Prevalence of Traumatic Brain Injury in the General Adult Population of the USA: A Meta-Analysis

Traumatic brain injury (TBI) is one of the leading causes of death and disabilities among civilians and military personnel worldwide [1]. It has been estimated that 64–74 million individuals experience TBI from all causes each year [2]. According to the Centers for Disease Control and Prevention (CDC), an estimated 1.7 million people sustain a TBI every year in the USA [3]. TBI can be associated with chronic consequences such as physical and psychological disorders [4]. In 2021, there were over 69,000 deaths because of TBI in the USA, with about 190 TBI-related deaths every day [5]. The risk of having a TBI is highest among adolescents, young adults, and the elderly [6]. The estimated 2016 overall healthcare spending attributable to non-fatal TBI among Medicaid, Medicare, and private health insurance was more than USD 40.6 billion in the USA [7]. In a meta-analysis in 2013 of 15 studies from developed countries, the prevalence of TBI was estimated to be 12.1% in the general population [8]. In another meta-analysis, the overall prevalence of TBI in the offender population was 60.25% [9] which was higher than the overall prevalence in the general population.

Although the incidence of TBI in the general population is available, there is scarce data regarding the lifetime prevalence of TBI in the general population of the USA. There are several problems in estimating the overall prevalence of TBI in the general population because available studies used different methods in reporting TBI, including self-report questionnaires and self-reporting through an interview which may be accompanied by recall bias in reporting a head injury that occurred many years in the past. Another problem is that some people do not have enough knowledge about defining a TBI which causes some bias in reporting a TBI based on available definitions which may result in either a lower or a higher estimation of TBI. There are different markers for defining the severity of TBI, including Glasgow Coma Scale (GCS) scores, duration of post-traumatic amnesia (PTA), and the presence or absence of loss of consciousness (LOC) at the time of head injury [10]. Using PTA as a marker of TBI is problematic because memories tend to diminish during the time of injury and people may have problems in retelling the event. While GCS is used as a reliable marker by healthcare professionals for defining TBI, self-reported GCS score is not valid when determining the presence of TBI by ordinary people. On the contrary, the presence of LOC may be a more valid and reliable marker for determining the TBI by ordinary individuals because it is more memorable for injured people or witnesses. In one study on 5,034 individuals, the prevalence of TBI with LOC was estimated to be 7.2% [11]. In another study on a small sample size of 20 African Americans, the prevalence of TBI was 60% [12]. Due to these variations in reporting TBI prevalence in the general population, in this study, we aimed to obtain a clearer understanding of the prevalence of TBI among the general adult population in the USA which can assist healthcare decision-makers in developing comprehensive TBI prevention programs and alleviate TBI-associated costs and burden on the healthcare system.

Our meta-analysis was performed using the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) checklist. The study protocol was registered with PROSPERO (CRD42024534598). A comprehensive literature search of PubMed from the National Library of Medicine and Google Scholar was performed from database inception to April 2024, using a combination of the following keywords and MeSH terms: “brain injury,” “traumatic brain injury,” “TBI,” “head injury,” “traumatic brain injury prevalence,” “TBI prevalence,” “TBI and the general population,” “TBI and healthy population,” “TBI and epidemiology.” The reference sections of the identified studies were also searched to identify further relevant articles. We included studies if they met all of the following features: (1) reported the lifetime history of sustaining TBI in adults (aged ≥18 years) from the US general population comprised of either men or women or of both sexes; (2) in which TBI had resulted in LOC as a reliable marker for self-reporting TBI; (3) written in English-language; (4) full-text available. We excluded studies containing fewer than 20 participants and those from some specific populations who have a higher prevalence of TBI than the general population such as homeless people, military personnel, and offenders. From source studies meeting inclusion criteria, we extracted the following information: first author, year of publication, total number of participants, TBI cases in total and separately for both sexes if available, number of males and females in each study, mean or median age of participants if available, study population, TBI measurement method, and mechanisms of injury if available. The risk of bias in identified studies was evaluated using the Newcastle-Ottawa Scale (NOS) (online suppl. Table 1; for all online suppl. material, see https://doi.org/10.1159/000540676). The NOS was developed to assess the quality of non-randomized studies. The risk of bias in each study was evaluated based on the following items: representativeness of the exposed cohort, selection of the non-exposed cohort, ascertainment of exposure, demonstration that outcome of interest was not present at the start of the study, comparability of cohorts based on the design or analysis, assessment of outcome, was follow-up long enough for outcomes to occur, and adequacy of follow-up of cohorts.

Analyses were conducted in R (version 4.3.2) using the meta and metafor packages. Statistical heterogeneity was assessed using the estimated Cochrane χ² test, Tau², and I² statistics. We classified heterogeneity based on computed I² as low (0–25%), moderate (25–50%), or high (≥50%) [13]. A meta-analysis was performed using a random-effects model because of possible differences between the included studies in terms of methodology and other confounding variables. Results are reported as pooled proportions with accompanying 95% confidence intervals (CI). We calculated the prevalence of TBI by dividing the total number of TBI cases by the total number of participants in included studies. We tabulated separate prevalence rates for males and females for studies in which the desired data were available for both sexes, to calculate the pooled odds ratio (OR) of TBI by sex.

