Risk Factors For Sports Concussion: Only Concussion History, Game Action Certain To Increase Risk, Study Finds
More research needed to identify other factors as highly certain to increase concussion risk.
Previous concussions and match play increase the risk of sustaining subsequent concussions, but the jury is still out on whether other factors, such as gender, playing position, playing level, style of play, environment and injury mechanism, also significantly increase risk of concussion in sport, finds a first-of-its-kind, evidence-based systematic review of the scientific literature published in the British Journal of Sports Medicine.
14 concussion risk factors
Analyzing the findings of 86 studies which met the critieria for
inclusion, South African researchers, led by Shameemah Abrahams, MS of the UCT/RMC Research Unit for Exercise Science and Sports Medicine in the Department of Human Biology at the University of Cape Town, identified 14 risk factors for
sports concussion and assigned each a level of certainty for risk
assessment purposes (see chart below):
1. Previous concussion (high level of certainty that increases concussion risk)
A history of previous concussion was measured as one or more sports concussion sustained prior to the observation period of the study in thirteen studies:
- Ten of 13 studies reported an increased risk of concussion in those athletes with a history of previous concussion.
- Four studies reported an increased risk of concussion in junior ice hockey players who had a history of previous concussion
- A greater risk was also observed in rugby union players with one or more previous concussions
- Four studies of American football observed a threefold to sixfold increased risk of sustaining a subsequent concussion.
- A cohort of athletes participating in various sports with at least one previous concussion were at 3 times greater risk of sustaining a concussion compared to those who had no previous concussion.
- Three studies of American football and soccer players showed a 2-fold to 11-fold increased odds of sustaining a concussion in those with a history of previous concussion.
- All three studies reporting no effect were lower quality retrospective level of evidence II studies (small sample sizes, weaker methodology, prospective and retrospective cohort studies) and III (case control)
- The ten that showed increased risk were well-designed and high quality prospective studies with four denoted as level I and the rest were level II)
- No studies found a decreased risk.
- While studies reporting concussion history were often constrained by the unreliability of patient recall, overall the published studies provide a good estimate of risk and are unlikely to be effected by future studies.
2. Gender (low level of certainty that a risk factor for concussion)
23 studies investigated whether there was a difference in concussion risk between male and female athletes:
- 4 studies found men to be at increased risk
- Men were at greater risk in youth alpine sports, youth American football, lacrosse, and when comparing different sport types, which introduces a bias as (i) men tend to play high collision sports with increased concussion risk; and (ii) men are often more willing to take more risks with the same sport.
- 10 studies showed that women had a greater risk of concussion
- Women were found to have 1.5 fold to 2.5 fold greater concussion risk in various levels of soccer
- Women had a 1.5 fold and 3-fold higher concussion risk in 5 studies with basketball populations
- 4 studies compared female softball and male baseball players, with 3 reporting greater concussion risk in softball players.
- Possible reasons for increased concussion susceptibility in women are:
- increased head acceleration during impacts in soccer compared to men, indicative of decreased neck strength and effective head mass
- increased head to ball ratio in female soccer players
- increased willingness of women to report injuries
- 9 studies found no difference in concussion risk between sexes in taekwondo (3), soccer (2), collegiate rugby (1), lacrosse (2) and when comparing different sport types (2)
- 2 studies found no difference between male and female soccer players but included only 5 and 29 concussions respectively and therefore lacked statistical power
- the difference in protective equipment between men's and women's lacrosse and rules with reduced physical contact in the female game possibly explain the result in the lacrosse studies
The results suggest, the study's authors concluded, that "in sports where rules and physicality are more equal between the sexes, such as soccer and basketball, women appear to be at greater risk. However, when all sports are collectively analyzed, there is a low level of certainty that sex is a risk factor for concussion." Abraham and her colleagues called for future studies to "include measures of exposures so that a reliable correlation between sex and concussion risk may be established."
