“Silent stroke”: An oxymoron meaning “dementia”

Brain MRI demonstrates the presence, in a large percent of elderly patients, of ischemic cerebrovascular disease (ICVD), usually as white-matter lesions and lacunar strokes. These lesions are considered clinically silent because they fail to produce sensory or motor deficits. Nonetheless, these lesions disrupt prefrontal-subcortical circuits that mediate executive control functions (ECF). The loss of ECF is manifested as behavioral and mood changes and as executive dysfunction leading to loss of autonomy in activities of daily living and may escape detection. Many cases are likely to be diagnosed as “Alzheimer disease” (AD) instead. This is because preclinical levels of AD pathology (ie, sufficient to affect hippocampal and memory function) are very common in the elderly. Mild-cognitive impairment (MCI) has been estimated to affect 40% of persons over the age of 65, in contrast to “dementia,” which affects at most 15%. However, memory impairment alone is insufficient for the diagnosis of dementia. Other cognitive domains must also be affected, and these must be sufficient to explain disability. It is very likely that comorbid “silent” frontal system ICVD converts a significant fraction of MCI cases to “AD.” This explains, first, the high prevalence of frontal system ICVD in clinically diagnosed AD; second, the inverse association between ICVD and Braak stage in clinically diagnosed AD; and third, the recent identification of traditional risks factors for ICVD as being also risk factors for AD.