Traumatic brain injury (TBI) is a significant public heath problem in the United States. Approximately 2.5 million Americans seek medical services annually for the treatment of mild to severe TBI (1). Given that mild TBIs (mTBIs) are thought to be a “silent epidemic” that often go unrecognized and underreported, there is no consensus on how many of these injuries occur annually (2). Within the sports literature alone, it is estimated that between 1.6 and 3.8 million mTBIs occur annually; however, this excludes injuries due to other causes (falls, accidents, assaults, etc.) and likely underestimates the total number of injuries that occur (3). Within the military, mTBIs have been described as the “signature” injury of the recent wars in Iraq and Afghanistan and have affected more than 300,000 service members since 2000 (4).

Mild traumatic brain injury occurs when a force to the head or body results in the transient disruption of brain functioning for less than 30 minutes, alteration of mental state for less than 24 h, and posttraumatic amnesia for less than 1 day (5, 6). These injuries can cause a wide variety of acute (lasting 1 month or less) and chronic (lasting several months or more) neurological symptoms. These include changes in cognition, emotion, and sensation as well as headaches, nausea, fatigue, and sleep disturbances (7).

Traditionally, mTBI has been viewed exclusively as a temporary change in brain functioning. However, recent neuroimaging evidence, using diffusion tensor imaging (DTI), indicates that these injuries may also result in structural changes to white matter pathways in the brain (8). DTI indicates that neurocognitive deficits after mTBI may lead to impaired executive functioning due to axonal injury in the frontal lobe (9). Changes in these pathways may contribute to the chronic symptoms of mTBI such as poor memory, poor concentration, and sadness (8, 10). For example, retired former professional football players with multiple mTBIs report more cognitive and behavioral symptoms and have higher axial diffusivity on DTI when compared to healthy controls, suggesting that an accumulation of mTBIs over a lifetime is associated with both white matter changes and overt symptoms (11).

In addition to white matter changes, mTBI is also associated with more severe long-term structural changes. For example, extensive atrophy and tauopathy have been observed post-mortem in former male athletes and military personal with a history of repetitive mTBI (12, 13). This neurodegenerative disease, termed chronic traumatic encephalopathy (CTE), is thought to be progressive and stem from exposure to repetitive brain injury.

In the majority of cases, the symptoms of an mTBI resolve within a couple of months. Repetitive exposure to mTBIs, such as those experienced by contact-sports athletes (14–16) and military personnel, has been linked to long-term problems with executive functioning. Executive functioning is the management, regulation, and execution of cognitive processes such as working memory, task switching, reasoning, and problem solving. Previous research suggests problems with executive functioning, within individuals with repetitive mTBIs, emerge by the fifth decade of life, even though the brain injuries occurred decades earlier (14). To date, no studies have examined the impact of lifetime accumulation of repetitive mTBIs on cognition, particularly executive functioning, and mood in a sample of young adults who were not elite athletes. Therefore, the present study examined the influence of repetitive mTBIs acquired over a lifetime on cognitive complaint, self-reported executive functioning, and symptoms of depression.

Histograms for each of the dependent variables demonstrated that the data had a non-normal distribution; therefore, non-parametric statistics were used. Mann–Whitney U tests revealed that participants with r-mTBI compared to those that reported no-mTBI reported more symptoms of depression (U = 246.00, p = 0.006), more change in cognitive functioning over the past 5 years (U = 241, p = 0.005), and more executive functioning problems (U = 269.00, p = 0.018). Given that significant group differences were observed on the overall scale on the BRIEF-A (GEC), follow-up tests were performed on the two subscales. Significant group differences were observed on both of these subscales: BRI (U = 274.50, p = 0.023) and the MI (U = 278.00, p = 0.027). In both cases, the r-mTBI group reported more symptoms than that of the no-mTBI group. Scores on the CCI, Beck Depression Inventory, second edition (BDI-II), and BRIEF-A are listed in Table 2.

