Search strategy and study selection

This PROSPERO-registered (CRD42022352780) systematic review and meta-analysis was reported in accordance with the guidelines for Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA)²⁶ and Meta-analysis Of Observational Studies in Epidemiology (MOOSE).²⁷ We identified relevant literature by searching PubMed (Medline) and Embase databases between their inception and April 23 2023. We used combinations of free text and controlled terms in 2 categories (Supplemental Box S1): the exposure (i.e., specific sports such as boxing, soccer, martial arts, and rugby), and the outcome (i.e., dementia and its subtypes such as Alzheimer’s disease and vascular dementia). We also scrutinised the reference sections of retrieved articles.

We included a published paper if it fulfilled the following criteria: utilised a cohort study design; a comparison of dementia occurrence was made between a group of former contact sports athletes and unexposed (general population) or lesser-exposed (former athletes from non-contact sports) controls; the diagnosis of dementia outcomes was made by a healthcare professional as part of a medical examination in either a research or clinical setting (studies based on self-reports of physician diagnosis and cognitive tests did not qualify); reported standard estimates of association (e.g., relative risk, odds ratios, hazard ratios) and variance (e.g., confidence interval, standard error); appeared in a peer-reviewed journal; and was published in English. Two authors (GDB and PF) independently screened the identified records first by title, then abstract, and, if necessary, the full paper. Any discrepancies were resolved on discussion.