD have a notable overlap with ID, particularly PDD-NOS (Bouras, Holt, Day, & Dosen, 1999; Kraijer, 1997; LaMalfa, Lassi, Bertelli, Salvini, & Placidi, 2004). Often, researchers and diagnosticians have had difficulty differentiating the symptoms of ASD versus ID, and this difficulty increases as severity of ID increases (Hare, Mellor, & Azmi, 2007; LaMalfa et al., 2004; Matson, Dempsey et al., 2008). Thus, a test that could differentiate adults with ID from those with autism or PDD-NOS and ID, particularly with individuals in the severe/profound range of ID, would be especially useful.
The present study was designed to test the convergent and discriminant validity of a new scale for differentially diagnosing ASD from ID as well as different ASD. A modification of the multitrait–multimethod approach, first described by Campbell and Fiske (1959), was used to establish the psychometric properties of the scale. This approach is considered to be a particularly stringent means of establishing the psychometric validity of a construct.
Section snippets
Participants
The sample for this study consisted of 307 adolescents and adults with profound (n = 235), severe (n = 40), moderate (n = 16), mild (n = 2), or unspecified ID (n = 14). Participants ranged in age from 16 to 88 with a mean age of 52 years. There were 168 males and 139 females in the sample. All of these individuals were residents of one of two developmental centers in central or south Louisiana, which varied in size from 350 to 550 residents. Participants were diagnosed by licensed psychologists at the
Results
The matrix of Pearson-product moment correlations among the five measures is presented in Table 3. The means and standard deviations of the scores on each test are also included. The convergent validity of the ASD-DA was demonstrated by correlating it with the DSM-IV-TR/ICD-10 checklist, MESSIER, and VABS. As predicted, the ASD-DA had a high positive correlation with the DSM-IV-TR/ICD-10 checklist (r = .60, p < .01) and high negative correlations with both the total score of the MESSIER (r = −.67, p <
Discussion
In the largest field trial with children using DSM-IV and ICD-10 criteria, both measures proved to have good sensitivity and specificity with children evincing ASD compared to other disorders (Volkmar et al., 1994). No such studies have been conducted for ASD adults with ID. However, by extrapolation from this and related child studies, it is the best available class assignment method to this point for adults (Charman, 2002, Myhr, 1998). Having said this, a scale with good reliability and
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