Relationship between depression and neurodegeneration: risk factor, prodrome, consequence, or something else? a scoping review

Background: The link between depression and neurodegeneration is complex and unclear. It is debated whether depression is a risk factor, a prodrome, a consequence, or unrelated. Objectives: This review examines these possibilities to clarify their connection, focusing primarily on Alzheimer’s disease, vascular dementia, Parkinson’s disease, and other highly comorbid neurodegenerative diseases. Methods: Eligibility criteria: The stud- ies included in this review focused on neurodegenerative diseases with high comorbidity with depression, published in peer-reviewed English-language journals, providing empiri- cal evidence on the link between the two conditions or theoretical frameworks that point to other studies. Non-human studies and those irrelevant to this connection were excluded. Source of evidence: AI-supported tools identified relevant articles. Results: Most studies suggest depression may contribute to neurodegeneration, but clinical, neuroimaging, and longitudinal evidence also support its role as a prodrome or consequence, indicating a bidirectional relationship. Conclusions: Despite extensive research, the connection remains unclear, highlighting the need for further investigation into the underlying mechanisms and interdependencies, focusing on longitudinal studies by examining causality.