Screening tools are used to identify people with possible dementia and to refer them for diagnosis and services. There is wide variability in cognitive functioning of individuals with IDD, therefore experts suggest that it is not advisable to use standardized neuropsychological tests to assess for decline in functioning with this population. Also, the results cannot be compared to general population norms, and many neuropsychological tests have not been validated for use in individuals with developmental ages less than 5 or 6 years (O'Caoimh et al., 2013). It is also difficult to discern dementia-related memory decline from a person with IDD because he or she may never have developed the specific level of cognitive skills measurable by standard memory tests (Aylward et al., 1997; McCarron et al., 2014; Pyo, 2011). Individuals with intellectual disabilities may not perform well on many neuropsychological tests because they require good communication and dexterity, intact sensory function, and good compliance by the individual undergoing the test (McCarron et al., 2014; Prasher, 2004). Sensory changes such as hearing and vision impairments are common in individuals with Down syndrome and may also affect their ability to participate in cognitive testing. It is important to use longitudinal screening approaches so that changes can be measured over time and to consider changes in not only cognition but also personality, behavior, and ADLs when evaluating for dementia (O'Caoimh et al., 2013).
There are a variety of screening instruments that are used with adults who have intellectual disabilities, and some instruments are not applicable for all persons with such disabilities. For instance, some measures may be at such a low level as to make the detection of initial cognitive changes very difficult to identify in individuals with intellectual disabilities who have higher intelligence. These types of measurement problems are called "ceiling effects" (Hutchinson & Oakes, 2011). Other instruments such as those used for the detection of dementia among the general population are not suitable for people with intellectual disabilities because of a "floor effect," meaning that the questions are too difficult for the individuals being tested (Deb et al., 2007b).
There are different types of dementia screening instruments for people with IDD, including observer rated scales, direct neuropsychological tests, and adaptive behavior measures. Some instruments are easily administered and can be used by family members or staff, and others are more complicated and require a provider who has undergone specialized training. Observer rated scales involve the reporting of behavioral changes by a knowledgeable informant. To establish whether there has been a change in cognitive status, observer rated scales should be used with caregivers or relatives who have known the person for a long period of time (Ball et al., 2004; Deb et al., 2007b). Observer rated scales appear to be more useful for the diagnosis of dementia in people with an intellectual disability than neuropsychological tests (Deb & Braganza, 1999). Observer rated scales include the Dementia Screening Questionnaire for Individuals with Intellectual Disabilities (DSQIID), Dementia Questionnaire for Persons with Mental Retardation (DMR), and CAMDEX, an adaptation of the Cambridge Examination for Mental Disorders of Older People with Down Syndrome.
Neuropsychological tests are objective and standardized measures of an individual's performance on prescribed tasks known to be linked to a specific brain structure or pathway. Some neuropsychological tests originally developed for the diagnosis of dementia in the populations without an intellectual disability have been modified for use in people with such disabilities (Deb & Braganza, 1999). Modified tests include the Severe Impairment Battery (SIB), Test for Severe Impairment (TSI), and Down Syndrome Mental Status Examination (DSMSE).
The inadequacies of using cognitive testing with people living with IDD led to the development of instruments that assess an individual's adaptive behavior. Adaptive behavior is defined as a person's ability to cope on a day-to-day basis with the demands of his or her environment (Hutchinson & Oakes, 2011). Adaptive behavior measures for assessing cognitive impairment in people with IDD use informants to report an individual's ability to function socially and perform ADLs. These measures include Adaptive Behavior Dementia Questionnaire (ABDQ), Adaptive Behavior Scale (ABS), and Daily Living Skills Questionnaire (DLSQ). Regardless of the choice of instrument, the National Task Group Recommendations for the Evaluation and Management of Dementia (Moran et al., 2013) emphasize the importance of using the same instrument consistently over time to recognize change in relation to the person's baseline.