A Study of Donanemab (LY3002813) in Participants With Early Cognitive Decline (TRAILBLAZER-ALZ 7)

Inclusion Criteria: * Have gradual and progressive cognitive decline for greater than or equal to ( ≥) 6 months. * Have least 1 core clinical feature of dementia with Lewy bodies (DLB). * Have a score ≥20 on Montreal Cognitive Assessment (MoCA). * Meet plasma P-tau217 criteria. * Have a cerebrospinal fluid (CSF) result consistent with the presence of brain amyloid pathology. * Have a CSF result consistent with the presence of alpha-synuclein pathology. * Have at least 1 reliable study partner who will provide written informed consent to participate, is in frequent contact with the participant, and is familiar with overall function and behavior, such as day-to-day activities and cognitive abilities. Exclusion Criteria: * Have a disease or condition that could interfere with this study or is a current serious or unstable illness. * Have, or is suspected to have, a significant neurological disease (other than the studied condition) that affects the central nervous system and may affect the individual's cognition or ability to complete the study. * Have a history of cancer that, in the investigator's opinion, has a high risk of recurrence and preventing the completion of the study. * Have clinically significant multiple or severe drug allergies, significant atopy, or severe posttreatment hypersensitivity reactions. * Have previously received amyloid-targeting therapy. * Active immunization against amyloid-beta. * Have a centrally read MRI that does not meet study entry criteria. * Have contraindication to MRI or PET scans. * Have any contraindication to lumbar puncture.