Semantic dementia and primary progressive aphasia: a problem of categorization?

The relationship between semantic dementia (SD) and primary progressive aphasia (PPA) has been the subject of debate ever since the syndromes were first described, in converging streams of research from the neuropsychological and neurologic communities. The most salient clinical features of SD are anomia with circumlocution and semantic paraphasia, single-word comprehension deficit, and reduced category fluency. Of critical importance is the fact that patients also show deficits on non-verbal tasks using visual, auditory, and other modalities, suggesting that the key impairment in SD is a breakdown in conceptual knowledge rather than a specific problem with language. The finding of item consistency between the various tests supports this view. The order in which the features appear can be explained by the variable degree of redundancy in access to semantic knowledge from the different perceptual modalities. Atrophy is seen in the anterior and inferior temporal lobe rather than in classic language areas, further distancing SD from aphasic syndromes. Semantic dementia and progressive non-fluent aphasia (PNFA) share some clinical and pathologic characteristics with frontal variant frontotemporal dementia, but there are also clear differences between the three syndromes. We believe that many patients described as having fluent primary progressive aphasia in fact have early SD. Semantic dementia is a well-defined syndrome, distinct from PNFA but related to it within the spectrum of frontotemporal lobar degeneration syndromes.