To obtain a comprehensive understanding of the implications of environmental and climate factors on sport-related concussion incidence in outdoor contact sports.
MEDLINE (via Ovid), Embase (via Elsevier), CINAHL Complete (via EBSCOhost), SPORTDiscus (via EBSCOhost), and Scopus (via Elsevier).
Studies that report incidence of sport-related concussion, assess data from athletes participating in outdoor contact sports, report on 1 or more climate or environmental factors, and report a diagnosis of concussion performed by a licensed medical professional were included. Reasons for exclusion included no report on extrinsic or environmental factors, no data on sport-related concussion incidence, and self-report of concussion diagnosis.
This systematic review was conducted using Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines using 2 reviewers at each phase and a third reviewer for conflict resolution.
A total of 7558 articles were reviewed, and 20 met the inclusion criteria. There was moderate to strong strength of evidence concluding no difference between surface type (grass versus artificial) on sport-related concussion risk. Moderate to strong strength of evidence was found supporting no difference in sport-related concussion incidence based on game location (home versus away). There was no consensus on the effects of altitude or temperature on sport-related concussion incidence. One high-quality study found a decreased risk of sport-related concussion when playing in wet versus dry conditions. Heterogenous populations and data collection methods prevented extraction and meta-analysis.
Although a consensus on specific environmental and climate factors that influence sport-related concussion incidence was limited, the majority of studies were of high quality and gave insight into opportunities for future investigation. Administrators of large injury surveillance databases should consider including specific environmental and climate factors to provide investigators with robust data sets to better understand potential associations with sport-related concussion.
There is a general lack of consensus on specific climate and environmental factors that influence sport-related concussion incidence in outdoor contact sports, and current injury reporting systems do not routinely document the environmental or climate factors that may be related to concussion injuries.
Nationwide injury surveillance systems may provide an opportunity to further investigate this relationship, as simple environmental and climate data could be added, such as field type, temperature, humidity, field conditions, location, and altitude.
Having a better understanding of this relationship will inform future decision-making and allow for improved preparation in environmental conditions that may be related to increased risk of concussion.
Sport-related concussion is a prevalent sports-related injury with rates as high as 0.47 per 1000 National Collegiate Athletic Association (NCAA) athlete-exposures1 and 0.51 per 1000 high school athlete-exposures.2 However, according to the Centers for Disease Control and Prevention National Concussion Surveillance System, only 1 out of every 9 sport-related concussions is identified,3 possibly because of underreporting or a poor understanding of symptoms by athletes. Studies have found that over the course of a collegiate sports career, 12% of athletes did not report sustaining a concussion, and 26% sustained an unrecognized sport-related concussion.4 Additionally, up to 50% of high school athletes sustain an unreported concussion.5,6 Immediate recognition and diagnosis is imperative for the long-term health and well-being of athletes. Risk factors that influence sport-related concussion incidence have been studied to facilitate improved awareness and diagnosis. Most commonly, these risk factors are intrinsic in nature, such as sport played, previous history of concussion, and gender, or are related to match play versus practice.4,7,8 The 2017 Berlin consensus statement found minimal evidence for effective injury prevention strategies that can address these intrinsic factors6 ; however, authors highlight the need for a clearer understanding of potentially modifiable factors to design prevention strategies and reduce the risk of sport-related concussion. To date, there is minimal research investigating extrinsic factors in relation to sport-related concussion incidence. However, there is evidence that factors such as weather, surface type, and altitude influence sport-related injury.9–12
Weather and temperature are nonmodifiable elements faced by athletes who play outdoor sports that can affect injury rates. Tobías et al assessed the possible effect of ambient temperature on emergency department visits resulting from sports injury and found a higher risk of emergency visits (orthopedic, neurologic, and internal diagnoses) when the weather was cold (5°C) and hot (28°C), which represent the 5th and 95th percentiles of the annual temperature range for that location, respectively.9 Orchard et al found that in American football, there was a lower risk of knee and ankle injury when playing in cold weather.10 However, Chalmers et al did not find an association between injury rate and temperature in rugby union football.13 In professional soccer players, Aoki et al conducted a 15-year prospective epidemiological study and found a lower injury rate on rainy days than on cloudy and sunny days.12 As demonstrated, most research has primarily focused on musculoskeletal injury, leaving a gap in our understanding of how weather may impact sport-related concussion incidence in this population.