3. Age (low level of certainty that a risk factor for concussion)
- 14 studies examined whether concussion risk was modulated by age:
- 8 studies identified the older age group to be associated with greater concussion risk
- 3 studies found the younger age group at greater risk
- 3 studies found no effect of age
- High school versus adult rugby
- 11-12 to 13-14-year-old ice hockey players
- male high school lacrosse athletes
- Collegiate cheerleaders were at 3-fold greater concussion risk compared to those younger than 18 (study hampered by small number of concussions (23) recorded in 9,022 participants). Increase concussion risk in collegiate athletes not limited to cheerleaders:
- Level III (cases control) retrospective study investigating 3,700 sport concussions for several sports found increased likelihood of concussion in 10-14 and 15-18 year olds compared with younger athletes (5-10 years)
- Significantly higher concussion rates observed in 11-16 year old ice hockey players compared to 9-10 year olds (but finding probably effected by effect of body checking only being permitted in 10-year-olds and above.
- Children younger than 6 years at greater risk of concussion in roller skating, ice skating, and rollerblading.
- Large scale level 1 prospective cohort study, including 1000 concussions, found that high school football players were at increased risk compared to collegiate players.
"Although more studies reported greater risk in the older cohort, only one of these studies was a high quality level 1 [study] and the remaining seven were either level II or III mainly due to small number of concussions and a weaker design. ... The outcomes of the appraised studies indicate age may play a role in concussion risk; however, contrasting findings confers as low level of certainty for concussion risk."
4. Genetics (low level of certainty that a risk factor for concussion)
The researchers found studies of the genetic association to concussion risk limited. The apolipoprotein E (APOE) gene (which encodes for the lipid carrier protein, apo E, which has been implicated in nerve damage) is, they said, the only genetic marker investigated so far with regard to concussion risk, with only one of the three qualifying studies showing increased risk.
- All three studies investigated the association between the APOE gene and concussion.
- A variant of the APOE gene was significantly associated with a history of one or more previous concussions in intercollegiate soccer and American football. Both studies which observed no association between the APOE ɛ4 allele, concussion history, and multiple concussions only involved a small number of concussions, prompting Abrahams and her colleagues to view the absence of an association "cautiously."
"Because of the few studies that were available and none with a high level of evidence of I, the genetic risk for concussion must be given a low level of certainty."
5. Behavior (low level of certainty that a risk factor for concussion)
A preference for aggressive action on the field was used as a measure of athlete behavior in four high quality level I and II studies.
- Three investigated concussion risk in ice hockey
- One found that athletes who played ice hockey to relieve tension and aggression were significantly more likely to sustain a concussion compared with players with less aggressive tendencies
- 2 found no effect of aggressive behavior preference and experience in an aggressive tactic on concussion risk
- One found that taekwondo athletes who used defensive blocking skills during competition where contact to the head is permissible showed a significantly decreased concussion risk
"Although there is an implication for aggressive behavior (including legal sport actions) increasing concussion risk in sports involving head and neck contact," the study said, "very few hig-quality studies result in a low certainty level assigned to behavior as a concussion risk."
6. Games versus practice (high level of certainty that a risk factor for concussion)
All of the 29 studies included for analysis found a higher risk of concussion in matches/games compared to practices, with the primary reason for the increased risk being the increased risk of high-impact collisions in match play.
While noting that there was a wide range of estimated increased risk, often due to low numbers of concussions in training sessions which skewed the statistics, the fact that all studies found increased risk made it unlikely that future research would alter this finding so as to justify assigning match play a high level of certainty to increase concussion risk.
7. Match/game period (low level of certainty that a risk factor for concussion)
Two studies examined the period of the game as a potential risk factor for concussion, one finding there were significantly more concussions in the second compared with the third period in elite ice hockey, the second finding no difference in concussion rates between the first and second halves of professional rugby league football.
Due to the limited number of studies, Abrahams and her colleagues assigned a low level of certainty to the time of the match as a risk factor.
8. Mechanism of injury (low level of certainty that a risk factor for concussion)
A collision with another player, either accidentally or illegally, was often the mechanism of injury in most studies, but a major limiting factor is the lack of risk estimation, with often only the percentage or proportion of concussion reported.
Of the two prospective cohort level II studies included for analysis, one study found a significantly different concussion incidence between unintentional and intentional collisions. Specifically, a hit to the head or helmet was a significantly common mechanism of concussion injury in ice hockey, soccer, and American football players, but no measure of concussion risk could be performed because no risk estimate was measured in either study.
