Underlying pathology identified after 20 years of disease course in two cases of slowly progressive frontotemporal dementia syndromes

We report two cases from the frontotemporal lobar degeneration (FTLD) spectrum with a remarkably slow progression. The first case demonstrated insidious-onset behavioral symptoms and personality changes resembling behavioral variant of frontotemporal dementia, followed by a benign course of over 26 years, and his brain autopsy revealed the diffuse form of argyrophilic grain disease. The second case presented with slowly progressive cognitive and motor deficits, reminiscent of the corticobasal syndrome, deteriorated very slowly over 22 years and his brain autopsy revealed FTLD-TDP with C9ORF72 pathology. These two cases confirm the notion of slowly progressive frontotemporal lobar degeneration caused by an underlying FTLD pathology, rather than a phenocopy.