This systematic review is relevant to sports medicine professionals working in outdoor environments and variable climate conditions, as no previous literature reviews have synthesized current research on these factors related to sport-related concussion incidence. We chose to focus this systematic review on climate-related factors specific to competitive contact sports with known increases in sport-related concussion risk to provide medical professionals with valuable insight to enable more informed clinical decision-making for possible sport-related concussion prevention. Our purpose was to enhance our understanding of the implications that climate and environmental characteristics may have on sport-related concussion incidence in outdoor contact sports to enable more informed clinical decision-making by sports medicine teams. Thus, the aim of this study was to systematically review the literature to identify whether a relationship exists between environmental and climate factors and sport-related concussion incidence.
METHODS
This review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines and was recorded in PROSPERO (international prospective register of systematic reviews), a registry of systematic reviews under registration number (CRD42022302464).
Search Strategy
A comprehensive search was conducted by a professional medical librarian within MEDLINE (via Ovid), Embase (via Elsevier), CINAHL Complete (via EBSCOhost), SPORTDiscus (via EBSCOhost), and Scopus (via Elsevier) from inception to December 22, 2021. Each search involved keywords and subject headings for the concepts of outdoor sports and sport-related concussions. Case reports, editorials, newsletters, conference abstracts, books, and book chapters were removed via search filters. Full search strategies are available in Appendix 1. The search identified a total of 16 281 references that were imported and uploaded into Covidence (www.covidence.org), a systematic review screening tool, for automatic deduplication. The duplicate citations (n = 8719) were removed in this manner before screening. In addition, a manual search of reference lists of the articles screened for inclusion was performed to find relevant articles not identified in the initial search.
Study Selection
Two independent reviewers screened all titles and abstracts. Conflicts were resolved by a third reviewer. The same process was repeated for full text review. Studies were included if they reported sport-related concussion incidence for an outdoor contact sport and included an environmental factor, such as surface type, game location, field condition, temperature, or altitude. The diagnosis of sport-related concussion must have been made by a licensed medical provider such as a physician or athletic trainer. Studies were excluded if no environmental factor(s) were reported, the study design was inappropriate, or concussions were self-reported.
Quality Assessment
The risk of bias was determined independently by 2 reviewers using a modified version of a tool created and validated by Hoy et al.14 The original described tool has a total of 10 items. One item was removed after a preliminary review of the literature found that this item lacked relevance to the types of studies meeting our inclusion criteria. This evaluation checklist contains 9 items pertaining to methodology of studies reporting on prevalence (Table 1). Items were scored as yes (0) or no (1). A total of 0 to 3 points indicated a low risk of bias, 4 to 6 points indicated a moderate risk of bias, and 7 to 9 points indicated a high risk of bias. Two raters independently scored each study, and a third rater resolved conflicts. The level of agreeability of the risk of bias between the 2 reviewers was calculated using the Cohen κ.
Data Extraction and Analysis
The primary outcome of interest was the reported incidence rates of sport-related concussion in athletes participating in outdoor contact sports. Data and key findings regarding sport-related concussion incidence across different sports, surface types (grass versus artificial), game locations (home versus away), altitudes, temperatures, and wet versus dry conditions were recorded and qualitatively categorized by 2 independent reviewers. Sample size, level of play, type of medical professional making the sport-related concussion diagnosis, and method of data collection (ie, use of an injury reporting surveillance system) were also recorded and considered in the qualitative analysis. Both the study design and level of evidence according to the Oxford Centre for Evidence-Based Medicine are included to further describe the quality of findings (Table 2). Heterogenous populations and data collection methods prevented extraction and analysis of data via meta-analysis.
RESULTS
A total of 7558 articles were initially reviewed, and 20 met the inclusion criteria (Figure). The level of agreeability of the risk of bias between the 2 reviewers was 0.89, with 55% (11/20) of articles having a low risk of bias and 45% (9/20) having a moderate risk of bias (Table 3). A description of the study results can be found in Table 2. Football followed by rugby, soccer, and lacrosse were the most common sports investigated by authors (14, 3, 3, and 2 studies, respectively). There was moderate to strong strength of evidence concluding no difference between surface type (grass versus artificial) on sport-related concussion risk. Moderate to strong strength of evidence was found supporting no difference in sport-related concussion incidence based on game location (home versus away). There was no consensus on the effects of altitude or temperature on sport-related concussion incidence. One high-quality study found a decreased risk of sport-related concussion when playing in wet conditions versus dry conditions.
DISCUSSION
Overall, we found a general lack of consensus on specific weather and environmental factors that influence sport-related concussion incidence in outdoor contact sports. Often, these factors are not documented after injury intake and warrant consideration by medical personnel working with athletes who play outdoors. Unique to outdoor contact sports is exposure to variable weather, temperatures, and surface types, and although these are often nonmodifiable, understanding risk factors can facilitate more appropriate and efficient diagnosis of sport-related concussion. This information may not influence teams and organizations to cancel games or practices; however, it may better inform decisions on scheduling, location selection, and facility design to better mitigate injury.
