Two distinct subgroups of senile dementia of Alzheimer type: quantitative study of neurofibrillary tangles.

The heterogeneity of senile dementia of Alzheimer type (SDAT) has been suggested by some authors clinically and neuropathologically. The heterogeneity of SDAT was investigated based on quantification of NFT combining Braak and Braak's neuropathological staging and the density of NFT in various areas of the cerebral cortex. Brain tissues were examined from 16 autopsy cases with clinically late onset dementia (> age 65) and neuropathologically diagnosed dementia of Alzheimer type (DAT). Gallyas-Braak staining was used for the quantification of NFT. The density of NFT was examined in the precentral gyrus, middle temporal gyrus (T2), parahippocampal gyrus and the amygdaloid nucleus. The 16 cases studied were divided into two groups depending on the number of NFT in the cortex (cut-off score: 5/mm2): the AD-like group (NFT > or = 5/mm2) and the common group (NFT < 5/mm2). The density of NFT in the precentral gyrus (t(3.225) = -9.007, P = 0.002) and T2 (t(3.365) = -3.774, P = 0.027) in the AD-like group was significantly higher than those in the common group. However, no significant difference was observed in the parahippocampal gyrus between the two groups (t(14) = -0.318, NS). Moreover, there were no significant differences between the two groups as regards age at onset and the duration of the illness. The present study revealed the possible existence of two subgroups in SDAT having significantly different NFT densities in various areas of the cerebral cortex without any significant difference in their duration of illness. This classification has no relationship to Braak and Braak's staging, which depends only on the distribution of NFT, irrespective of their density. Arai et al. revealed that the NFT density in AD was significantly higher than in SDAT. We suggest that the neuropathological findings of the AD-like group in SDAT resemble those of presenile AD.