State of the Field: Differentiating Intellectual Disability From Autism Spectrum Disorder

Abstract

The topic of this special issue on secondary versus idiopathic autism allows for discussion of how different groups may come to manifest autism spectrum disorder (ASD) or ASD-like symptoms despite important etiological differences. A related issue is that, because many of the social communication deficits that define ASD represent a failure to acquire developmentally expected skills, these same deficits would be expected to occur to some extent in all individuals with intellectual disability (ID). Thus, regardless of etiology, ASD symptoms may appear across groups of individuals with vastly different profiles of underlying deficits and strengths. In this focused review, we consider the impact of ID on the diagnosis of ASD. We discuss behavioral distinctions between ID and ASD, in light of the diagnostic criterion mandating that ASD should not be diagnosed if symptoms are accounted for by ID or general developmental delay. We review the evolution of the autism diagnosis and ASD diagnostic tools to understand how this distinction has been conceptualized previously. We then consider ways that operationalized criteria may be beneficial for making the clinical distinction between ID with and without ASD. Finally, we consider the impact of the blurred diagnostic boundaries between ID and ASD on the study of secondary versus idiopathic ASD. Especially pertinent to this discussion are findings that a diagnosis of ID in the context of an ASD diagnosis may be one of the strongest indicators that an associated condition or specific etiological factor is present (i.e., secondary autism).

The Clinical Dilemma of Autism Spectrum Disorder Diagnosis in a Child with 9p Deletion

“…3 The specific criteria developed within the DSM-5, including deficits in social communication and interaction, lack of nonverbal communication for social interaction, and restricted, repetitive patterns of behavior or interests (RRB), are difficult to differentiate from developmental delays in young children who are at a developmental age where social communication skills are not yet significantly developed. 3 Current clinical diagnostic tools such as ADOS, a semi-structured play-based tool, are not as reliable when the developmental age of a child is less than 18 months. 4 Additional limitations of the ADOS are found in the validity of scores for children with significant language delays and deficits in verbal communication.…”

“…6 Consideration of the overall platform of developmental skills may prove to be useful in the operationalization of this criterion since uniform delays in all developmental domains may point toward behavioral symptoms being attributed to low developmental age. 3 Delayed motor skills may also limit abilities to gesture, initiate social overtures, and interact in reciprocal play activities. The ADOS-2 Module 1 was conducted to obtain some further observation of specific behaviors that may be consistent with an ASD diagnosis; however, observed skills such as directing vocalizations toward caregivers, joint attention, making simple conventional gestures, and a lack of stereotypic mannerisms were in concordance with her developmental age.…”

“…further cognitive, language, and motor skills, the social developmental trajectory may become clearer. 3 In children with chromosomal differences, ongoing evaluation of developmental status should be completed to ensure that appropriate specialized educational support programs and behavioral interventions are offered to support the child's development.…”

Psychometric Study of the Social Responsiveness Scale in Phelan–McDermid Syndrome

“…The evaluation and improvement of existing assessment tools are particularly important in populations with severe ID such as PMS, but is also challenging due to verbal communication challenges, comorbid medical conditions, and atypical skill progression [Mohr & Gray, ; Redin et al, ; Soorya, Leon, Trelles, & Thurm, ; Wright et al, ]. Diagnostic criteria mandate that ASD should not be diagnosed if symptoms are better explained by ID or developmental delay, however, diagnostic boundaries are often blurred and few operationalized criteria exist to identify clinical distinctions [Thurm, Farmer, Salzman, Lord, & Bishop, ]. Moreover, genetic syndromes are often accompanied by co‐morbid medical conditions, which may change the clinical presentation of ASD [Thurm et al, ].…”

“…Diagnostic criteria mandate that ASD should not be diagnosed if symptoms are better explained by ID or developmental delay, however, diagnostic boundaries are often blurred and few operationalized criteria exist to identify clinical distinctions [Thurm, Farmer, Salzman, Lord, & Bishop, ]. Moreover, genetic syndromes are often accompanied by co‐morbid medical conditions, which may change the clinical presentation of ASD [Thurm et al, ]. The Characteristics of Assessment Instruments for Psychiatric Disorders in Persons with Intellectual Developmental Disorders (CAPs‐IDD) was developed as a set of Intellectual and Developmental Disability‐specific guidelines to aid in the evaluation and adaptation of instruments for use in these populations [Zeilinger, Nader, Brehmer‐Rinderer, Koller, & Weber, ].…”