Olfactory centres in Alzheimer's disease: olfactory bulb is involved in early Braak's stages

In Alzheimer's disease (AD), neurofibrillary tangles spread from the entorhinal cortex to the limbic system, then to neocortical areas, according to the Braak's stages. Olfaction is impaired in early stages of AD. The aim of this study was to describe the pathology of the cortical olfactory centres in relation to Braak's stages determining the earliest site of pathology. We examined 15 control and 15 AD cases. The primary olfactory cortices were involved in more advanced Braak's stages, while olfactory bulbs were damaged in very early (i.e. Braak's stage 0 or 1) stages. These results are supporting the fact that olfaction might be an early marker in AD and argues against the hypothesis that AD pathology is spreading through the olfactory system.