Adult Manifestations of Pervasive Developmental Disorder Not Otherwise Specified (PDD-NOS): A Combined Epidemiological and Case Register Study.

Background: Autism Spectrum Disorder is a highly heterogeneous disorder which is currently defined on the basis of childhood behavioral repertoire. Limited information is available on features of the autistic phenotype in adults. Knowledge on the outcome for individuals with normal intelligence with milder forms of ASD is scarce.

Objectives: The aim of this study is to describe autistic and comorbid psychopathology in young adults, with a previous clinical diagnosis of PDD-NOS. Based on scores on dimensional and categorical diagnostic measures their profile of ASD and comorbid psychopathology is compared to that of young adults in specialist psychiatric healthcare without an ASD diagnosis.

Methods: Participants from the TRacking Adolescents’ Individual Lives Survey (TRAILS-clinical cohort) with a PDD-NOS (n=148) or other (n=250) diagnoses as registered in the Psychiatric Case Register North Netherlands between age 11 and 19 were compared, at age 19, on ASD symptomatology (Autism Diagnostic Observation Schedule (ADOS) module 4, Adult Social Behavior Questionnaire (ASBQ) self- and other-report versions) and comorbid diagnoses (Composite International Diagnostic Interview (CIDI) and ADHD self- and other checklists).

Results: Young adults with an PDD-NOS or Asperger diagnosis in childhood on average scored higher on the ADOS and ASBQ than clinical controls, but the majority (72%) did not exceed the ASD cutoff according to the ADOS. Eighty-one percent of the ASD group compared to 97 % of the clinical controls fell outside the ASD spectrum according to the ADOS Calibrated Severity Scale. Differences between ASD and control groups were larger on ASBQ other-report than on ASBQ self-report. On all subscales of the ASBQ differences between ASD and clinical control groups scores reached statistical significance, apart from self-report on the violation of social conventions and sensory stimulation / motor stereotypies subscales. Number of comorbid psychiatric disorders according to the CIDI did not differ significantly between the groups. The ASD group was more dependent on mental health care in and psychiatric medication than the clinical comparison group.

Conclusions: A clinical PDD-NOS diagnosis in early adolescence in most cases did not lead to an ASD classification according to ADOS in young adulthood in our sample. Compared to a clinical comparison group, young adults with a previous PDD-NOS diagnosis had on average higher scores on ASD measures, had an equal level of psychiatric comorbidity and were more dependent on mental health care. In adolescence, milder expression of ASD symptomatology may be transitory and might form part of a multidimensional early syndrome with important variation in diagnostic outcome.