Part 1 of this post summarized three diagnoses: Intellectual Disability (ID), Autism spectrum disorder (ASD), and Global developmental delay (GDD). Here, we discuss some of the similarities and differences between them, and how clinicians decide whether a child should be diagnosed with one, the other, or both.
It is important to note that a significant proportion of individuals with ASD have co-occurring GDD/ID (prevalence rates vary from 30 to 80 percent). Therefore, clinicians should consider whether a child with ASD may have co-occurring ID.
When I see children in our clinic, I try to separate ASD and ID/GDD by considering the child’s “mental age” (eg, their mental age equivalent based on cognitive abilities, and adaptive skills), and then look at their social-communication skills through that lens. Standardized cognitive tests and adaptive skills questionnaires have “age equivalence tables” which help clinicians understand the “age equivalent” for a given child’s performance.
As an example (note: this is a fictional scenario not based on any one person), let’s say I see a 6-year-old child named Chris. Chris’ IQ falls below 70, and the standardized test manual notes that his performance is equivalent to that of a 3.5-year-old child. Parent questionnaires about his adaptive skills show a similar pattern—his adaptive skills are around the 3-year-old level. This information leads me to believe that he likely has ID.
However, his parents are concerned about possible ASD since his social-communicative skills are delayed and he displays some unusual interests and body movements. Based on this information, I should look at Chris’ social-communicative skills from a 3-3.5-year-old lens when considering whether he might meet the diagnostic criteria for ASD. That is, I should expect his social-communicative skills to look like those of a 3-3.5-year-old since that is his approximate “mental age.”
Most 3 to 4-year-old children can talk about past and future events, request, give information when asked, and initiate conversations. However, they are less able to display skills expected from a 6-year-old, including the use of metaphors/sarcasm/non-literal language, displaying a wider range of feelings, and understanding the needs of the listener in conversations. Chris’s social-communicative skills are likely delayed compared to his chronological age due to his ID. It is my job, therefore, to determine whether his social-communicative skills are delayed compared to his mental age (eg, if they are below what would be expected for a 3-3.5-year-old child) or not.
If his social-communicative skills are approximately what would be expected for a 3-3.5-year-old child, I am unlikely to diagnose Chris with ASD, and would only diagnose him with ID. Similarly, if his interests are unusual or immature for a 6-year-old, but are normative for a 3-3.5-year-old, that aligns with my expectations given his ID. Further, although it is uncommon for a 6-year-old to display repetitive body movements (eg, clapping, jumping, and spinning when excited), those behaviors are more common in a 3-3.5-year-old.
If, however, Chris’ social-communicative skills were significantly below what would be expected for a 3-3.5-year-old child (eg, his speech was limited and/or repetitive) and he was displaying restricted and/or repetitive interests (eg, if he was only lining up blocks rather than playing with them), I would be more likely to diagnose him with both ID and ASD.
It is important to remember the following when considering whether a child has ASD, ID/GDD, or both:
- Children with ID/GDD will generally have delayed social communicative skills. ID/GDD is defined (in part) by delays in multiple areas of intellectual and adaptive functioning, which includes social skills. Therefore, clinicians should not automatically assume a child has ASD and ID simply because the child has delayed social communication skills.
- Keeping a child’s “mental age” in mind is important, as it has implications for how to interpret specific behaviors. For example, toddlers and young children often jump up and down and clap their hands repeatedly when they are excited, or insist on specific routines or rituals for bedtime/bath time. Those behaviors are not considered “restricted and/or repetitive” at such a young age and would not necessarily be interpreted as signs of ASD. However, as children get older, those behaviors are less common and may be interpreted as “red flags” for ASD due to their restricted and/or repetitive nature. Therefore, if a clinician is evaluating an 8-year-old child whose mental age is closer to 3 years old, the clinician should interpret behaviors according to age expectations for a 3-year-old.
- A substantial proportion of children with ASD have co-occurring ID, but clinicians need to make sure that they are considering all of the above before assigning this dual diagnosis.