Traumatic Brain Injury and Risk of Amyotrophic Lateral Sclerosis.

Importance: History of traumatic brain injury (TBI) or repetitive head impacts is associated with an increased risk of neurodegenerative disease. This association has attracted attention in recent years through the relationship between contact sports participation and the increased risk of a number of neurodegenerative diseases, including motor neuron disease or amyotrophic lateral sclerosis (ALS). However, to date, the association between TBI in the community and ALS risk remains uncertain. Objective: To leverage population-level health records to explore the association between a history of TBI and subsequent ALS risk. Design, Setting, and Participants: This retrospective cohort study accessed UK-wide electronic health record (EHR) data from individuals 18 years or older with TBI history and age-, sex-, and area deprivation-matched general population comparators. EHR data were available from January 1, 2005, to December 31, 2020, with database interrogation performed on February 11, 2021, and data analysis conducted between June 1, 2023, and October 3, 2024. Exposure: Documented history of TBI. Main Outcomes and Measures: Outcomes were obtained by individual-level linkage to EHR data available via Clinical Practice Research Datalink. Risk of ALS was evaluated using Cox proportional hazards regression models to investigate its association with TBI. Results: Overall, 85 690 adults with a history of TBI and 257 070 matched adults with no history of TBI were included, for a total of 342 760 participants (50.1% male; mean [SD] age, 50.7 [17.6] years). During a median 5.72 (IQR, 3.07-8.82) years of follow-up, providing 2.13 million person-years of follow-up, 150 incident ALS cases were recorded, resulting in 7.05 cases per 100 000 person-years. Risk of ALS was higher among individuals with a history of TBI compared with individuals without a TBI history (hazard ratio [HR], 2.61; 95% CI, 1.88-3.63). However, this association was time dependent, with risk confined to the 2 years following TBI (HR, 6.18; 95% CI, 3.47-11.00), but not thereafter. Conclusions and Relevance: In this retrospective cohort study of 342 760 adults, an association between TBI and subsequent risk of ALS was identified. However, this association was confined to the 2 years immediately following injury. As such, the association between TBI and higher ALS risk may indicate reverse causality, with TBI in some individuals perhaps reflecting a consequence of early, subclinical ALS.