Objective To summarise the literature on the barriers to athletes seeking mental health treatment and cultural influencers of mental health in elite athletes.
Design Systematic review
Data sources PubMed, Cochrane, Scopus, SportDiscus (Ebsco), and PsycINFO (ProQuest) up to November 2018.
Eligibility criteria for selecting studies Qualitative and quantitative original studies of elite athletes (those who competed at the professional, Olympic, or collegiate/university levels), published in any language.
Results Stigma, low mental health literacy, negative past experiences with mental health treatment-seeking, busy schedules, and hypermasculinity are barriers to elite athletes seeking mental health treatment. Cultural influencers of mental health in elite athletes include: (1) the lack of acceptance of women as athletes; (2) lower acceptability of mental health symptoms and disorders among non-white athletes; (3) non-disclosure of religious beliefs; and (4) higher dependence on economic benefits. Coaches have an important role in supporting elite athletes in obtaining treatment for mental illness. Brief anti-stigma interventions in elite athletes decrease stigma and improve literary about mental health.
Conclusion There is a need for various actors to provide more effective strategies to overcome the stigma that surrounds mental illness, increase mental health literacy in the athlete/coach community, and address athlete-specific barriers to seeking treatment for mental illness. In this systematic review, we identified strategies that, if implemented, can overcome the cultural factors that may otherwise limit athletes seeking treatment. Coaches are critical for promoting a culture within elite athletes’ environments that encourages athletes to seek treatment.
- mental health
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What is already known?
Symptoms of mental health disorders are prevalent in elite athletes.
Elite athletes have a low rate of seeking treatment for mental health symptoms and disorders.
Elite sport culture includes some features that increase the likelihood of athletes suffering mental health symptoms and disorders.
What are the new findings?
Stigma, low mental health literacy, negative past experiences when seeking treatment for mental health symptoms and disorders, and busy schedules create barriers to elite athletes seeking mental health treatment.
Coaches have an important role in supporting elite athletes who are considering getting professional mental health support.
The lack of acceptance of women as athletes in some cultures is associated with poor mental health in elite athletes.
Brief anti-stigma interventions can benefit elite athletes and improve mental health literacy.
Mental health symptoms and disorders are prevalent in elite athletes, occurring in 5–35% of elite athletes annually.1–7 Anxiety, depression, sleep-related problems, alcohol misuse and eating disorders are highly prevalent in screening studies of elite athletes.1–5 Elite athletes appear to experience levels of many mental health symptoms and disorders similar to the general population, while some conditions such as eating disorders are more common in elite athletes.5 However, mental health treatment-seeking is low among elite athletes.8 Media portrayals convey some challenges with mental health treatment-seeking that elite athletes face: the image of the ‘strong, winning athlete’ contrasts with the image of the ‘weak, depressed human being’.9 10 Despite such portrayals, increasing numbers of studies are emerging that investigate factors related to the recognition of mental health symptoms and disorders and mental health treatment-seeking among elite athletes.10 However, no systematic reviews to date have summarised barriers to mental health treatment-seeking in elite athletes and negative cultural influencers of mental health in this population.
Many factors place the athlete at increased risk for mental health symptoms and disorders. The demands of a heavy training schedule, both physically and mentally, and a constant push to improve performance can be stressful.11 12 Many elite athletes live near or at training sites with other athletes, and are often separated from their families and friends. They often follow strict regimens with diet, schedules and habits, while striving to improve performance.12 13 Coaches and others push athletes to improve skills and abilities while closely monitoring their activities.14 Some subgroups, such as women and minorities, may experience lack of acceptance from their culture of origin or training environment.15 Unexpected events (eg, death of a family member, health/injury-related problems) can trigger mental health symptoms and disorders.16 Furthermore, increased risks of mental health symptoms and disorders occur in those who experience involuntary and/or undesired retirement from sport because of de-selection or injury.17 Former athletes are also at risk for mental health and substance use-related symptoms and disorders. Lack of retirement planning, high levels of athletic identity, lower educational attainment, post-sport unemployment, chronic pain, and adverse life events can impair the lives of former athletes and lead to mental health symptoms and disorders.18
Within this context, the present systematic review aims to summarise the findings of studies that investigated barriers to treatment-seeking among elite athletes, and cultural influencers that impact mental health symptoms and disorders in this population.
