Memory impairment, but not cerebrovascular disease, predicts progression of MCI to dementia

1report a cohort of subjects with mild cognitive impairment (MCI), investigating the impact of cerebrovascular disease on progression of MCI to dementia. The authors conclude that cerebrovascular risk factors did not predict progression to dementia. Given the methodologic limitations of the data analytic techniques employed in the evaluation of the relationship between cerebrovascular risk factors and progression to dementia, the assertion that cerebrovascular risk factors did not predict progression to dementia is incautious. First, the sample size in the investigation was limited to 52 subjects with 17 (33%) of those progressing to dementia and 35 (67%) remaining nondemented at the end of the observation period. As the sample was limited to a very small number of MCI converters to dementia, sophisticated statistical analytic tools such as logistic regression analysis cannot be used to examine the relationship between increasing cerebrovascular risk and incipient dementia. Instead, the authors employed t-test as a method of comparison of baseline cerebrovascular risk in the groups of dementia converters vs nonconverters, a methodology that lacks the sensitivity of logistic regression to evaluate relationships between multiple independent variables and discrete dependent variables (dementia and nondementia). Second, the authors created an ordinal scale of composite vascular risk drawn from six cerebrovascular risk factors, and calculated these as a sum from 0 to 6 of the presence of any of these factors. The authors have assumed, given the equal interval nature of their scale, that the difference in risk between having no cerebrovascular risk factors and having two is the same as the difference between having four cerebrovascular risk factors and having six. This may not be true. The authors comment that only 36% of subjects were free of cerebrovascular risk factors and 22% had three or more vascular risk factors, yet the mean composite risk for the dementia converters was 0.94 with SD 1.03 and for the non-converters was 1.67 with SD 1.60. Given the high degree of variance evident from the SDs, using the median of the entire sample for evaluation of central tendency and comparing “hi” and “low” interquartile ranges of cerebrovascular risk nonparametrically for differences in the number of subjects progressing to dementia may have yielded more information. Larger studies are needed to accurately estimate in the population the extent to which cerebrovascular risk impacts conversion from MCI to dementia.