Abstract
Alzheimer's disease (AD) is slowly progressive with prominent memory disturbance in early stage. As the disease progresses, other cognitive domains become involved. Although current diagnostic criteria require memory impairment as a feature of ‘typical’ AD, there are ‘atypical’ forms of AD in which the most salient impairments are in non-memory domains. Such phenotypes include posterior cortical atrophy, logopenic variant of progressive aphasia, and frontal variant of AD. These findings have recently been extended to the non-amnestic mild cognitive impairment (MCI) as a dysexecutive form or visuospatial variant of MCI. Especially, some executive dysfunctions are detected early in the course of AD and MCI, and may be useful to predict conversion from MCI to AD and progression of AD.
Dementia with Lewy bodies (DLB) tends to exhibit greater visuoperceptual, attentional and executive impairment and less pronounced memory impairment than AD. In the clock drawing test (CDT) , both AD and DLB patients tend to be equally impaired in drawing from memory, whereas DLB patients are more impaired in copying than AD. This pattern indicates a primary memory problem in AD and primary visual-perceptual and executive problems in DLB. Actually our SPECT studies reveal that the CDT impairment in AD is associated with decreased activity in the left posterolateral temporal cortex, which has an important role in semantic memory processing. In contrast, the CDT impairment in DLB is closely related to dysfunction of the frontal-subcortical network relevant to control of visuospatial attention and arousal, involving the frontal eye fields, supplementary eye fields, putamen and the thalamus. These features support the concept that DLB is a cortical-subcortical dementia, whereas AD is a cortical dementia.