Original studies of elite athletes (defined here as those competing at professional, Olympic, or collegiate/university levels), published in any language, reporting (1) barriers to elite athletes accessing mental health resources and/or (2) cultural considerations (eg, gender, sexual orientation, race, ethnicity, socioeconomic status, religion) on elite athlete mental health were included in the search. Excluded from this review were: meeting proceedings; reviews; commentaries; letters; opinion articles; case reports; position statements; book chapters; editorials; non peer-reviewed articles; articles focusing on the general population; articles focusing on subgroups in the general population (eg, just men or just women, youth, college students); articles focusing on other specific populations (eg, minorities, individuals with mental health disorders, individuals with a specific disease); articles focusing on professional dancers; articles focusing on elite drivers; articles investigating sport as a treatment; articles focusing on non-mental health issues or problems in elite athletes (eg, nutrition, performance, orthopaedics, cardiovascular, respiratory, other health issues); articles focusing on performance sports psychology; articles focusing on elite athlete staff (eg, coaches, managers, other professionals); and articles not focusing on barriers or cultural considerations.
We searched the PubMed, Cochrane, Scopus, SportDiscus (EBSCO), and PsycINFO (ProQuest) databases up to November 2018 to identify relevant studies.
Search (Identification phase)
Figure 1 shows the flow diagram for identification, screening, eligibility, and inclusion of studies in the present systematic review following the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) statement.19 One author (MH) searched databases to identify articles that discussed treatment-seeking behaviour, or the barriers thereof, of elite athletes seeking mental health services. The first search was completed in February 2018; duplicated citations from the multiple databases were removed, resulting in a total of 254 unique articles. The search was not limited to a specific date range. A second search conducted in October 2018, using the same terms and databases, resulted in 139 additional articles. A third and final search was conducted in November 2018, resulting in no additional articles specifically focusing on athletes. A total of 393 unique studies were identified in the searches.
The initial search string used within PubMed was: ((((“Mental Health Services”(Mesh) OR “mental health services” OR counselling(tw) OR “psychiatric help” OR “psychiatric assistance”)) AND (((((“Help-Seeking Behaviour”(Mesh)) OR (“Attitude”(Mesh) OR “Attitude to Health”(Mesh))) OR “mental toughness” OR “Social Stigma”(Mesh)) OR “Stereotyping”(Mesh) OR stigma(tw) OR “help-seeking” OR “Athletes/psychology”(Mesh) OR “barrier to” OR “refusal for” OR aversion OR attitude OR perception(tw)))) AND (athlete OR sport OR athletic)). However, additional terms such as “Health Knowledge, Attitudes, Practice”(Mesh) or “Athletic Performance/psychology”(Mesh) could also be used interchangeably. Using this set of terms as the base search string, each subsequent search within separate databases was modified slightly to allow for controlled vocabulary and keyword searching. Online supplementary appendix 1 presents the search terms for Scopus, SportDiscus (EBSCO), and PsycINFO (ProQuest). Endnote X9 software was used to manage and organise the citations.
In the screening phase, the first and the last author independently read the abstracts of all studies found in the search (n=393). After applying the inclusion and exclusion criteria, 333 articles were excluded. Around 80% of exclusions were based on the following nine exclusion criteria: reviews (n=33); commentary/letter/opinion/case/position statement/book chapter (n=27); general population (n=34); general young population (n=22); other specific populations (n=20); other problems/issues in athletes (n=48); no focus on barriers (n=22); no focus on health seeking (n=33); and focus on sport psychology to improve performance (n=24). Articles recommended for exclusion by one author at this stage were included for further evaluation in the next phase. In the eligibility phase, the first, second and third authors assessed the full-text articles (n=60) for eligibility. The first author determined inclusion or exclusion in cases of disagreement between the second and third authors. Eight articles were excluded based on the inclusion/exclusion criteria. Eventually, 52 studies were included in the present systematic review.
Data collection process
The first, second, third and last authors read all 52 included studies, independently. The second and third authors tabulated data from half of the studies each. Then the tabulated data were evaluated by the other three authors.
