Frontotemporal dementia and semantic dementia: anatomic variations on the same disease or distinctive entities?

Research into the non-Alzheimer dementias has exploded over the last 2 decades, and frontotemporal lobar degeneration has emerged as the most common cause of dementia in patients below the age of 65. In 1998, an international consortium of investigators focusing on this disease entity met and agreed upon the classification of 3 subtypes of frontotemporal lobar degeneration, using regional atrophy to distinguish them. These are: a frontally predominant form called frontotemporal dementia, a temporally predominant form called semantic dementia, and a left-frontally predominant syndrome, called progressive nonfluent aphasia. With the rise of genetic and neuropathologic findings, however, the usefulness of this subtype classification has been called into question. This paper discusses the major pertinent findings, and the implications of classifying the disease on the basis of atrophy versus genetic or molecular mechanisms.