One in five adolescents reported sustaining a lifetime TBI that resulted in a loss of consciousness for at least five minutes or an overnight hospitalization due to symptoms. Males reported more TBIs in their lifetime by six percentage points compared to females. The most commonly reported cause of TBI for our sample was team sports [8]. Our prevalence estimate of 20.2% is some 10 percentage points lower than the retrospective self-reported lifetime TBI estimates seen in some previous studies of older adolescents and young adults employing non-representative or clinical samples [12], [42]–[44]. This estimate is also about 10 percentage points lower than a birth cohort study from New Zealand who captured the hospitalized as well as non-hospitalized TBI events of 1265 individuals from the time they were born to age 25 [1]. The former difference may be due to two factors; our wider and younger age range (11 to 20 years old) that includes early and mid-adolescence, as well as our definition of TBI, which excluded forms of injury not involving loss of consciousness or at least one overnight hospital stay. Further, the OSDUHS exclusion of students in special education classes may have excluded groups with elevated TBI. The latter difference, may be due to cross-cultural differences as well as differences in the age sampling selection (5 years difference between our studies' highest age included). Possible cross-cultural differences between our samples are suggested by the mechanism of injury reported. In our sample, most injuries were due to team sports (63% for males and 46.9% for females), falls (24.7% for females, 5.1% for males) and bicycle accidents (8.1% for males, 1.8% for females) [8]. In the New Zealand sample, for individuals 0–15 years of age, most injuries were due to falls (66.96%) and being hit by an object (10.13%). For youth between 15–25, most injuries happened while playing rugby (21.2%), due to assaults (19.9%) or as a result of motor vehicle accidents (23.4%) [1]. However, it is important to note that mechanisms of injury in the New Zealand sample reflect lifetime injuries, whereas in our sample they only reflect injuries reported in the past 12 months.

Adolescent males and females who reported lifetime TBI also reported past year concurrent daily smoking, drinking, binge drinking, using cannabis, drug use problems, cyberbullying, bullying others, poor current marks in school, and being treated for physical injuries, compared to their peers who did not report a TBI. These results indicate that adolescents with TBI are vulnerable to a range of psychological and behavioural harms that co-occur with their history of a TBI. The association between the range of adverse correlates identified here and TBI suggests that the adverse correlates could represent a coping mechanism to deal with the effects of TBI, or that they may predispose adolescents to TBI or perhaps both [8], [17], [45]. For example, early substance abuse has been found to be a mediating factor for criminal involvement of children and youth with a history of TBI [44]. The potential for negative synergistic effects of combined TBI and mental health and substance problems, including impact on academic performance, social and vocational failure, suggests that this should be a priority area for further investigation [1], [6], [8], [17], [20].

Similarly, a cross-sectional study of 5000 adults from New Haven, Connecticut, found that 7.2% of the adults interviewed recalled a blow to the head, which caused confusion or unconsciousness and these individuals were twice as likely as non injured individuals to suffer from alcoholism, drug abuse, major depression and other psychiatric conditions, along with increased rates of panic attacks and suicide attempts [46]. Since these conditions may take time to develop, parents and children may forget a prior TBI that preceded their occurrence and not link them during medical appointments and reviews of the history leading to the emergence of these conditions, hence medical vigilance on this matter is warranted and recommended.

