Intellectual Disability in Autism Spectrum Disorder: Investigation of Prevalence in an Italian Sample of Children and Adolescents

Background: Studies published so far have reported highly variable rates of Intellectual Disability (ID) prevalence in Autism Spectrum Disorder (ASD), ranging from 16.7% to 84%. It is worth to note that most of these studies investigated the epidemiology of ASD in general, and not specifically the prevalence of ID in ASD. Moreover, studies on the prevalence rates of comorbid ID in ASD in Italy are still lacking. In fact, to our knowledge so far only one study conducted in Italy investigated the co-occurrence of ASD and ID. In more detail, La Malfa et al. assessed the prevalence of Pervasive Developmental Disorder (PDD) in an Italian sample of 166 residents with ID, and reported a PDD rate in people with ID of 39.2%. Therefore, new insights on the prevalence of ID in ASD in this country are needed in order better understand the rates of comorbidity and their implication for treatment.

Objectives: The aim of the present study was to investigate the prevalence of ID analyzing developmental and cognitive test data (Griffiths Mental Developmental Scale-Extend Revised- GMDS-ER and the Leiter International Performance Test-Revised-Leiter-R) in a large sample of children and adolescents with ASD referred to an Italian National Children Hospital tertiary referral center from January 2010 to December 2013.

Methods: All patients referred to the Child Neuropsychiatry Unit for a diagnostic assessment were included in the present study. All of them underwent a complete diagnostic evaluation throughout a multidisciplinary team. The ASD diagnosis were based on clinical assessment, and in the majority of cases were corroborated by the ADOS-G. To assess the developmental and cognitive ability we used the GMDS-ER and the Leiter-R. Indeed, children’s adaptive skills were assessed through the Vineland Adaptive Behavior Scale-Survey Form (VABS-SF).

Results: Recruitment procedure and participation rates are depicted in Figure 1. A total of 7927 patients referred for a diagnostic assessment (n=5375 for a first diagnosis, n=2552 for a diagnostic follow-up). Of them, 666 subjects were diagnosed with an ASD. 592 subjects with ASD performed a developmental or cognitive evaluation (n=360 the GMDS-ER, n=232 the Leiter-R). A total of 282 (47.6%) reported a DQ/Brief IQ <70. All of these subjects reported equivalent age below their chronological age in the VABS-SF skill domains.  

Conclusions: To our knowledge, this study is the first epidemiological survey on the prevalence of ID in a large sample of Italian individuals with ASD, documenting the rates of a country for which these data were missing. Shedding light on the epidemiology of ID in ASD is a crucial issue for clinicians and researchers in order to better define methodological and conceptual problems that needs to be further addressed. In fact, underestimate intelligence ability in individuals with ASD could affect their opportunity in everyday life. Finally, given that researchers have reported that IQ is one of the good predictor of outcome, a reliable measurement of intelligence ability in autism is essential in order to help clinicians to choose the most appropriate intervention and treatment strategy.