Previous concussions and match play increase the risk of sustaining subsequent concussions, but there is not enough high quality research evidence to determine 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.^[1]

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."