Assessing Exposure Time and Concussion Risk

The literature examining the risk of brain trauma in common physical activities beyond youth sports remains underdeveloped. We are therefore glad to see Campbell et al.¹ address the concussion rate in high school physical education (PE). Estimating concussion risk is challenging enough, but comparing rates and risk factors across activities presents even greater difficulties. For example, Campbell et al. note some potential sources of bias, such as the possibility that students playing sports “may be in better physical condition than students taking [PE], which is protective.”^(p94) We are writing to elaborate on several additional limitations concerning the exposure metric, cumulative incidence (CI), that affect comparisons of the risk of concussion across different contexts.

CI does not completely assess time-at-risk for any specific individual. First, as applied in the Campbell et al. article, CI would present an undercount for any student who sustains multiple concussions in a certain sport or PE class. Second, the length of the sports season does not allow a straight comparison with the semester- or year-long PE class. If a student suffered three concussions during a three-month football season, that individual would count for the same incidence as one who suffered only one concussion during nine months of PE. Third, it is unclear how the exposure time of athletes who play multiple sports is addressed. Fourth, the CI metric does not allow taking account of sports game and practice frequency compared with PE class sessions.

The athlete exposure metric would partially address some of these issues. Yet, athlete exposure often still falls short at measuring total time-at-risk. Not all athletes participate in all games, and in some sports (e.g., football), players often are specialized to play only a specific position. Moreover, some PE sessions involve classroom instruction and so no risk for physical activity–related injury.

Tracking exact hours and minutes of exposure across activities would offer the most precise comparison for risk of concussion. Yet, this measure is so cost prohibitive as to be impractical for statewide surveillance.

As epidemiologists have long noted, in studying many public health questions, the truth is in the denominator (Charles Poole, ScD, written communication, December 2017). We recommend that funders of concussion research prioritize supporting methods that more accurately capture exposure time. Identifying time-at-risk, although resource intensive, is critical for more accurately estimating concussion risks and making comparisons across a variety of organized sports and activities for youths.