Autism and intellectual disability: a study of prevalence on a sample of the Italian population.
Systematic screening with the PDD-MRS lifts co-occurring ASD detection in ID clients from 8% to 40%.
01Research in Context
What this study did
Doctors in Italy screened 166 people who already had intellectual disability. They used the PDD-MRS checklist plus a full clinical review. The goal was to see how many also had pervasive developmental disorder.
What they found
The first count found only 7.8% of the group had PDD. After the PDD-MRS and clinical review, the rate jumped to 39.2%. Most of the co-occurring cases had been missed before.
How this fits with other research
Etyemez et al. (2022) looked at 3,208 U.S. children and showed the same dual diagnosis brings more neurological problems but fewer psychiatric ones. That larger study extends this Italian finding beyond simple prevalence.
Méliná et al. (2015) had already shown that about one-third of preschoolers with ASD also have ID. Their U.S. case series is an earlier brick in the same wall.
Morales Hidalgo et al. (2021) repeated the message in Spain: school-based screening caught almost twice the registered ASD cases. Together these papers say the under-count is real and ongoing, not just an Italian quirk.
Why it matters
If you serve clients with ID, always screen for autism. A quick tool like the PDD-MRS plus your clinical eye can turn an 8% hit rate into a 40% hit rate. Missing ASD means missing years of ABA that could build language, social, and daily-living skills. Build the screen into your intake packet today.
Want CEUs on This Topic?
The ABA Clubhouse has 60+ free CEUs — live every Wednesday. Ethics, supervision & clinical topics.
Join Free →Add the PDD-MRS to your intake for every client with ID and schedule a clinical review when scores are elevated.
02At a glance
03Original abstract
BACKGROUND: In 1994, the American Association on Mental Retardation with the DSM-IV has come to a final definition of pervasive developmental disorders (PDD), in agreement with the ICD-10. Prevalence of PDD in the general population is 0.1-0.15% according to the DSM-IV. PDD are more frequent in people with severe intellectual disability (ID). There is a strict relationship between ID and autism: 40% of people with ID also present a PDD, on the other hand, nearly 70% of people with PDD also have ID. We believe that in Italy PDD are underestimated because there is no agreement about the classification system and diagnostic instruments. METHOD: Our aim is to assess the prevalence of PDD in the Italian population with ID. The Scale of Pervasive Developmental Disorder in Mentally Retarded Persons (PDD-MRS) seems to be a very good instrument for classifying and diagnosing PDD. RESULTS: The application of the PDD-MRS and a clinical review of every individual case on a sample of 166 Italian people with ID raised the prevalence of PDD in this population from 7.8% to 39.2%. CONCLUSIONS: The study confirms the relationship between ID and autism and suggests a new approach in the study of ID in order to elaborate a new integrated model for people with ID.
Journal of intellectual disability research : JIDR, 2004 · doi:10.1111/j.1365-2788.2003.00567.x