What Are the Major Differences between Autism & Intellectual Disability?

The notion of overlap between autism and genius has been culturally popularized. However, a significant portion of those on the spectrum also test within the intellectually disabled (ID) range with an IQ of 70 or less. There have been ongoing debates as to whether traditional IQ tests are an adequate means of testing intellectual potential, especially in people who have deficits in areas such as language or attention that are vital for proper testing.

But even though we don’t have a definitive idea of what portion of the autism population truly struggles with issues of ID, it is likely that there is a significant minority of those who are intellectually disabled in addition to being autistic. This is especially the case concerning those individuals who have identifiable genetic disorders.

People with ID often have impairments in language and can display repetitive motor mannerisms that are not unlike those sometimes seen in autism. So where does intellectual disability end and autism begin? What are the key behavioral differences between a person with ID and a person with ID and autism? In essence, what defines autism?

Thankfully, a collaborative group of researchers have addressed precisely those questions [1]. They decided to look at cases of ID with and without autism and determine which, if any, DSM criteria for autism differentiated the two well. (The researchers used DSM IV criteria for greater inclusion of individuals who had been diagnosed using the old system.)

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Interestingly, they found that criteria within the sociocommunicative domain (A, above) were the most sensitive criteria for autism. This was especially the case for deficits in language and social/emotional reciprocity.

However, many individuals with ID but without autism also fulfilled these same criteria, meaning that while they were sensitive to autism, they were not necessarily specific to it.

In fact, the criteria that had the highest combined sensitivity and specificity were those within the restricted and repetitive interest domain (B, above). In particular, encompassing preoccupation with one or more stereotyped patterns of interest and inflexible adherence to routines were the strongest predictors of autism.

Why is the sociocommunicative domain so nonspecific to autism? Perhaps one explanation is that these characteristics in autism and other conditions like intellectual disability reflect a loss of function in these major behavioral domains. It’s theorized that social and language deficits in autism are a loss of communication and coordination between disparate brain regions, often reflected in a dearth of long-range white matter connectivity [2]. As such, there may be different means for creating that loss of function that are common to a variety of developmental conditions, leading to the symptom overlap we see.

Meanwhile, the restricted/repetitive interests domain reflects a gain of function due to an excitatory/inhibitory imbalance that leads to neuronal overexcitation. This pervasive excitatory/inhibitory imbalance may be something unique to autism.

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Examples of scenarios promoting excitatory/inhibitory imbalance in autism at the level of the synapse. Borrowed from here.

Leo Kanner, the father of autism, set the stage for modern autism research. Isn’t it ironic then that his seminal paper, Autistic Disturbances of Affective Contact, predisposed the world to view autism largely as a social deficit when in fact such deficits are not specific to autism at all?

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