The most common studies of cognition in VaD have been attempts to distinguish it from AD. This line of research has most often investigated whether semantic abilities in AD are worse than in those with VaD and whether the deficits noted in VaD cases are consistent with a frontal-subcortical dementia (Mast, MacNeill, & Lichtenberg, 2002). A number of studies have attempted to differentiate dementia groups via cognitive patterns and overall abilities. These are reviewed more thoroughly in the Mast et al. study. Overall, the studies suffer from several methodological deficiencies. First, most of the studies use small sample sizes and, therefore, are underpowered. Second, and perhaps most importantly, the MMSE is used as a control for overall levels of dementia. The use of a screening measure to control for overall level of impairment is problematic. It is no surprise, then, that the results of the studies reviewed by Mast and colleagues exhibit no clear pattern.
Mast et al. (2002) demonstrated a new methodology for neuropsychological differential diagnosis research and applied this methodology to investigating for differences between those with dementia and stroke and those with dementia and no stroke. The MIMIC model (multiple indicators, multiple causes) is an extension of confirmatory factor analysis and a specific application of structural equation modeling. The model contains one or more latent variables, which are simultaneously identified by multiple endogenous indicators and multiple exogenous variables. Thus, in their study, the unique contributions of stroke to dementia is measured by controlling the latent variable of global impairment as calculated across a variety of neuropsycho-logical tests. In their study of 217 persons with dementia, stroke did not exhibit unique effects on cognition. Given the overlap between AD and VaD, this finding was not surprising.
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