Initially, the search returned 2,723 articles, of which 2,707 were excluded during the screening process (Fig. 1). Sixteen studies [11, 12, 14‒27] that evaluated the US general population met the inclusion criteria. During the screening process, the common reasons for exclusion were failure to report the presence or absence of LOC in people with a history of head injury, reporting TBI cases in pediatrics or adolescents, and evaluating TBI in populations that are at higher risk for TBI such as homeless people, prisoners, mentally ill people, and military personnel. The characteristics of the included studies are summarized in Table 1. Mechanisms of injury were available for only four studies [12, 14, 18, 20]. The total sample consisted of 27,491 individuals, of which 4,453 reported a lifetime history of TBI with LOC (18.2%, 95% CI 14.4–22.7%) (Fig. 2). Among males (11 studies), 1,843 individuals out of 8,854 reported a lifetime history of TBI with LOC (20.8%). Based on the data in nine studies for females, 1,363 individuals out of 11,943 reported a lifetime history of TBI with LOC (11.4%). Nine studies reported TBI cases separately for both males and females, which allowed us to calculate the OR of TBI by sex. The odds of sustaining TBI in males were higher than in females with moderate heterogeneity between studies (OR = 2.09, 95% CI 1.85–2.36, p < 0.01, I² = 40%) (Fig. 3). Although the Egger's test may lack the statistical power to detect bias when the number of studies is small (i.e., k < 10), the Eggers’ test results for pooled odds of TBI by sex do not indicate the presence of publication bias (p < 0.458) (Fig. 4).

We performed a sensitivity analysis by excluding one study with a sample size of less than 50 [12]. The overall prevalence of TBI was estimated to be 17.1% (95% CI 14–20.7%) and the heterogeneity between studies remained high (I² = 98%) (Fig. 5).

While the incidence of TBI is readily available, the prevalence of TBI in the general population is still debated. To our knowledge, this is the largest meta-analysis to identify the overall prevalence of TBI resulting in the LOC in the general adult population of the USA. Our analysis showed an overall prevalence of TBI of 18.2%, which was higher than the overall prevalence of 12.1% reported in the previous meta-analysis by Frost et al. [8]; however, they used the samples from the general population of four developed countries including Australia, Canada, the USA, and New Zealand. Our results show that the odds of experiencing TBI with LOC are 2.09 times higher in males compared to females which is approximately similar to the previous meta-analysis (OR = 2.22) [8]. This suggests that the male gender is a risk factor for sustaining a TBI. According to the reports, nearly 6.2 million US citizens are living with TBI-related disabilities [28], and approximately 35% of the 230,000 hospitalized TBI survivors experience the onset of long-term disability [29]. Therefore, knowing that approximately 18.2% of the general population has sustained a TBI with LOC has significant implications for public health and may change some health-related policies. In a recent meta-analysis by Hunter et al. [30], on people impacted by the criminal legal system, the overall estimate of TBI prevalence was 45.8% (95% CI 37.8–54.1%) which is higher than our estimate of prevalence in the general population. Another meta-analysis of incarcerated groups found that 41.2% of these individuals reported a history of TBI [31]. These findings suggest that there may be a relation between TBI and criminal actions and imprisonment; however, due to different confounding variables and methods of assessing TBI, further studies are needed to prove this association.

There was substantial heterogeneity among the included studies in reporting TBI prevalences, due to different population groups such as college students, hospital staff, and adult individuals, which can explain a degree of heterogeneity between studies. Additionally, some studies employed varying methodologies, like structured interviews or TBI questionnaires, which could lead to biased collection of pertinent data. One more factor causing heterogeneity is that due to age differences, some patients may not remember the brain damage that happened years ago, resulting in discrepancies in reporting TBI incidents. Due to a lack of sufficient data on different age groups, further analyses could not be performed to determine the differences in TBI prevalence among different age groups.

It has been shown that TBI survivors are at a higher risk for major depression, generalized anxiety disorder, post-traumatic stress disorder, and long-term disabilities [32, 33], which can affect the quality of life of TBI patients and their families. In terms of the economic burden, TBI can pose direct and indirect costs to the society and health system [34]; therefore, considering the prevalence of TBI in the general population may help decision-makers modify current public health policies. In comparison with the general population, a greater percentage of psychiatric patients report TBI and they are at increased risk for multiple brain injuries [18], which can be associated with higher costs of care. Based on our analysis, males are about twice as likely as females to experience a TBI with LOC, which is consistent with the results of previous studies [8, 23, 35, 36]. Although this may be due to higher risky activities in men such as substance abuse, alcohol consumption, and engaging more in contact sports, in this study, we are unable to confirm this relationship because the injury mechanisms were not available in most of the identified studies. Contrary to the previous meta-analysis [8], we did not find two studies eligible for entering into our analysis. In the study by Boswell et al. [36], since all participants were from emergency department patients, they may not reflect the general population, additionally, the data regarding adult TBI patients with LOC were not available. Concerning another study by Holmes and Buzzanga [37], we recognized that it is not about TBI followed by LOC, and the authors collected TBI cases separately from LOC cases.

