The majority of patients will have an underlying degenerative disease, whether it is Alzheimer’s disease, frontotemporal dementia, or dementia with Lewy bodies. As such, many of the investigations are essentially normal, but this can be difficult for the patient and the family to understand. Normal means that many other abnormalities have been excluded. Neither brain scan nor the other investigations can provide a definitive diagnosis of the underlying degenerative disease. This can only be established with certainty by examination of brain tissue. However, with careful clinical assessment, supplemented by investigations, a diagnosis can be made with approximately 80% accuracy. The 20% of uncertainty tends to be in identifying other degenerative diseases rather than missing a major reversible cause.
Increasingly, MRI scans can demonstrate tissue loss, which is common among degenerative diseases, and in a characteristic distribution, as in a frontotemporal diagnosis.
It is important to remember that the classification of diseases is there to help manage elderly patients. Elders and their problems are classified either at the level of the clinical presentations, such as memory impairment or dementia; or at a molecular level, in terms of specific DNA mutations in familial disease or particular abnormal proteins that are deposited.
Which classifications are chosen to determine when changes in cognition are abnormal is determined by what is helpful to clinicians and patients, as well as their families, in managing the distressing problems and behaviors that may develop.