Tau-negative amnestic dementia masquerading as Alzheimer disease dementia.

OBJECTIVE: To describe the phenomenon of tau-negative amnestic dementia mimicking Alzheimer disease (AD) clinically and radiologically and to highlight the importance of biomarkers in AD research.
METHODS: Eight participants with amnestic mild cognitive impairment or AD dementia were evaluated by a behavioral neurologist and had a standardized neuropsychological battery performed. All participants completed structural (MRI) and molecular (amyloid and tau PET) imaging. AD-signature thickness and adjusted hippocampal volume served as structural biomarkers, while standardized uptake value ratios (SUVRs) from validated regions of interest for amyloid and tau PET were used to determine molecular biomarker status.
RESULTS: All participants were thought to have AD as the primary driver of their symptoms before any PET imaging. All participants had hippocampal atrophy, and 2 participants fell below the AD-signature thickness cutoff for elderly controls (2.57), with a further 3 falling below the more stringent cutoff based on young controls (2.67). Four participants were amyloid positive (SUVR >1.42), and all were tau negative (SUVR <1.33).
CONCLUSIONS: The participants presented here were clinically impaired, with structural imaging evidence of neurodegeneration, in the absence of any significant tau accumulation. Therefore, AD is unlikely as a cause of their clinical presentation and neurodegenerative imaging findings. Several implications are discussed, including the need to establish amyloid and tau positivity in N+ participants before enrolling them in trials of disease-modifying therapy agents for AD.