Time Delta Head Impact Frequency: An Analysis on Head Impact Exposure in the Lead Up to a Concussion: Findings from the NCAA-DOD Care Consortium

Abstract

Sport-related concussions can result from a single high magnitude impact that generates concussive symptoms, repeated subconcussive head impacts aggregating to generate concussive symptoms, or a combined effect from the two mechanisms. The array of symptoms produced by these mechanisms may be clinically interpreted as a sport-related concussion. It was hypothesized that head impact exposure resulting in concussion is influenced by severity, total number, and frequency of subconcussive head impacts. The influence of total number and magnitude of impacts was previously explored, but frequency was investigated to a lesser degree. In this analysis, head impact frequency was investigated over a new metric called 'time delta', the time difference from the first recorded head impact of the day until the concussive impact. Four exposure metrics were analyzed over the time delta to determine whether frequency of head impact exposure was greater for athletes on their concussion date relative to other dates of contact participation. Those metrics included head impact frequency, head impact accrual rate, risk weighted exposure (RWE), and RWE accrual rate. Athletes experienced an elevated median number of impacts, RWE, and RWE accrual rate over the time delta on their concussion date compared to non-injury sessions. This finding suggests elevated frequency of head impact exposure on the concussion date compared to other dates that may precipitate the onset of concussion.

Keywords: Concussive threshold; Head impact exposure; Sport-related concussion; Subconcussive; Traumatic brain injury.

Figure 1

Violin plots showing each athlete's time delta (time from the first head impact of their session to the concussive event) for injuries occurring during each the three different session types: Competition, Regular Season Practice, Preseason Practice. Violin plots shown use width to show density of data points. A box plot is overlaid showing the 1st, 2nd, 3rd, and 4th quartiles. ***p-value < 0.005, **p-value < 0.01.

Figure 2

Violin plots comparing the percentile at time of injury across the population of 61 concussions for the four different time delta analyses presented, (A) Head Impact Frequency, (B) Head Impact Accrual Rate, (C) Cumulative RWE Over the Time Delta, (D) RWE Accrual Rate. Violin plots shown use width to show density of data points. A box plot is overlaid showing the 1st, 2nd, 3rd, and 4th quartiles.

Figure 3

Intra-athlete analyses of the day of injury time delta impact exposure vs. all other non-injury time deltas. The number of impacts over each time delta window for every session during the season are normalized to the number of impacts over the time delta on the injury date. Each row shows the HIE history for each concussion from the season of injury. The vertical red line represents the normalized exposure on the day of injury, each dot represents a session an athlete participated in, and the shading is used to reflect the number of sessions at an exposure level. The darker the circle is equivalent to more sessions at a specific exposure level.

Figure 4

Violin plots showing the number of head impacts at the time of concussion (inclusive of concussive impact) for injuries occurring during each of the three session types: Competition, Regular Season Practice, Preseason Practice. Violin plots shown use width to show density of data points. A box plot is overlaid showing the 1st, 2nd, 3rd, and 4th quartiles.