Spearman correlation coefficients were used to compare overall scores on the BDI-II, CCI, and BRIEF-A. Number of r-mTBIs correlated positively with the CCI (rho = 0.513, p < 0.000), BDI-II (rho = 0.477, p < 0.000), and the overall score on the BRIEF-A (rho = 0.472, p = 0.010). A significant correlation was also observed on the BRIEF-A MI subscale (rho = 0.460, p = 0.012), but not on the BRI subscale (rho = 0.346, p = 0.066). In all cases, as the number of mTBIs increased, the number of symptoms associated with each measurement increased as well. Figures 1–3 show scatter plots for the relation between self-reported mTBIs and overall scores on the BDI-II, CCI, and BRIEF-A.

Young otherwise healthy undergraduate students with a history of repetitive mTBI compared to those that reported no history of mTBI reported more change in cognitive functioning over the past 5 years, more executive functioning problems over the past 30 days, and more symptoms of depression over the past 2 weeks. These findings suggest that a lifetime accumulation of two or more concussions as compared to a history of no reported concussions may result in worsening symptoms of cognitive function, and increased executive dysfunction and depression. In general, the most common cognitive complaints included remembering names and faces of new people, remembering without notes or reminders, remembering things compared to those within the individuals’ age group, and the perception that people who know the individual would say that their memory is worse. In regard to symptoms of depression, the most common symptoms reported were changes in sleep patterns and appetite, as well as tiredness or fatigue. Interestingly in athletes, fatigue in the acute stage has previously been identified as risk factors for long-term cognitive and behavioral symptoms (21). In regard to executive function, the weaknesses were spread between both metacognitive and behavioral functions.

Consistent with another report in college and professional football players (14), the present study found significant correlations between number of self-reported mBTIs and self-reported rating of cognitive change, executive functioning, and symptoms of depression. Both studies found that as the number of self-reported mTBIs increases, so does self-reported symptoms. Given the recent publicity surrounding CTE in football players, the findings from the former study may have been influenced by only symptomatic individuals volunteering for the study. Given the population in the present study, the effects of this media coverage is likely lessened and therefore provides more evidence indicating support for a relation between number of mTBIs and changes in mood and cognition.

Overall, the present findings are consistent with a growing body of evidence indicating that exposure to mTBIs is associated with long-term problem in mood and cognition, particularly in athletes and military personnel. Within the military, TBI has been linked to self-reported physical and cognitive problems in veterans from the recent wars in Iraq and Afghanistan (22, 23). As these veterans continue to return home and reintegrate into civilian life, these findings suggest they may experience subtle symptoms associated with these injuries.

Though we believe that this study has many strengths, there are also several limitations that should be acknowledged. Given that the sample size was relatively small (58 undergraduate students) and that the participants were from a single urban New England institution, the results may not generalize to all undergraduate students. Given the recent media attention on the long-term effects of mTBIs on mood, cognition, and behavior, it is possible that participants believed there should be a relation between these factors and therefore responded accordingly. Additionally, participants may have had a tendency toward a “yea saying” bias and future studies should include an innocuous questionnaire about an unrelated topic (e.g., food preference) to address this potential bias. Variables such as socioeconomic status, learning disabilities, and average amount of sleep were not specifically addressed in the survey for this study, which may have contributed to the results collected between the BDI-II, CCI, and BRIEF-A.

This studied relied on self-report data, which may or may not correlate with objective neuropsychological testing. Therefore, a follow-up study using neuropsychological testing may be helpful in better understanding the relation between lifetime mTBIs and cognition. Finally, participants’ reporting of number of lifetime mTBIs may be inaccurate. It should be highlighted that in the present study, participants were provided a definition of mTBI that has been used in previous studies to assess the number of lifetime injuries (14, 24) and has been shown to affect the number of injuries reported overall (17). Though this increase may be attributed to demand characteristics, it is important to highlight that a previous study found that providing this definition did not change the number of self-reported mTBIs for 25% of the sample (17). Nonetheless, future studies may consider using a structured clinical interview, such as the Ohio State University TBI Identification Method – Interview Form.

In sum, young otherwise healthy undergraduate students with a history of repetitive mTBI reported more cognitive problems and symptoms of depression when compared to individuals who have not experienced an mTBI. These findings suggest that a lifetime accumulation of mTBIs may result in subtle changes in mood and cognitive symptoms even in highly functioning individuals.

All authors contributed significantly to the study design, data collection, analysis, and interpretation as well as to prepare the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.