Amyloid Clearance and Transient CSF Aβ40 Reduction in a Case of ARIA-E/H Following Lecanemab Treatment

We herein report a case of amyloid-related imaging abnormality (ARIA)-E/H following lecanemab treatment in a 70-year-old man with mild cognitive impairment due to Alzheimer's disease. Generalized seizures occurred after the third infusion and were accompanied by FLAIR hyperintensity, microbleeds, and a minor acute infarct. Amyloid PET revealed focal clearance of amyloid plaques in the ARIA-affected regions. CSF Aβ40 levels transiently decreased by ~30% during the ARIA episode, whereas Aβ42 remained stable, thereby increasing the Aβ42/40 ratio. These findings suggest that ARIA may facilitate focal amyloid clearance and that CSF Aβ40 reduction may serve as a potential biomarker for ARIA onset and resolution.