Pervasive Developmental Disorder, Not Otherwise Specified (PDD-NOS) is the most frequently diagnosed pervasive developmental disorder (PDD), yet it is the least well characterized. Current diagnostic criteria for PDD-NOS are vague and difficult to use reliably, as specific numbers and types of symptoms are not provided. The few studies that have investigated PDD-NOS as a diagnostic category provide little support for its reliability or validity. The objective of the current study was to clarify PDD-NOS as a diagnostic category by specifying its boundaries with Autistic Disorder (AD) and similar disorders on parent-reported adaptive and behavior problems as well as PDD symptoms.
The sample consisted of 162 children with PDD-NOS, AD, or other developmental disabilities (DDs). Participants were between the ages of 24 and 146 months (mean=65.2; SD=30.5). Individuals with PDD-NOS were matched to those with AD and DD on age, gender, ethnicity, and non-verbal IQ (NVIQ). The three groups were compared on select subscales of the Pervasive Developmental Disorder Behavior Inventory (PDDBI), the Child Behavior Checklist (CBCL), and the Scales of Independent Behavior-Revised (SIB-R). To measure the impact of age on PDD symptoms, behavior problems, and adaptive behavior, matched groups were separated by age into preschool (2-5 years), and school age (6-12 years) groups. Cluster analysis of PDD symptoms was performed to determine how classification based on multivariate heuristics would compare to clinical diagnoses. Discriminant function analysis was used to determine which PDD symptoms would most effectively predict diagnostic group membership.
MANOVAs indicated that no significant differences existed between PDD-NOS and AD groups on measures of PDD symptoms or adaptive behavior. The only differences to emerge between PDD-NOS and AD groups were significantly higher rates of anxiety and affective symptoms in the PDD-NOS group. The DD group had significantly fewer PDD symptoms and less-impaired adaptive behavior as compared to the PDD groups. Significant diagnosis-by-age interactions were not found. The cluster analysis revealed that a three-cluster solution best fit the data. The clusters were labeled “Subthreshold PDD Symptoms,” “Language Impaired, Ritualistic,” and “Typical Autism.” The clusters did not differentiate diagnostic groups. The discriminant function analysis revealed that a set of three PDD symptoms differentiated the DD group from the PDD groups with a 67% correct classification rate, and that a set of two symptoms differentiated PDD-NOS, AD, and DD groups with a 52% correct classification rate.
Overall, these results suggest that previously-reported differences in symptomatology between PDD-NOS and AD might be better accounted for by differences in NVIQ. Level of functioning was a salient contributor to the heterogeneity observed in PDDs. Results of this study support proposals by other researchers to incorporate level of functioning into PDD classification systems.