Neural correlates of episodic memory in behavioral variant frontotemporal dementia

Impaired episodic memory is currently an exclusion criterion for behavioral variant frontotemporal dementia (bv-FTD), although prior studies have shown that neuropsychological memory performance varies from very impaired to intact in such patients. Our study investigated i) whether this variability might be due to the admixture of true bv-FTD and phenocopy syndrome patients and ii) the neural correlates of episodic memory deficits in bvFTD. Groups of patients with true bvFTD (n = 14), phenocopy syndrome (n = 6), Alzheimer's disease (AD) (n = 14), and healthy controls (n = 15) underwent memory testing and had MRI scanning with ratings of regional brain atrophy. Phenocopy patients did not differ to controls on memory scores or atrophy ratings. By contrast, bvFTD and AD patients were impaired on both measures in comparison to controls and more importantly, bvFTD and AD did not differ on memory scores. Atrophy patterns differed, with AD showing typical medial temporal lobe atrophy, while bvFTD patients had predominantly prefrontal cortex atrophy. In bvFTD neuropsychological memory performance correlated with frontal atrophy ratings while in AD significant correlations were found between memory and both medial temporal lobe and frontal atrophy ratings. Taken together, out data shows that bvFTD patients can show a similar degree of episodic memory impairment on neuropsychological tests to AD patients, however, the neural correlates differ. The previously variable reported memory performance in bvFTD is likely due to the inclusion of phenocopy patients, who are mostly undistinguishable from controls. These findings have implications for the diagnosis of bvFTD.