There are several noteworthy limitations to our study. One of the limitations of this study is the possibility of recall bias in the included studies, which can be caused by difficulties in remembering previous events or experiences by the participants, which makes the current findings difficult to interpret. In our analysis, we included studies that defined the TBI as having a TBI followed by LOC as the most reliable and valid marker for determining TBI by ordinary people, because it can be more easily recalled by injured people than PTA. In most studies, TBI is classified based on the GCS, but when it comes to ordinary individuals, self-reported GCS may not be understandable or a valid marker for defining TBI. On the other hand, we may miss some cases of TBI by defining TBI as having a head injury with LOC, because in contact sports, most head injuries may not be followed by LOC, leading to an underestimation of the prevalence of TBI in the general population. Since we restricted our analysis to TBI with LOC, people who have experienced a mild TBI may be missed because as reported most of the TBIs are mild [38] and they may not result in LOC in most of the cases. Accidental falls are one of the main causes of TBI in the elderly [39], and since most of the included studies were conducted on college students, our findings may estimate the prevalence of TBI lower than the actual rate in the general population. The most common causes of TBI have been reported to be motor vehicle accidents, falls, and violence [40]. In our meta-analysis, detailed data on the causes of TBI were not available for all studies; therefore, we are not able to make a correct conclusion in this case, although it is an important factor for interpreting the results. Another limitation of our study is the exclusion of studies targeting specific populations such as patients participating in contact sports, as the probability of sustaining TBI has been shown to be greater in sports/recreational activities compared to the general population [41], potentially resulting in an overestimation of the TBI prevalence in the general population. Due to the self-reported nature of TBI cases in identified studies, it was not possible to determine the severity of TBIs in this analysis.

Loss of productivity and delay in return to work after TBI can cause a notable economic burden due to its frequent occurrence [42]. In the USA, healthcare costs linked to non-fatal TBIs in 2016 were over USD 40.6 billion across Medicaid, Medicare, and private health insurance [7]. Considering the socio/economic burden of TBI on society, prevention programs can be designed to reduce the occurrence and severity of TBI. Reducing the risk of TBI decreases the necessity for post-injury treatment, lowering healthcare expenditures, as well as minimizing the effects of TBI on individuals, and their families. Even though TBI is a common cause of emergency department visits, there is still a lack of clear understanding about it among individuals and its possible consequences, underscoring the need to increase awareness and knowledge. Although during the past decades, many advances have been made in the field of understanding the symptoms, mechanisms, and biology of TBI, there are still shortcomings in the complete treatment of TBI. Therefore, based on what is currently known, prevention is far more effective than cure. There are some ways to TBI prevention, for instance, raising awareness among the public and healthcare professionals about the extent and characteristics of TBI. When it comes to young adults, wearing a seat belt while driving, obeying the rules, and not driving while intoxicated with alcohol or drugs can assist in reducing TBI cases. In certain groups like those participating in contact sports, wearing well-maintained equipment, adhering to play rules, identifying TBI as soon as possible, protecting injured players in the field from additional hazards, and managing head-injured players properly could be effective prevention strategies. Falls are the leading cause of TBI in the elderly. A variety of prevention interventions can be done to reduce the risk of falling and further injuries in older adults which have been shown to lower healthcare costs [43, 44]. For the elderly, improving home safety, using bars along staircases, and regularly assessing their mental status, balance, and memory could be recommended TBI prevention strategies.

To increase the robustness and reliability of present results, future epidemiological studies should use larger sample sizes from different population groups using more standardized questionnaires or structured interviews. Given social differences between states and differences in state laws, further studies in all states are needed to strengthen the current results. These findings could be used to advocate for policy changes, such as improved safety regulations, enhanced healthcare service accessibility for all TBI survivors, and increased funding for TBI research and prevention programs.

Despite all limitations, we found a prevalence of TBI in the US general population of 18.2%, making it a major public health concern. In addition, males were more than twice as likely as females to sustain TBI with LOC. Considering the irreparable long-term consequences of TBI on survivors, their families, and the healthcare system, implementing effective prevention strategies can result in substantial reductions in TBI-related permanent disabilities and medical care costs.

An ethics statement is not applicable because this study is based exclusively on the published literature.

The authors have no conflicts of interest to disclose.

No funding was received for this work.

A.K. was responsible for screening articles, data extraction, quality assessment, data analysis, and drafting the manuscript. A.S. and B.L.-W. were the senior authors responsible for study supervision and manuscript revision.

All required data are provided in Table 1.