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Measuring Cognitive Change in Individuals With Intellectual Disabilities

By Jamie Santa Cruz

Accurately identifying cognitive decline in individuals with intellectual disabilities is an important aspect in ensuring their optimal quality of life.

Individuals with intellectual disabilities (ID) are increasingly living lifespans typical of the general population. As a result, a larger percentage of these individuals is reaching ages at which cognitive decline presents challenges. The issue of cognitive decline is particularly significant for individuals with Down syndrome, who are at high risk for early onset dementia. Although assessing cognitive change in individuals with ID has traditionally been difficult, new research from the University of California (UC) Davis suggests that one battery of tests in particular may be reliable for tracking change in the ID population.1

The study, led by David Hessl, PhD, a professor in the department of psychiatry and behavioral sciences at UC Davis, enrolled participants with varying forms of ID, including Down syndrome, fragile X syndrome, autism, and other disabilities. It made use of the National Institutes of Health (NIH) Toolbox Cognitive Battery, which measures processing speed, executive function, episodic memory, word/letter reading, receptive vocabulary, and working memory. The battery was developed primarily using data from typically developing people, Hessl says, but because the test performs well in children as young as 3 years old, the researchers thought it could be useful for individuals with ID who are older than 3 but functioning at a much lower level.

According to preliminary results, published in the Journal of Neurodevelopmental Disorders, the NIH Toolbox battery is indeed feasible for individuals with ID who have a mental age of about 3 or 4 and above. The results are stable (with strong test-retest reliability over a four-week period), and they align well with other assessments that tap into similar areas of cognitive function.

As part of the ongoing project, all participants in the study will be retested at a two-year follow-up to see whether the battery is able to detect growth and intellectual development among the child participants. Although results from that two-year follow up are forthcoming, previous research on the NIH Toolbox battery in neurotypical children suggests that the test should be able to detect such change over time. "If you look at the pattern of results—if you look at 3-year-olds, 4-year-olds, 5-year-olds, 6-year-olds—it's a very step-wise increase in performance, so there is reason to believe from the studies of typical kids that these tests should be sensitive to changes in people with ID," Hessl says.

Because the researchers' primary interest was in measuring improvements in cognition, the study enrolled only participants up to the age of 25. The relevance of the findings for measuring declines in cognition in older adults is thus unclear. However, Hessl believes the test likely has utility for tracking changes related to aging. "I think that the toolbox battery has potential in that area. It's just a question of doing more research to ensure that it's sensitive to changes that happen in those patients."

Challenges of Measuring Cognitive Decline in Individuals With ID
Currently, assessing cognitive change in individuals with ID is a difficult task. A variety of developmental neuropsychological batteries can be used. According to Wayne Silverman, PhD, director of intellectual disabilities research in the department of behavioral psychology at Kennedy Krieger Institute, individuals with ID may be tested either with the adult versions of the tests or with versions designed for children, depending on the specific disability and degree of impairment. Such assessments can be used to establish a baseline for language, executive control, processing speed, memory, and other aspects of cognitive function.

Theoretically, once a baseline has been established, it is possible to detect cognitive change over time through reassessment with these same tests. However, according to Hessl, many of the available assessments are not designed to detect longitudinal change, and there are few data in the literature to show which tests might be reliable for this purpose.

Further, it can be difficult to establish baselines in the ID population using standard cognitive batteries. Individuals with ID are not as heterogeneous in their performance on cognitive assessments as those in the general population. Among neurotypical individuals, there is a band of ability, but it is relatively narrow, says Lucille Esralew, PhD, clinical administrator of CARES & S-COPE at Trinitas Regional Medical Center in Elizabeth, New Jersey. Among individuals with ID, by contrast, the range of cognitive capacity is extreme, going from individuals with profound intellectual disability (IQ of less than 20) up to those with mild disability (IQ as high as 69). Not only is there a broad range of ability among individuals with ID, but the same person may perform differently from one assessment to the next. "Nobody understands exactly why, but the consistency with which people perform is a little bit less than what you see in the neurotypical population," Silverman says.

With regard to dementia screens in particular, a variety of standard measures are used to evaluate adults in the general population who have suspected dementia, but all are calibrated for people of typical intelligence, Silverman says. This means that individuals with conditions such as Down syndrome often perform below the floor of the test, even if they do not have dementia, he says. Such is the case, he says, with tests like the Mini-Mental State Examination, one of the most widely used measures of cognition.

