**2.8 Neuropathology**

As discussed in the section above, a definitive diagnosis of AD can only be made at postmortem examination. Unfortunately the number of autopsies has declined markedly in the last three decades depriving clinicians and researchers alike of an important source of medical knowledge. Neuropathological examinations are now largely restricted to a research environment with the potential issue that clinically confirmed cases are not be representative of the AD spectrum in the greater population.

As with the clinical diagnosis, there are established criteria for the pathological diagnosis of AD (Hyman and Trojanowski, 1997) although these too are currently being revised. Interestingly, although postmortem examination is required for a definitive diagnosis of AD the pathological criteria again, only suggests that AD is 'probable' or 'possible'. In order to understand how these criteria have been derived it is useful to give a brief overview of the characteristic or pathognomonic aspects of AD.
