Diagnosing intellectual disability

[Posted on 23 October, 2019 by Bibbi Hagberg & Eva Billstedt]

Intellectual disability (ID) is an important diagnosis. Not only does it legally entitle individuals to aid through the Act concerning Support and Service for Persons with Certain Functional Impairments, it also grants them the right to utilise the special school curriculum. Being diagnosed with ID is often disheartening to both the affected individual and their family, regardless of whether learning difficulties had already been recognised and accepted prior to diagnosis. It limits certain future opportunities, particularly academically, and also establishes a need for support, primarily during childhood but to some extent throughout adulthood as well.

Diagnosing ID requires both medical and psychological assessment to ensure that there are no other underlying causes behind the learning difficulties and/or to identify any other concurrent difficulties/conditions. The psychological evaluation is crucial when diagnosing ID, and includes both an assessment of the patient’s cognitive functions (through testing) and an assessment of whether these functions are in harmony with the patient’s adaptive function (everyday function and support needs). The patient’s circumstances at home and at school form a significant part of the basis for the assessment of the individual’s adaptive function. In other words, test results alone can never be sufficient grounds for diagnosis; all the information gathered must be taken into account. In uncertain cases, a follow-up examination within the following year is usually recommended.

Health care services diagnose ID and subsequently inform schools, habilitation services and other relevant institutions to ensure that the patient gets whatever support they’re entitled to through the Act concerning Support and Service for Persons with Certain Functional Impairments. However, sometimes the diagnosis isn’t “accepted” by one or more of these institutions, even when the patient’s aggregated assessment score corroborates it. This usually occurs when the testing profile is highly uneven, where one function is shown to be operating at a level beyond the scope of ID. When these disagreements occur between health care services and other social institutions, patients and their families end up caught in the middle, taking on an even greater burden than they’re already faced with.

Aside from the emotional charge associated with ID, some might argue that diagnosing ID should be a relatively straightforward process. Cognitive tests should make it fairly easy to conclude that someone has ID, right? Well, in many cases it isn’t much more complicated than that, but there are cases that aren’t as clear-cut. Some patients have unevenly distributed intelligence, where their verbal function might clearly indicate ID while their non-verbal visuo-spatial function is somewhat higher and thus outside the typical scope for diagnosis. What then?

The Wechsler scales are the most frequently used cognitive tests for assessment of ID, and consist of subsections indexed according to which cognitive function is being tested, e.g. verbal function, working memory etc. It’s important to note that the Wechsler scales have been changed in recent years to include more indices but fewer subsections per index, which in turn has caused greater statistical uncertainty. Moreover, studies have shown that full scale scores (which include subsections from all indices, i.e. Full Scale IQ/FSIQ) are more stable over time (i.e. test-retest reliability, Watkins & Smith 2013) and that the correlation between full scale scores and school performance is greater than that between index scores and school performance (Watkins et al 2007; Freberg et al 2008).

The full scale score is also the Wechsler measure that best reflects general intelligence. In a recent study of the latest version of the Wechsler scales (WISC-V), Dombrowski et al (2018) conclude that “Overall, primary interpretive emphasis of the WISC-V should be placed upon the FSIQ with only secondary consideration given to the four index score areas for ages 6-14 and five index areas for ages 15-16”.  Other studies (Canivez et al 2019) have also emphasised the importance of full scale scores over index scores.

Aside from the test’s structure, studies have shown the indices of the Wechsler scales to be uneven, particularly among individuals with suspected ID. For example, speed index scores (measuring things like processing speed) tend to be higher than other index scores for people with ID (Bergeron & Floyd 2013).

When assessing ID, if one part of the cognitive profile deviates from the rest – for example if the result of one specific subsection greatly influences the index result – that should be viewed in a larger context. Is the function covered by that particular subsection important in everyday life, and might it serve to alleviate any other difficulties the affected individual has?

We as psychologists must consider each potential case of ID carefully. Before diagnosing, we always aim to gather a full and comprehensive view of the child’s strengths and weaknesses. In order to diagnose accurately and ensure proper intervention measures, it is crucial that not only we who make these assessments but also those who receive our evaluations understand the tests that form the basis of ID diagnoses.

[This is a blog. The purpose of the blog is to provide information and raise awareness concerning important issues. All views and opinions expressed are those of the writer and not necessarily shared by the GNC.]