Female adolescents with lifetime TBI may be more vulnerable for the co-existence of adverse psychological and behavioural conditions than male adolescents with TBI. We found that adolescent females who reported a lifetime TBI were significantly more likely to report four more health-compromising outcomes than males even when holding constant the effect of age. These adverse associations were current elevated psychological distress, past year suicidal ideation, being bullied at school in the past year and past-year cigarette smoking. Further analyses describing the interactive effects among TBI, sex and age showed earlier risk for girls with lifetime TBI to also report daily cigarette smoking, compared with girls without TBI, an effect that was not evident for boys. In the total sample, smokers report smoking their first cigarette at an average age of 14 [29]. Our analysis of the TBI positive group shows that girls, but not boys, with TBI had an earlier onset of smoking compared with girls without TBI. Higher odds associated with past year drinking were evident among adolescent girls and boys with TBI among ages 11 through 13 as well as 14 through 16 compared with those who did not report any TBI. Among 7th graders, 13% had their first drink before the end of grade 6 [29]. We observe here that drinking at an early age is particularly prevalent among adolescents with TBI. The highest odds associated with physical injuries treated by a physician were observed in early adolescence (11 through 13) for girls with TBI and later adolescence (17 through 20) for boys with TBI compared with their counterparts without TBI. In 2011, 44.2% of males and 39.3% of females reported physical injuries that were treated by a physician [29]. We observed here that students with TBI were at higher risk for physical injuries that required treatment by a physician. This risk is particularly higher among young girls age 11 through 13 and older boys age 17 through 20.

Our results are concordant with those of a meta-analysis showing elevated vulnerability for post-TBI symptoms among adult women following mild forms of TBI in adult investigations [19]. Possible bio-social factors that may be responsible for gender differences in outcomes post TBI include premorbid differences, post TBI differences in treatments, hormonal differences, differences in cognitive abilities and psychosocial factors, or a combination of these factors [19]. Future studies should investigate whether these differences could be explained by neurochemical, neuroanatomical or developmental changes post-TBI between sexes. Further, the circumstances around the history of the TBI should be investigated and examined to see if childhood trauma such as a history of abuse or neglect may be associated with TBI and its adverse correlates (46,47). Higher risk for related comorbidities following TBI may also affect the long-term quality of life of these individuals and should be examined by future research [49]. Long-term outcome studies have shown an elevated rate of smoking and drinking among female TBI survivors in relation to the general population [51]. The strong associations among smoking, alcohol consumption or both and breast cancer in women are well known [49]. Data from adult studies, and now from adolescent data, show that women and girls with TBI report concurrent smoking, drinking and binge drinking conditions compared to women and girls without TBI [8], [16], [19], [30], [48]. It is possible that binge drinking rates for girls in this study may have been underestimated compared to other studies. The definition of binge drinking that has been used in the OSDUHS is the same for both males and females (5 or more drinks on an occasion). However, the National Institute on Alcohol Abuse and Alcoholism has suggested that a binge occasion for women be defined as 4 or more drinks in one setting for girls/women, and 5 or more drinks for boys/men [49]. A recent report by the CDCP, revealed that binge drinking (using the NIAAA operational definition of 4 or more drinks in one setting, for women) among young girls was around 20% (one in five) and for women around 24% (one in four) in the US [50]. Nonetheless, our study provides additional support for sex and gender based analyses in TBI research [22], [27], [51].

Further analyses describing the interactive effects between TBI, sex and age showed that late-adolescent males with a lifetime TBI report elevated daily smoking and injuries, whereas late-adolescent females with a lifetime TBI report elevated alcohol use. Female adolescents with TBI had daily cigarette smoking estimates that were 1.4 percentage points different between the upper age categories: 14 to 16 years olds and 17 to 20 years olds. For males, however, the increase was 8.9 percentage points higher between the 14 to 16 years old and 17 to 20 years old age categories. TBI differences among medically treated injuries were lower by 16.2% between the 11–13 year old age group and the 17–20 years old age group for females, but were higher by almost 20% for males. Lastly, the transition from 14 to 16 years to 17 to 20 years widened the sex difference with females reporting higher alcohol consumption than males (89.3% v 81.65). The clinical implications here include promoting vigilance and screening for potential substance use and physical injuries in adolescent patients with TBI. Primary physicians should also inquire about past TBIs among adolescents reporting these conditions during initial consultation. Efforts to prevent TBI during adolescence and intervene at an early stage may reduce injuries and comorbid problems in this age group. Predicting outcomes and providing care for adolescents with TBI remains problematic, in the absence of clinical data. To address this public health issue, a refocusing of research efforts on this population is justified to prevent the re-occurrence of TBI over time and to discern unique care requirements to facilitate best outcomes.