Cumulative Head Impact Exposure Predicts Later-Life Depression, Apathy, Executive Dysfunction, and Cognitive Impairment in Former High School and College Football Players

Abstract

The term "repetitive head impacts" (RHI) refers to the cumulative exposure to concussive and subconcussive events. Although RHI are believed to increase risk for later-life neurological consequences (including chronic traumatic encephalopathy), quantitative analysis of this relationship has not yet been examined because of the lack of validated tools to quantify lifetime RHI exposure. The objectives of this study were: 1) to develop a metric to quantify cumulative RHI exposure from football, which we term the "cumulative head impact index" (CHII); 2) to use the CHII to examine the association between RHI exposure and long-term clinical outcomes; and 3) to evaluate its predictive properties relative to other exposure metrics (i.e., duration of play, age of first exposure, concussion history). Participants included 93 former high school and collegiate football players who completed objective cognitive and self-reported behavioral/mood tests as part of a larger ongoing longitudinal study. Using established cutoff scores, we transformed continuous outcomes into dichotomous variables (normal vs. impaired). The CHII was computed for each participant and derived from a combination of self-reported athletic history (i.e., number of seasons, position[s], levels played), and impact frequencies reported in helmet accelerometer studies. A bivariate probit, instrumental variable model revealed a threshold dose-response relationship between the CHII and risk for later-life cognitive impairment (p < 0.0001), self-reported executive dysfunction (p < 0.0001), depression (p < 0.0001), apathy (p = 0.0161), and behavioral dysregulation (p < 0.0001). Ultimately, the CHII demonstrated greater predictive validity than other individual exposure metrics.

Keywords: behavior; cognition; concussion; football; long-term impairment; subconcussive impacts.

FIG. 1.

Schematic of the dose-response model with a constant baseline risk of later-life impairment (baseline gradient of slope = 0) below the cumulative head impact threshold dose and with increasing probability of impairment (dose-response gradient of slope >0) above that threshold dose. Color image is available online at www.liebertpub.com/neu

FIG. 2.

Each dichotomous outcome measure (A–F) was fit using a bivariate probit, an instrumental variable model adjusted for both age and education as well as our predictor variable of interest (cumulative head impact index [CHII]). Baseline risk refers to the risk of impairment at the CHII dose range below the thresholds identified in Table 6. Above these thresholds, a significant linear dose-response relationship between CHII exposure and later-life clinical impairment was found for all outcomes. See Table S1 for tabular data (see online supplementary material at http://www.liebertpub.com)