The complete absence of any high-quality studies ascertaining risk based on the mechanism causing concussion, combined with poor data analysis result in a low level of certainty for mechanism of injury and concussion risk, the study said.
9. Playing position (low level of certainty that a risk factor for concussion)
Playing position as a risk factor for concussion has frequently been investigated in American football, ice hockey, and rugby:
- 6 studies (2 of which are high-quality level 1 studies) showed no effect of playing position on concussion risk
- 2 studies (one a level 1, the other a level 2) found an increased and decreased risk for specific playing positions, showing that quarterback in football had the highest risk of sustaining a concussion compared with all other playing positions, with a risk 1.7 to 5 times greater compared with other offensive positions (with the exception of a wide receiver), as well as an almost twofold and threefold increased risk compared with defensive positions.
- 2 American football studies found no effect of individual playing positions on concussion risk.
- 1 of 3 ice hockey studies reported an almost 3-fold decreased risk of concussion among goalies compared with defensemen and forwards;
- Conversely, no difference in concussion risk was observed when comparing goalies agianst defensemen and forwards in the other two studies of competitive youth and professional ice hockey players, although no risk estimation could be determined for the professional hockey study.
- Both rugby studies reported no effect of playing position on risk, although few concussions were reported, which explains the absence of a risk effect.
"Owing to inconsistent findings among the reviewed studies," Abraham and her colleagues concluded that "certainty that playing position is a risk factor for concussion is low."
10. Playing level (low level of certainty that a risk factor for concussion)
The 6 studies assessing concussion risk at different playing levels within the same age-group yielded varied findings:
- 2 prospective cohort studies (levels 1 and II) found concussion rates increased in descending divisions of college football, with one finding that there was a lower risk in Division I football compared to D-2 and 3, which the authors attributed to poor quality of protective equipment, decreased skill levels or players having to play both offensive and defensive positions;
- Conversely, a study on professional rugby leagues found that increasing playing level tended to increase concussion risk, but the difference was not statistically significant.
- 3 studies (professional rugby, 2 ice hockey) found no difference in concussion risk between different playing levels.
"From among the selected studies no consistency in results was found, therefore a low level of certainty was given to playing level as a risk factor for concussion. Further level I prospective cohort studies are required to improve the level of evidence," concludes the study.
11. Protective equipment (low level of certainty that a risk factor for concussion)
Thirteen studies (6 mouth guard, five padded headgear, 3 face visors) investigated the ability of protective gear to reduce concussion risk
- Mouth guards:
- The effect of mouth guards is unclear. "Surprisingly, a trend for increased concussion risk was shown in American football players wearing mouth guards, possibly explained by the phenomenon of "risk compensation," which occurs when protective equipment use induces an increase in risky behavior that may negate the possible effect of such equipment.
- 5 of the 6 studies found no significant effect of mouth guard use on concussion risk.
- 1 level II study showed a decrease in concussion rate after introducing customized mouth guards in 28 American football players, although the study's methodology has been heavily criticized, including the fact that the players were older when they used the customized mouth guards and there was an overrepresentation in concussion rates when using the standard mouth guards as both match and practice injuries were included compared with only match injuries for the customized mouth guards.
- The relation between mouth guards and concussion prevention is constrained by the use of static cadaver skulls to mimic dynamic biomechanical and biological processes, the limited evidence for an association between force transduction and concussion induction, and few sport concussions resulting from mandibular impact.
- Padded headgear was show to decrease the risk of abrasions and lacerations, but its effectiveness in preventing concussions needs to be determined.
- A trend for padded soccer headgear in a level II prospective cohort of adolescent soccer players showed a 2.5-fold greater concussion risk, but the study's authors said the finding needed to be interpreted "cautiously" because no 95% confidence index was given and concussions were based on recall of symptoms. There is also biomechanical evidence that commercially available soccer headgear products do not have the structural ability to prevent concussions.
- Studies of rugby headgear yielded mixed results:
- 2 studies indicated that headgear had no significant effect on concussion risk;
- 1 large level II prospective cohort study of adult amateur rugby playersd found that the use of padded headgear decreased concussion risk.
- 1 large level I study that included 81 concussions found that rugby headgear did not significantly decrease concussion risk in professional rugby union teams.