Another method for detecting cognitive change, often used in tandem with direct assessments, is measurement of adaptive behavior using a scale such as the Vineland Adaptive Behavior Scale. With this approach, Silverman says, family members or professional caregivers are asked a series of questions designed to describe an individual's cognitive function and determine his or her capacity for independent functioning.

But adaptive behavior scales have significant limitations, according to Seth Keller, MD, cochair of the National Task Group on Intellectual Disabilities and Dementia Practices. They involve reliance on information provided by family members such as parents, who may be biased and may not be keen observers of behavior or on information provided by professional caregivers, who may not have not known the disabled individual for a long period. In the neurotypical population, Keller says, it would be possible to ask questions of the individual directly, but this option is often not available with individuals with ID, as they typically can't provide the information clinicians need.

Even when individuals with ID are known to be experiencing cognitive decline, determining the cause of that decline may be difficult. All individuals, whether neurotypical or not, may show declines in their cognitive abilities throughout adulthood due to the normal processes of aging. Even in the general population, Silverman says, "there is no consensus as to what instrument could be used to differentiate mild cognitive impairment from the normal kinds of changes you would see with aging in the absence of Alzheimer's disease." It's an area of current research, he explains, but for now, there is a large reliance on longitudinal follow-up to validate preliminary diagnoses of mild cognitive impairment—even more so in the ID population than in the general population.

Clinical Relevance of the NIH Toolbox
Given the difficulties currently involved in measuring cognitive change in the ID population, Esralew says the findings from UC Davis are of interest, even though the research has not involved older adults to this point. Any tool that can measure specific skill sets such as attention, memory, or executive function could, if normed in an older population, be very useful to both psychologists and physicians as they seek to measure the impact of interventions for patients with ID. "Right now, I don't have a tool that helps me gauge whether someone has benefited from an intervention," Esralew says.

According to Keller, however, the clinical utility of the NIH Toolbox for evaluating cognitive change in adults with ID is likely limited, for pragmatic reasons. "The problem in a lot of respects with these neuropsychological tests is who is going to perform them. Who has the ability to do the test; who's going to pay for the tests?" he says. Many individuals with ID, especially older adults, receive their care from community providers, not university physicians or academic centers, he says, and practitioners in these settings need to be able to offer cognitive tests that are relatively quick and easy. "A lot of the neuropsychological tests [such as the NIH Toolbox] are exhaustive and time-consuming, and not a lot of people, not a lot of locations, can do them on a regular basis."

In a clinical setting, Keller says, a better tool for assessing cognitive decline and screening for dementia in the ID population is the Early Detection Screen for Dementia (EDSD) developed by the National Task Group on Intellectual Disabilities and Dementia Practices (NTG). The EDSD, which is designed for adults with ID, including Down syndrome, contains an adaptive behavior rating scale completed either by family or professional caregivers with the idea of tracking changes in cognitive function and behavior that are likely indicative of dementia. Although the tool is not designed for diagnosis, it can alert clinicians to the need for further investigation. According to Keller, the NTG is currently pushing for all adults with Down syndrome to be assessed using the EDSD beginning at the age of 40 in order to determine baseline scores; baselines are recommended for individuals with all other intellectual disabilities beginning at the age of 50.

Importance of Accurate Testing for Cognitive Decline in the ID Population
Whether or not the NIH Toolbox battery in particular may have clinical utility for older adults with ID, the issue of measuring cognitive change and screening for dementia in individuals with ID is an important one, Esralew says. Currently, she says, diagnostic overshadowing is a significant problem. "There is a tendency to attribute everything to the person's developmental disorders and thereby to miss other factors such as depression, delirium, anxiety, extreme reaction to psychosocial stressors, and of course, neurocognitive disorders."

Ultimately, an accurate diagnosis is important for quality of life, because it impacts both treatment and support options, Silverman says. "It's very important to think about the strategies for ensuring that everybody has the highest quality of life that could be achieved for them," he says.

— Jamie Santa Cruz is a freelance writer based in Englewood, Colorado.

1. Hessl D, Sansone SM, Berry-Kravis E, et al. The NIH Toolbox Cognitive Battery for intellectual disabilities: three preliminary studies and future directions. J Neurodev Disord. 2016;8(1):35.