There has been an increase in the use of artificial grass, with almost half of the National Football League (NFL) stadiums being turf,15 and with that, a growing amount of evidence identifying an altered risk of sport-related injury depending on surface type. Several studies have found an increase in knee ligamentous injury and foot and ankle injuries when playing on artificial surfaces compared with natural grass in football and rugby.11,16,17 Balazs et al conducted a systematic review to determine the risk of anterior cruciate ligament rupture on natural grass versus artificial turf and found an increased risk of anterior cruciate ligament injuries on synthetic playing surfaces in football but not in soccer.18 Unfortunately, many studies did not report on turf generation or age of the turf used for training and games. Unlike weather, there have been studies looking at the relationship between surface type and sport-related concussion injury risk. O’Leary et al conducted a systematic review and found no association between incidence of sport-related concussion and playing on artificial turf in soccer.19 However, this review only included soccer as well as studies that reported “head/neck” injuries without defining concussion, which may have skewed their results There is a lack of confidence in our understanding of whether surface type may influence sport-related concussion incidence, necessitating further investigation.
Altitude is one of the few environmental factors that has been studied in relation to sport-related concussion incidence in sport; however, the body of literature is conflicting. Some authors have found a reduction in sport-related concussion injury risk when playing at a higher altitude,20–22 but other studies have found no association between the two.23–25 Lack of consistency in population and methodology still leaves this question up for debate. Other factors, including game location and field condition, warrant further investigation as these are unique to outdoor sports, and the environment can be much more variable and inconsistent than environments within stadiums or inside. This also warrants investigation to determine if there is a relationship between these factors and sport-related concussion.
There were limitations to our study, which should be noted. Only 4 of the articles that met the inclusion criteria included female athletes26–29 participating in soccer and lacrosse. American football was the most common (14 studies) sport assessed. This limits the generalizability of the results to other sports and female athletes. Several studies used NFL concussion incidence data from the FRONTLINE “Concussion Watch,”20,21,30–32 which has not been validated as a reliable injury surveillance database. Some injury epidemiology studies, such as, for example, articles using NFL data, did not identify or differentiate domed/indoor stadiums versus outdoor stadiums in their analysis when reporting other environmental variables of interest. Lastly, the methodology for collecting environmental and climate-specific factors was not homogenous across studies. Different ranges for altitude and what was high versus low altitude varied.20,21,24,31,33 Binney et al recently published a study challenging the “statistical flaw” of studies creating categories of altitude as seen in the studies included in our systematic review.34 The authors analyzed concussion rate data from the NFL during the season from 2012 to 2019 and assessed the relationship between altitude and concussion rates continuously as well as categorically as previously done to compare results. They found that the continuous plots did not demonstrate any association between sport-related concussion rate and altitude. However, when using a cutoff point (644 feet, as used in Myer et al20 ), a significant difference in sport-related concussion rate was observed. These limitations may elucidate why we were unable to find specific trends in sport-related concussion incidence and identify the need for consistency when defining environmental characteristics to understand their influence more accurately on sport-related concussion incidence.
Due to the lack of consensus on the influence of environmental factors on sport-related concussion incidence revealed by this systematic review, there is an opportunity to better use injury reporting systems by adding simple environment-specific data. Sport-related concussion diagnosis is challenging in the athletic environment, and understanding all potential factors that influence concussion incidence opens the door to develop more comprehensive and effective clinical decision-making for medical professionals. Nationwide injury surveillance systems may provide a fruitful opportunity to further investigate this relationship, as simple environmental and climate data could be included, such as field type, temperature, humidity, field conditions, location, and altitude. Participation in injury surveillance systems can result in a substantial sample of data to better identify trends in injury incidence in sports.35 Further study of environmental and climate factors may motivate sports medicine decision-makers to consider changes to modifiable factors related to games and practices to improve safety and mitigate injury risk in this population.
REFERENCES
Appendix 1. Search Strategy
Search Strategy Report
Date of completed search: December 22, 2021.
Total number of articles (before deduplication): 16 281.
Total number of articles (after deduplication): 7562.
Database: MEDLINE (OVID)
Database: Embase (Elsevier)
Note: all searches were conducted in the “results” tab.
Database: CINAHL Complete (EBSCOhost)
Database: SPORTDiscus (EBSCOhost)
Database: Scopus (Elsevier)
