2.1 Standard Protocol Approvals, Registrations, and Patient Consents

The study was approved by the Vanderbilt Institutional Review Board (VUMC IRB #230651). Written informed consent was obtained from all participants in the study.

2.2 Participants and Procedures

A cross-sectional survey study was conducted. Recruitment for the study was done via ResearchMatch [17], a national health volunteer registry that was created by several academic institutions and supported by the US National Institutes of Health as part of the Clinical Translational Science Award program. ResearchMatch has a large population of volunteers who have consented to be contacted by researchers about health studies for which they may be eligible. Potential participants were invited to be part of a study titled “Assessing Brain Health in Adults.” There was no mention of sports, traumatic brain injuries, concussion, or CTE in the study information to reduce potential participation bias. All participants were at least 18 years of age. No other exclusion criteria were applied. There was no financial compensation associated with this study.

2.3 Survey

The survey sent to participants assessed for all features of TES as defined in the consensus criteria [14]. Additional information participants were asked to provide general demographic information, concussion history, sports participation history, general health history, and any current symptoms of any condition as described in the following categories. All data were self-reported.

2.3.1 Repetitive Head Impacts

A detailed history of participants’ involvement in sport was queried. Participants reported each organized sport they competed in. For each sport they reported playing, they specified how many years they played at each level (i.e., number of years before high school, during high school, during college, semi-professionally, and professionally). Questions assessing other potential repetitive head impacts (e.g., military, law enforcement service, or domestic violence) were also included in the survey. For individuals who reported having experienced domestic violence or law enforcement/military service, further information was gathered about the number of times they engaged in combative training (in days), number of explosions/breaches they experienced overpressure, number of rounds of heavy weapons, number of controlled detonations, number of improvised explosive devices (IEDs), and number of instances in which they experienced domestic violence with strikes to the head.

2.3.2 Concussion History

The recently published TES consensus criteria do not include a history of concussion(s) as part of its research diagnostic criteria. However, we believed this was important to assess. The survey presented this definition of concussion: “We define a concussion as a blow to the head or whiplash that caused any one or more of the following: (1) witnessed loss of consciousness (being “knocked out” and someone seeing it), (2) loss of memory for events immediately before and/or after the injury, or (3) feeling dazed and confused for at least 30s.” Participants reported the number of concussions they experienced and the date of their most recent concussion.

2.3.3 Proposed Core Clinical Features of TES

The TES consensus criteria outline three core clinical features, which were assessed. Participants were asked if they had significant problems with (1) episodic memory (“significant problems with my memory for specific events that I have experienced, such as recent conversations or important things I have done in the past two weeks”), (2) executive functioning (issues with “planning things in my daily life; organizing my daily schedule; flexible thinking; inhibiting my impulses; shifting between tasks; multitasking; problem solving”), and (3) neurobehavioral dysregulation (“significant problem controlling my emotions and behavior” such as issues with explosiveness, impulsivity, rage, violent outbursts, having a “short fuse,” “mood swings”) for 1 year or more. Answer choices were: no; yes to some degree; and yes definitely. If participants endorsed either “yes” option, a follow-up question assessed if they feel this problem has become worse in the past year to assess the progressive nature of each feature.

2.3.4 Other Current Symptoms

Supportive features of TES were also assessed, such as dysarthria, ataxia, imbalance, and tremor over the past year (response options: never, rarely, sometimes, often, always). Lifetime history of anxiety (yes/no) and depression (yes/no) were also queried based on the participant endorsing if a healthcare provider told them they had either diagnosis. Current symptoms of depression and anxiety during the past 2 weeks were assessed using the Patient Health Questionnaire-9 [18] (PHQ-9) and the Generalized Anxiety Disorder-7 [19] (GAD-7). Functional status was gauged using the follow scale: independent, slightly reduced performance/functioning, definite impairment, not independent, and cannot participate (see Table 1 of the Electronic Supplementary Material for a detailed explanation of functional status). Though not a part of the TES consensus criteria, frequency of sleep difficulties (i.e., “trouble falling or staying asleep”), chronic pain (i.e., pain in one or more parts of my body”), and migraine headaches were assessed over the past year (response options: never, rarely, sometimes, often, always).

2.4 Sport Exposure Criterion

Based on the TES consensus criteria, participants were placed into groups based on their history of involvement in high-exposure contact/collision sports that was likely associated with substantial repetitive head impacts. Consistent with Katz et al. [14], substantial exposure to contact sport (hereby referred to as the Sport Exposure Criterion) was defined in this study as having ≥ 2 years of contact sport participation during high school (or beyond) and ≥ 5 years of contact, collision, or combat sport participation during their lifetime. Football, soccer, lacrosse, boxing, hockey, rugby, martial arts, and wrestling were included and considered high-risk contact sports in this study. Participants who did not meet the Sport Exposure Criterion and denied other forms of repetitive head impacts were used as a comparison group. Participants who did not meet the Sport Exposure Criterion but endorsed a history of other repetitive head impacts (e.g., military service, law endorsement, domestic violence) were separated into a “Other Head Impacts” group and not included in the main statistical analyses given that the minimum threshold for these head impacts has not yet been established.

2.5 Statistical Analyses

Descriptive statistics were conducted to provide an overview of the sample, including the proportions of individuals who met the Sport Exposure Criterion. Additionally, the proportions of the sample that reported each core clinical feature of TES were presented, along with the proportions of those who reported other health problems unrelated to TES. The χ2 analysis was employed to assess the relative proportions of individuals who fulfilled the core clinical features of TES, categorized based on whether they met the Sport Exposure Criterion versus those who did not meet this criterion or endorse other head impacts. In the same manner, a χ2 analysis was utilized to examine the proportions of individuals who manifested health issues unrelated to TES, grouped according to whether they met the Sport Exposure Criterion or not. Two binary logistic regressions were performed to predict the presence of one or more progressive core clinical features of TES using several independent variables. In both regressions, our primary independent variable was the Sport Exposure Criterion. The first regression included covariates that are independent of the consensus TES criteria (i.e., age, gender, chronic pain, and sleep problems). In the second model, we extended the variables from the first model and introduced psychiatric features, which are considered supportive features of the consensus TES criteria. It was important to ensure that these psychiatric features did not fully explain the TES core clinical features. We operationalized the psychiatric features by considering a history of psychiatric disorders (i.e., depression, anxiety, post-traumatic stress disorder, and substance abuse) as well as screening positively for current depression or anxiety (i.e., PHQ-9 ≥ 10 or GAD-7 ≥ 10). In a sub-analysis including individuals with no history of head impacts (i.e., no prior concussion history, as well as no contact sport exposure, military exposure, or history of head trauma from domestic violence), we present the proportions of those who reported TES features. These proportions are stratified based on the significant covariates included in our second multivariable model. All analyses were conducted using SPSS version 28.0 (IBM, Armonk, NY, USA).

2.6 Data Availability

The survey, statistical analyses, and underlying data supporting the conclusions of this article will be made available by the authors to qualified researchers, without undue reservation.