In the last 2 years, there have been 3 successful trials of antiamyloid antibodies in Alzheimer disease (AD): aducanemab, now controversially US Food and Drug Administration-approved under the accelerated approval pathway1; lecanemab, now FDA-approved2; and donanemab, now going through the approval process.3 All 3 share a common broad mechanism, that is, antibody-mediated removal of β-amyloid (Aβ) from the brain, and this is almost certainly the basis of their therapeutic action.4 When used in the earliest symptomatic stages of AD, all have modest clinical effects, all clear Aβ from the brain, and all show evidence for some changes in molecular markers believed to be downstream of Aβ accumulation in keeping with disease modification.4 However, all these drugs—and several other antiamyloid immunotherapies that failed to show positive effects in clinical trials (e.g. bapineuzemab and gantenerumab)5,6—have the troubling adverse event of antibody-related imaging abnormalities (ARIA). ARIA can take the form of vasogenic edema or sulcal effusion (ARIA-E) or haemosiderin deposition due to hemorrhage (ARIA-H).7 In vivo, ARIA is detected using MRI: ARIA-E is visible on fluid attenuation inversion recovery sequences; ARIA-H is best seen on iron-sensitive (T2* or susceptibility-weighted imaging) as microbleeds and/or superficial hemosiderin deposition. The pathophysiology of ARIA has yet to be fully determined but may result from antibody-mediated breakdown of amyloid plaques releasing Aβ which is deposited in vessels leading to increased cerebral amyloid angiopathy or alterations in perivascular clearance or inflammation, possibly through complement activation.8
