Comorbid neuropathologies but not Braak stage influence cognitive impairment in primary age-related tauopathy

Primary age-related tauopathy (PART) is a β-amyloid-independent tauopathy, thought by some to be a distinct process from Alzheimer disease neuropathologic change (ADNC). Two categories of PART have been defined: definite PART (those with complete absence of β-amyloid deposition) and possible PART (those with minimal and restricted β-amyloid distribution). It is unclear whether there is any significant cognitive effect of "isolated" or "pure" PART, as opposed to ADNC, in which cognitive decline mirrors pathologic progression. We evaluated the effects of neurodegenerative pathologies on longitudinal cognitive decline using a combination of univariate analysis, multivariable logistic regression analysis, and variance decomposition in patient cohorts with neuropathologically confirmed definite PART (n = 174) and possible PART (n = 182). ADNC-related pathologies did not significantly contribute to cognitive impairment in either cohort. Cognitive decline in definite PART was instead dependent on the presence and severity of TDP-43 pathology/limbic-predominant age-related TDP-43 encephalopathy (LATE) stage, Lewy body pathology, and arteriolosclerosis, while cognitive impairment in possible PART was primarily affected by hippocampal sclerosis and infarcts. Additionally, 67.8%-75.7% of variance in cognitive decline was unaccounted for by these neurodegenerative pathologies. These results indicate that PART pathology in isolation does not significantly impair cognition, which is instead primarily influenced by comorbid neuropathologic features.