Differing patterns of temporal atrophy in Alzheimer's disease and semanticdementia

Objective: To characterize and quantify the patterns of temporal lobe atrop hy in AD vs semantic dementia and to relate the findings to the cognitive p rofiles. Medial temporal lobe atrophy is well described in AD. In temporal variant frontotemporal dementia (semantic dementia), clinical studies sugge st polar and inferolateral temporal atrophy with hippocampal sparing, but q uantification is largely lacking. Methods: A volumetric method for quantify ing multiple temporal structures was applied to 26 patients with probable A D, 18 patients with semantic dementia, and 21 matched control subjects. Res ults: The authors confirmed the expected bilateral hippocampal atrophy in A D relative to controls, with involvement of the amygdala bilaterally and th e right parahippocampal gyrus. Contrary to expectations, patients with sema ntic dementia had asymmetric hippocampal atrophy, more extensive than AD on the left. As predicted, the semantic dementia group showed more severe inv olvement of the temporal pole bilaterally and the left amygdala, parahippoc ampal gyrus (including the entorhinal cortex), fusiform gyrus, and the infe rior and middle temporal gyri. Performance on semantic association tasks co rrelated with the size of the left fusiform gyrus, whereas naming appeared to depend upon a wider left temporal network. Episodic memory measures, wit h the exception of recognition memory for faces, did not correlate with tem poral measures. Conclusions: Hippocampal atrophy is not specific for AD but is also seen in semantic dementia. Distinguishing the patients with semant ic dementia was the severe global but asymmetric (left > right) atrophy of the amygdala, temporal pole, and fusiform and inferolateral temporal gyri. These findings have implications for diagnosis and understanding of the cog nitive deficits in AD and semantic dementia.