Memory resilience in Alzheimer's disease with primary progressive aphasia

Objective To determine whether memory is preserved longitudinally in primary progressive aphasia (PPA) associated with Alzheimer's disease (AD), and to identify potential factors that maintain memory despite underlying neurofibrillary degeneration of mediotemporal memory areas.

Methods Longitudinal memory assessment in 17 PPA patients with autopsy or biomarker evidence of AD (PPA-AD) and 14 patients with amnestic dementia of the Alzheimer type with AD at autopsy (DAT-AD).

Results In PPA-AD, episodic memory, tested with nonverbal items, was preserved at initial testing and showed no decline at retesting 2.35 ± 0.78 years later, at which time symptoms had been present for 6.26 ± 2.21 years. In contrast, language functions declined significantly during the same period. In DAT-AD, both verbal memory and language declined with equal severity. Although imaging showed asymmetric left sided mediotemporal atrophy in PPA-AD, autopsy revealed bilateral hippocampo-entorhinal neurofibrillary degeneration at Braak stages V and VI. In comparison to DAT-AD, however, the PPA-AD group had lower incidence of Apolipoprotein E4 and of mediotemporal TDP-43 pathology.

Conclusions Memory preservation in PPA is not just an incidental finding at onset but a core feature that persists for years despite the hippocampo-entorhinal AD neuropathology that is as severe as that of DAT-AD. Asymmetry of mediotemporal atrophy and a lesser impact of Apolipoprotein E4 and of TDP-43 on the integrity of memory circuitry may constitute some of the factors underlying this resilience. Our results also suggest that current controversies on memory in PPA-AD reflect inconsistencies in the diagnosis of logopenic PPA, the clinical variant most frequently associated with AD.