Position paper on diagnostic criteria for Alzheimer disease.

The lesions of Alzheimer disease (AD) consist of synapse and neuron loss associated with progressive deposition of amyloid as diffuse and neuritic plaques and accumulating tau abnormalities in the form of neurofibrillary tangles and neuropil threads. Diagnostic criteria for Alzheimer disease constitute arbitrary cut-off levels above which AD is deemed to exist, and below which lesser amounts of the same abnormalities are relegated to the nebulous category of aging changes. Demanding neocortical tangles for a diagnosis of AD sacrifices sensitivity on the altar of specificity, since, while such lesions usually represent an advanced stage in the orderly evolution of AD, lighter burdens of plaque-predominant AD pathology with tangles confined to the medial temporal lobe can cause dementia when associated with concomitant synapse loss. Such muted AD pathology typifies the Lewy body variant of AD, and it serves to segregate it from pure Lewy body disease. We endorse the semiquantitative neuritic-plaque based criteria from CERAD for routine diagnosis, and Braak staging with descriptive profiling of AD lesions in a research context.