In a series of 128 subjects with prospective neuropsychological and neuropathological evaluation, Crystal et al1 identified 20 demented patients who did not meet the pathological criteria for well-characterized degenerative dementias, such as Alzheimer disease (AD) or dementia with Lewy bodies (DLB). These patients were diagnosed with dementia of unknown etiology (DUE), including 11 patients with hippocampal sclerosis, 7 with leukoencephalopathy and/or multi-infarct dementia, and 2 others. The mean ± SD age of these subjects was 89.1 ± 5.8 years, compared with 79.9 ± 11.4 years for subjects with AD and 86.7 ± 6.1 years for nondemented controls. While 5% of demented patients who died in their 70s and 21% of those who died in their 80s had DUE, 48% of those who died in their 90s had DUE.
Jellinger K. Frequency of "Dementia of Unknown Etiology" Increases With Age. Arch Neurol. 2001;58(9):1498–1499. doi: