Specifying and Differentiating

Pervasive Developmental Disorders (PDD), also called Autism Spectrum Disorders (ASD), are defined in terms of abnormalities in social and communication development in the presence of marked repetitive behaviour and narrow interests (APA, 1994). The DSM-IV (APA, 1994) and ICD-10 (WHO, 1993) provide diagnostic criteria for autism and related disorders such as Asperger syndrome (AS), Rett’s, and childhood disintegrative disorder. Unfortunately, the diagnostic category of pervasive development disorder-not otherwise specified (PDD-NOS) does not have specific criteria and is often seen as a catchall diagnosis for children who do not fit the criteria for one of the other pervasive developmental disorders (Filipek et al., 1999). According to Cohen & Volkmar (2005) classification systems should aim at improving communication, through their features (internal consistency, use easiness, good definition of categories) and being widely accepted. The accuracy of early diagnosis, as well as developmental pathways that are observed in young children with ASD have both theoretical and practical importance (Luyster et al., 2005). An empirically developed dimensional approach that defines the spectrum on multiple dimensions may offer several advantages. It may, for example, result in more correspondence between the results of genetic research and the phenotype of autistic disorders, provided the pathology can be summarized by empirical and valid behavior dimensions (Volkmar et al., 2004; van Lang et al., 2006; Hus et al., 2007). It is now well recognized that children with PDD vary in the number and severity of symptoms (Szatmari et al., 2002). In DSM-IV, a diagnostic category within PDD, which is called “pervasive developmental disorder-not otherwise specified” (PDD-NOS), defines children with symptoms such as restricted social interaction, poor verbal and non-verbal communication skills, strict and/or stereotypical behaviors but without full diagnostic criteria of autism (APA, 1994). Epidemiological data suggest that PDD-NOS is at least twice as common as autism in the general community (Chakrabatri & Fombonne, 2001). One or more of the following conditions may lead to a PDD-NOS diagnosis (1) onset of the disorder after 3 years of age, (2) atypical symptoms with regard to the 12 criteria of autism specified in DSM-IV, (3) fewer than 6 criteria and thus subtreshold (Walker et al., 2004). A categorical system like DSM-IV can be very useful for diagnosing prototypic manifestations of a disorder, but it is less useful in encompassing what may be, in its broader manifestations, a