After reading the full text of the 52 articles included in the present systematic review, the first and last author identified the most relevant topics for data collection. These topics were presented to and discussed with a panel of experts during the International Olympic Committee Consensus Meeting on Mental Health in Elite Athletes in Lausanne, Switzerland in November 2018. The panel included psychiatrists, psychologists, primary care sports medicine physicians, a neurologist, a neurosurgeon, an exercise scientist, a social worker, and elite athletes; and represented a diversity of geographical locations (Australia, Brazil, Canada, China, India, Italy, the Netherlands, South Africa, South Korea, Turkey, the UK and the USA). Based on expert consensus during this discussion of findings from the 52 articles, this review aims to summarise the literature with respect to:
Barriers, facilitators, influencing factors, preferred characteristics of counsellors, and interventions regarding elite athletes accessing mental health resources;
Cultural issues that impact the mental health of elite athletes, including gender, gender identity, sex, sexual orientation, race, ethnicity, socioeconomic status, and religion.
No method of handling data and combining results of studies was carried out because of high heterogeneity among the included studies, as follows: different types of studies; non-similar measures; intervention heterogeneity; and sampling and design heterogeneity.
Tables 1–6 present the main results for barriers, facilitators, influencing factors, preferred characteristics of counsellors, and interventions regarding access of elite athletes to mental health resources, and the main results for cultural influencers of mental health for elite athletes.
American and non-American quantitative cross-sectional studies are presented in tables 1 and 2, respectively. Tables 3 and 4 present the results of the qualitative and mixed-method studies, respectively. Findings of multinational studies are presented in table 5, and intervention studies in table 6.
Among the 52 included studies, the majority were published in the last 10 years (n=40), with almost half of the studies published in the last 5 years (n=24). There were 35 quantitative, 12 qualitative, and five mixed-method studies. The majority were cross-sectional (n=43). Four randomised controlled trials were included among the nine prospective studies. Intervention studies were few (n=7). More than half of the studies were from North America (n=28), followed by Europe (n=11), Oceania (n=5), and Asia (n=3). The five remaining studies were cross-national multi-continent studies. Almost all studies were from high-income countries, except for two studies from Malaysia and India. No studies were found from Africa or South/Central America. In total, the studies included 13 255 elite athletes, with >90% of those athletes from quantitative studies (n=12 596). Elite athletes from at least 71 sports were included in the review. Football (soccer) was the most common sport, being included in 18 studies. The instruments used most often were: the Attitudes Toward Seeking Professional Psychological Help Scale20; the Athletic Identity Measurement Scale21; the Expectations about Counselling Questionnaire22; the Attitudes Toward Seeking Sport Psychology Consultation Questionnaire23; and the Sport Psychology Attitudes–revised form.24
The present review identified barriers for elite athletes seeking mental health treatment. Stigma attached to mental health symptoms and disorders appears to be the strongest barrier in quantitative and qualitative studies in sports, supported by 18 studies in the present systematic review.23 25–41 Elite athletes often believe mental health symptoms and disorders are a sign of weakness, or report stigma associated with mental health symptoms and disorders. They also report a lack of knowledge and understanding of mental health symptoms and disorders.32 Elite athletes appear to have higher levels of stigma compared with non-athlete peers.26 Lower openness and lower conscientiousness predicted greater stigma towards sport psychology consulting.39 Public stigma (stigma endorsed by the general public) and self-stigma (individuals’ own stigmatised attitudes related to internalisation of public stigma) predict a significant detrimental impact on treatment-seeking by athletes for mental health symptoms and disorders.8 33 34 Greater perceived public stigma than self-stigma was found in elite athletes.26 However, public stigma appears to be less of a deterrent for collegiate athletes to seek treatment than in the past.28 42
A study of collegiate athletes showed that they had a significantly lower mean score than non-athlete peers on scales assessing attitudes toward mental health, reflecting less willingness to seek mental health treatment.43 Concerns regarding how elite athletes will be perceived by their peers, coaches and sport managers could be a barrier even for those with positive attitudes toward mental health treatment-seeking who express a willingness to seek treatment.33 44 45
Other reported barriers include lack of mental health literacy, negative past experiences with mental health treatment-seeking, and busy schedules.32 34 35 41 42 46–49 Lack of mental health literacy (knowledge and beliefs about mental health disorders that aid their recognition, management or prevention) was a barrier in different levels and types of sports in qualitative and qualitative studies.32 41 47 50 Time constraints also influenced many elite athletes, especially collegiate athletes, in not seeking mental healthcare.42
