Contrasting metabolic impairment in frontotemporal degeneration and early onset Alzheimer's disease

[18F]FDG positron emission tomography (PET) scans of 14 patients comprising the clinical prototypes of dementias that are considered to be associated with frontotemporal lobar degeneration (FTLD) were compared to a population of 15 patients with early onset Alzheimer's disease (EOAD). The FTLD group included patients with frontotemporal dementia (FTD), semantic dementia (SD), and primary progressive aphasia (PPA). A voxel to voxel group comparison identified metabolic impairment in the bilateral ventromedial frontal area, the left anterior insula, and inferior frontal cortex, and indicated the right middle temporal gyrus to exhibit increased activity in FTLD compared to EOAD patients. All identified cortical structures are considered to be critically involved in neuropsychological features associated with FTLD (altered social behavior, aphasia) and EOAD (impaired linguistic and visuo-constructive abilities). In conjunction with recent insights from neuropathologic investigations, these results implicate that the a priori heterogeneous prototypes of FTLD (FTD, SD, PPA) may share more common ground than previously assumed, and therefore would become distinguishable as an entire group from EOAD.