Associations Between Cumulative Head Trauma and Self‐Reported Parkinsonism and Parkinson's Disease in Former Soccer Players

BACKGROUND: Cumulative head trauma, encompassing repetitive head impacts (RHI) and traumatic brain injury (TBI), has been associated with later-life neurodegenerative disease. Studies on parkinsonism/Parkinson's disease (PD) have evaluated RHI and TBI in isolation, yielding mixed results. Cumulative head trauma may better predict PD risk. OBJECTIVE: To examine independent and cumulative effects of RHI through soccer participation and TBI history on PD diagnosis and age at diagnosis. METHODS: We used a cross-sectional cohort design with data from the Fox Insight study. Participants completed self-report questionnaires on PD diagnosis, TBI (no TBI, TBI with loss of consciousness [LOC], TBI without LOC), and sport participation. Multivariable logistic regressions examined the effects of soccer, TBI group, and their interaction on PD diagnosis. Cox proportional hazards examined associations with time to PD diagnosis. Covariates included age, sex, race, education, income, body mass index, heart disease/diabetes, and family PD history. RESULTS: Among 1231 participants (mean[SD] age = 66.12[10.60]), 278 played soccer (953 non-contact and collision sport [CCS] athletes). Participants reported no TBI (390[33%]), TBI without LOC (495[43%]), or TBI with LOC (258[23%]). Overall, 768(62%) reported a PD diagnosis (mean[SD] age at diagnosis = 59.02[9.86]). A significant soccer × TBI interaction indicated that soccer players with TBI with LOC had 2.6× higher odds of PD compared to non-CCS athletes without TBI (OR [95% CI] = 2.600[1.085-6.305], P = 0.033). Former soccer players with TBI with LOC had a 15.6% higher hazard of early PD diagnosis (HR = 1.469, P = 0.027) compared to non-CCS athletes with no TBI. CONCLUSION: Cumulative head trauma (RHI + TBI) increased PD risk and lowered age at onset.