Several additional factors are associated with negative attitudes about mental health treatment-seeking, including: identification as male; hypermasculinity; younger age; black (vs Caucasian) race; US (vs European) nationality; lower measures of openness and higher measures of conscientiousness (personality factors); gender role conflicts; and participation in physical contact sports.23 28 34 37 39 43 51–55
Conversely, other factors can facilitate elite athletes seeking mental healthcare, including: availability of individual mental health treatment services inside the university or training facilities; positive previous interactions with mental health providers; having an established relationship with a mental health provider; perception of benefits to seeking treatment; strong positive coping skills; higher neuroticism, higher conscientiousness, and higher openness (personality factors); a sense of self-efficacy to seek treatment; and positive attitudes of others, especially coaches and family members, regarding seeking mental health treatment.8 27 29 32–34 36 39 43 45 53 56–58 Coach support for mental health treatment-seeking is an important facilitator for elite athletes.32 Moreover, athletes with stronger positive coping skills generally are more supportive of seeking mental health treatment.31 Finally, athletes have strong preferences for counsellor characteristics, such as familiarity with their sport, same gender, older but still close enough in age to understand their lives, and geographic proximity to the sports facility.28 54 55
We identified seven intervention studies designed to decrease barriers to athlete mental health treatment-seeking. Despite improving mental health knowledge and decreasing stigma attached to mental health disorders, these studies reported no effects on mental health treatment-seeking.40 41 49 50 58–60 Generally, the interventions were short-term, aiming to increase awareness and understanding of mental health, decrease stigma attached to it, and reduce overall barriers to accessing mental health supports and services for athletes. Bapat et al 40 tested an 8 hour training programme. Beauchemin et al 41 tested an integrative outreach model, which consisted of a one-session workshop, as well as three- or five-session components of a larger class focusing on overall wellness. Donohue et al 59 tested two semi-structured interview formats: one interview focused on discussing the athlete’s experiences in sports, and the other focused on describing sport psychology and its potential benefits to the athlete. Donohue et al 60 conducted a subsequent study to determine interest in participating in one of two goal-oriented programmes. Donohue et al 58 also tested a sport-specific optimisation approach to concurrent mental health and sport performance, which consisted of a longer intervention, with 12 performance meetings of 60 to 90 min within 4 months. Gulliver et al 50 conducted a study with three brief, fully-automated, internet-based mental health treatment-seeking interventions. Kern et al 49 tested an internet-based intervention, which consisted of nine presentations and two videos.
Several cultural influencers that are both associated with and impact the mental health of elite athletes have been described. Gender issues were reported in five studies.48 61–64 Specifically, discrimination and segregation involving female athletes have been reported.48 62 Hammermeister and Burton61 found that sex role stereotypes and role expectations predispose men and women to cope with stress differently. Women are socialised to use more emotion-focused coping strategies, particularly seeking emotional social support, while men are socially reinforced for using more problem-focused approaches to coping.61 Generally, male athletes are more prone to doping.64 However, female elite football (soccer) players have reported a higher likelihood of doping than male elite football players.
Recent studies have reported less homophobia within sport than earlier studies.57 65 Some athletes argue that an athlete’s ability to play is the only criterion on which they should be judged, and their sexuality is of little consequence.57 However, professional football (soccer) players suspect that there are no openly gay players in their sport because of financial considerations (ie, the sport would be perceived as less marketable with openly gay players).57
Regarding race and ethnicity as cultural factors within elite sport, a qualitative study found that younger, largely non-white team members appeared to feel more pressure to fit in by hiding aspects of themselves that diverge from the perceived ‘norm’.66 British athletes had lesser stigma towards mental health treatment-seeking, greater personal openness, and lesser preference for a consultant of the same race or culture than Singaporean athletes.39 Lawrence67 found a fairly broad representative distribution of race and wealth in summer Olympic athletes. However, some sports have important race differences. Participants in equestrian, modern pentathlon, road cycling, rowing, and sailing were more likely to be white. Conversely, non-white athletes were more likely to participate in athletics (track and field), basketball, gymnastics, taekwondo, table tennis, and judo. Lawrence67 found that sports associated with white athletes were also associated with the athletes having a private education. Torres Colon et al 68 reported that those more dependent on financial benefits of sport have different attitudes towards concussion (ie, more willingness to risk multiple concussions). Walseth63 found that sports can play an important role for social capital accumulation in young female athletes in the Middle East.