- Two studies observed the effects of facial protection on concussions in ice hockey:
- No difference was found in concussion rates in players wearing full, partial or no facial protection.
- One study found that players who complained that the face mask obscured their vision were 5 times more likely to be concussed.
The researchers concluded that the "overall effect of protective equipment is inconclusive," thus warranting assigning a low level of certainty for protective equipment as a concussion risk.
12. Body checking (low level of certainty that a risk factor for concussion)
Body checking is thought to increase concussion risk, especially in younger players.
- 4 of the 6 studies showed a greater concussion risk due to body checking in ice hockey players ages 11 to 16
- A prospective study of 986 ice hockey players reported an increased concussion risk for those permitted to body check (13-14 and 15-16 years) compared to those who were not (9-10 years), but the age difference between the comparative groups makes drawing conclusions about the effect of checking on concussion risk difficult.
- Although prohibiting body checking at younger age groups is thought to increase injury risk due to poor technique, one study found no difference in concussion risk between players with body-checking experience (checking allowed at 9-10 years old) and novices (no checking alllowed)
- Only 2 of the 5 studies on body checking were of a high level of evidence of I, with the remaining studies being level II and III.
A limitation is that most lacked age-matched comparative groups, thus a low level of certainty was given for body checking as a concussion risk estimate.
13. Environment (low level of certainty that a risk factor for concussion)
3 studies analyzed whether environmental factors affect concussion risk:
- Neither of two level II studies comparing concussion risk on natural and artificial grass in elite soccer players found a significant difference in concussion risk between the two surfaces, although one noted a tendency for match concussion to be reduced on the artificial turf. Both lacked the large numbers of concussion often required to identify significant associations.
- A large scale level II retrospective study found that youth football-related concussions were more likely to occur at school compared with at a recreational facility or home.
"Owing to the limited number and size of studies observing the effect of playing environment, a low level of certainty was assigned," the study said.
14. Other factors (low level of certainty that a risk factor for concussion)
The following risk factors that were assessed by a single study were assigned a low level of certainty:
- rugby union players with less training and below average body mass had higher concussion rates
- junior ice hockey players in the lowest body weight quartile were at increased concussion risk;
- players who used a wheelchair sustained less concussions
- trends for fewer concussions were reported in youth ice hockey winning teams and football players with low aerobic fitness
- a trend has been identified between the temporal side of the head and concussions in soccer and football, which may be due to the fact that impacts to the side of the head are often outside of the player's field of vision, limiting the abilty to engage the neck muscles necessary to decrease head acceleration after impact, thus increasing concussion risk; the side of the head may also be biomechanically more vulnerable to an impact force.
Level of Certainty (high-low) Definitions Used for Risk Assessment
|Level of Certainty
|The available evidence includes consistent results from level 1 studies (randomized controlled trials and high quality (large sample sizes, robust methodology) prospective cohort studies). These studies provide a good estimate of risk and are unlikely to be strongly affected by future studies.
|The available evidence includes sufficient evidence to determine that there is risk associated with the injury, but confidence in the estimate is constrained by factors such as the sample size and quality of the studies, as well as inconsistency of findings of individual studies, as well as inconsistency of findings across indiviual studies. As more information becomes available, the magnitude of risk could change or even alter the conclusion.
|| The available evidence is insufficient to assess risk. Evidence is insufficient because of the limited number or size of studies, and inconsistency of findings across individual studies. More information may allow an estimation of risk.
"The investigation of risk factors is an important step twoards understanding [what causes] concussion. Although several risk factors were identified in the appraised studies, poor study methodology caused constrained estimation of concussion risk for almost all of the investigated risk factors, except previous concussion and match play," Abraham writes.
1. Abrahams S, McFie S, Patricios J, Posthumus M, September AV. Risk factors for sport concussion: an evidence-based systematic review. Br J Sports Med. 2014;48:91-97. doi:10.1136/bjsports-2013-0927734.
Risk Factors For Sports Concussion: Only Concussion History, Game Action Certain To Increase Risk, Study Finds
Previous concussions and match play are all but certain to increase the risk of concussions, but the jury is still out on whether other
factors, such as sex, playing position, playing level, style of play,
environment and injury mechanism, also increase concussion risk, finds a first-of-its-kind, evidence-based
systematic review of the scientific literature.