Blodgett et al 66 and Harkness62 reported on religion as a factor in elite sport. Despite its importance as a cultural dimension for some athletes, especially non-whites and Muslims, religion is not usually disclosed among teammates, while it is common for teammates to share other aspects of their culture.62 66 Religious convictions also predicted lower likelihood of doping.64
This review aimed to systematically review the barriers to elite athletes seeking mental health treatment and the main cultural influencers that may impact their mental health symptoms and disorders. As in the general population, stigma emerged as the most commonly reported barrier to treatment-seeking for elite athletes, along with low mental health literacy, negative past experiences with mental health treatment-seeking, busy schedules, and other factors such as hypermasculinity. Lack of acceptance of women as athletes emerged as a commonly reported cultural influencer of mental health in elite athletes, as did race and ethnicity, religiosity, and socioeconomic status.
Although elite athletes report more stigma to mental health treatment-seeking than the general population, stigma is declining in younger cohorts, which accompanies a similar trend in discrimination against people with mental health symptoms and disorders in the general population.69 However, elite athletes continue to have more difficulty in disclosing apparent signs of ‘weakness’, which is how some perceive mental health symptoms and disorders; this attitude can be an important barrier to any anti-stigma intervention in this population. Athletes fear, possibly rightly so, that disclosing mental health symptoms or disorders would reduce their chances of maintaining or signing a professional team contract or an advertising campaign. In addition, athletes fear negative reactions from their teammates and coaches if mental health symptoms or disorders were to be disclosed. Although brief anti-stigma interventions in collegiate athletes did not impact long-term mental health treatment-seeking behaviour, brief anti-stigma interventions reported good initial results, and may be an important initial step to overcome barriers.
Increasing mental health literacy, including among elite athletes, is a global challenge.70 Increased knowledge about the most prevalent mental health symptoms and disorders (eg, anxiety, sleep disorders, depression, eating disorders, and alcohol and other substance misuse) would be important for current and former elite athletes. In addition, access to confidential mental health services geographically close to elite athletes, and with counsellors familiar with the type of sport and associated stressors experienced by target athletes, could help to minimise barriers. The preference of athletes for counsellors of the same gender and closer age should be carefully evaluated. It is important to optimise the comfort level of athletes seeking mental health treatment, but not to create stigma regarding mental health providers of a different sociodemographic background and thus further limit availability of providers.
Broad-reaching cultural influencers impact elite athletes in different ways. Their own cultural identities—which include gender, gender identity, sex, sexual orientation, race, ethnicity, socioeconomic status, and religion—can play a role. Some athletes may experience performance disadvantage and higher likelihood of mental health symptoms or disorders associated with cultural influences; importantly, there is no evidence that this is because of any inherent vulnerability associated with certain cultural identities, but rather, it likely relates to discrimination based on cultural factors.71 While not meeting our strict criteria for inclusion in this systematic review, some reports suggest that female athletes engaged in progressively higher levels of elite sport face varying degrees of cultural acceptance.72 Moreover, women participating in traditional ‘male’ sports may face being marginalised and stereotyped and may experience unequal training opportunities and resources. Elite sport opportunities for women are influenced by ethnic beliefs, religion, sexualisation, and traditional gender roles.73 Considering religious rules about the body and presentation in public, combining more traditional roles with being an elite athlete can be problematic.73 For example, tension may exist between what is functionally optimal attire for elite women athletes and what is deemed culturally acceptable.74 75 Gender stereotyping in the media may influence how women athletes view themselves.74 Women athletes may be stereotyped as ‘lesbian’ to keep them from playing certain sports, or from playing for certain coaches or teams.74 Some professional female athletes must train outside their native countries and may struggle to find a support network and cultural understanding from teammates in their new location.
Masculinity and sexuality are also common issues for male athletes. Like women, men may be excluded and patronised.76 Gay men and lesbian women athletes face various degrees of acceptance, which can negatively impact performance. Despite apparent improvements in ease of disclosing gender identity and gender expression within sport, other studies (not meeting our strict criteria for inclusion in the systematic review) attest to negative experiences of non-heterosexual athletes in sport, specifically including worse mental health relative to heterosexual athletes.77–79 Transgender athletes often have negative experiences in sports and struggle to be accepted.77 Religion is a taboo subject of conversation among elite athletes, contributing to social isolation of athletes from minority religious backgrounds. Racial disparities—including those related to exploitation, player-coach tension, and prejudicial treatment—along with socioeconomic inequities form barriers that prevent equal opportunities.80
Finally, coaches were identified as having an important role for supporting elite athletes in mental health treatment-seeking. They are in frequent contact with athletes, and thus can support and encourage acknowledgement and treatment of mental health symptoms and disorders by creating a destigmatising environment wherein mental health treatment-seeking is a core function of training and self-care. This kind of support could be especially important in adolescent athletes, who might be even more vulnerable to coaches’ positive or negative attitudes regarding mental health treatment-seeking than adult athletes. Parents and other caregivers might also promote positive attitudes toward mental health treatment-seeking and toward those affected by mental health symptoms and disorders.79
Limitations and future studies
There was great heterogeneity among the studies we included (eg, quantitative vs qualitative, cross-sectional vs prospective, sampling differences, and different measures and endpoints). As a result, we did not perform meta-analyses. Additionally, many athletes come from countries where there are few, if any, mental health services, and where there may also be ways of understanding and treating mental health symptoms and disorders outside evidence- and biomedically-based ones.81 82 The present review did not aim to summarise evidence on the role of these alternative methods in overcoming barriers to mental health treatment-seeking.
Future studies in low- and middle-income countries using and/or validating the most important scales from studies included in the present review (eg, Attitudes Toward Seeking Professional Psychological Help Scale, the Athletic Identity Measurement Scale, the Expectations about Counselling Questionnaire, the Attitudes Toward Seeking Sport Psychology Consultation Questionnaire, and the Sport Psychology Attitudes–revised form) are needed to generate greater understating of barriers to mental health seeking at a global level. In addition, intervention programmes to decrease discrimination against those with mental health symptoms and disorders should focus on the most problematic subgroups of elite athletes, such as those with marginalised cultural identities within elite sport. Studies testing longer interventions, supported by already-piloted brief interventions, in select subpopulations might be a helpful next step. Non-collegiate elite athletes should also be the focus of such interventions, considering that previous studies were carried out in collegiate athletes. Additionally, already-tested brief interventions could be tested in larger samples.
Stigma, low mental health literacy, negative past experiences with mental health treatment-seeking, busy schedules, and hypermasculinity are important barriers to mental health treatment-seeking for elite athletes. The lack of acceptance of women as athletes, lower acceptability of mental health disorders among non-white athletes, non-disclosure of religious beliefs, and higher dependence on economic benefits are also important features to be considered regarding vulnerability to mental health symptoms and disorders in elite athletes. More effective strategies for overcoming stigma and increasing mental health literacy for elite athlete populations are needed. Focused and tailored interventions on problematic subgroups identified by the present systematic review would be an important next step. Coaches could be important agents for supporting positive mental health attitudes within the elite athlete environment, including fostering an environment of mental health treatment-seeking. A better understanding of sport as a sub-culture within society is needed, including which elements of that sub-culture are particularly conducive to positive mental health outcomes.
The authors thank the other participants in the 2018 International Olympic Committee Consensus Meeting on Mental Health in Elite Athletes, including Cindy Miller Aron, David Baron, Antonia Baum, Abhinav Bindra, Richard Budgett, Niccolo Campriani, Alan Currie, Jeff Derevensky, Lars Engebretsen, Ira Glick, Paul Gorczynski, Michael Grandner, Doug Hyun Han, David McDuff, Margo Mountjoy, Aslihan Polat, Rosemary Purcell, Margot Putukian, Simon Rice, Allen Sills, Torbjorn Soligard, Leslie Swartz, and Li Jing Zhu, for their input on the development and interpretation of this research.
Contributors We confirm that all authors listed on this manuscript meet requirements for authorship credit. Specifically, all authors have participated in the following ways: Substantial contributions to the conception and design of the work, and the acquisition, analysis and interpretation of data. Drafting the work and revising it critically for important intellectual content. Final approval of the version published. Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; internally peer reviewed.
Data availability statement All the data collected in the present systematic review have been presented in the present manuscript.
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