This study sought to assess the relationship between chronic headaches and concussions, analyzing the role of historic concussions on chronic headaches, as well as that of premorbid headaches on future concussion incidence, severity, and recovery. We found a history of concussions was a significant contributor to headache burden among adolescents and young adults. However, those with chronic headaches were not more likely to be diagnosed with a future concussion, despite presenting with more severe concussions that had protracted recovery. Our findings not only suggest the need for conservative management among adolescents and young adults with chronic headaches, it also indicates a potential health care gap in this population. Those with chronic headaches may be referred for concussion diagnosis and management at lower rates than those with no such comorbidities.

The prevalence of chronic headaches among our cohort of student-athletes was 15%. High-quality studies estimate a 5 to 6% prevalence of chronic headaches among the general population [1, 2]. A longitudinal national survey out of Norway found 6% of young adults aged 26 to 28 experienced headaches multiple times a week [1]. A narrative review found a 5% prevalence of chronic headaches in 59 of 347 studies analyzed [2]. Specifically among subjects from 14 studies aged 10 to 19 the prevalence of chronic headaches was 3%. We report a 15% prevalence of chronic headaches among a group of student-athletes with an average age of 15. Our cohort may have a higher incidence of chronic headaches because it only includes student-athletes. Even though our analysis of a specific sub-population may not represent the national chronic headache burden among all adolescents, our findings are relevant to an increasingly athletic youth population, with approximately 60% of American adolescents involved in school-affiliated youth sports [8].

We found that chronic headaches were independently associated with female gender (OR 1.44, 95%CI [1.25–1.67], P < 0.0001), depression/ anxiety (OR 2.90 [2.26–3.71], P < 0.0001), positioning as a lineman (OR 1.59 [1.34–1.88], P < 0.0001), and a history of concussions (OR 2.31 [1.98–2.71], P< 0.0001). Female gender and depression/ anxiety have both been previously implicated in large-scale studies as significant contributors to the national pediatric headache burden [5, 6]. Though chronic headaches are well-reported as post-concussion sequalae [7], a history of concussion has not been similarly implicated in large-scale studies [5, 6]. We found a history of concussions had a greater magnitude (OR 2.31) of effect in its association with chronic headaches than gender (OR 1.44), but not greater than depression or anxiety (OR 2.90), likely making it a moderate contributor to the national pediatric headache burden. Specifically, among those with chronic headaches, a greater number of past concussions (r2 = 0.95) as well as concussions characterized by loss of consciousness (P < 0.0001) were associated with more severe chronic headache and symptom burden. Our findings strengthen the correlation between the two conditions, demonstrating that a history of more severe and repetitive head trauma likely significantly contribute to adolescent headache burden.

We included patients with both post-traumatic headaches and primary migraines in the “chronic headaches” cohort. We believed this would increase the external validity of our findings, especially in clinical settings, where the classification of headache, whether post-traumatic or primary migraine, is not always clear [26,27,28]. Additional sub-group analysis demonstrated that those with chronic headaches classified as primary migraines by physicians also had a significant history of concussion (OR 1.81) [Supplementary Table 3], though to a lesser extent than the entire chronic headaches group (OR 2.31). Even though this does not mean concussions were the cause or primary contributing factor to these patients’ migraines, it suggests head trauma may play a role in the development of physician-diagnosed primary migraines, or vice versa. This piece of evidence adds to the cannon of literature emphasizing the difficulty of differentially diagnosing primary migraines and post-traumatic headaches as defined by the ICHD-3 [26,27,28]. A more specific definition for post-traumatic headaches may be warranted.

Though we initially found the CH cohort had a greater incidence of future concussions than the NH cohort (56 vs. 43 concussions per 100 patient years, P < 0.0001), multivariate analysis controlling for significant demographic, clinical, and sport-related variables yielded no such effect (OR 0.99 [0.85–1.14], P = 0.85). Additionally, more severe chronic headaches were not associated with increased incidence of future concussion (r2 = 0.11), further strengthening the lack of correlation. However, the CH did have greater deviations in Symptom Score from baseline to PI (OR 1.05 [1.01–1.09], P = 0.01), suggesting greater concussion severity as compared to the NH cohort. Increased severity seems to be driven by more severe headaches (1.73 ± 0.06 vs. 1.58 ± 0.03, P = 0.05), more irritability (0.60 ± 0.06 vs. 0.47 ± 0.02, P = 0.03), sleeping more (0.48 ± 0.06 vs. 0.38 ± 0.02, P = 0.05), as well as sleeping less (0.22 ± 0.06 vs. -0.06 ± 0.02, P < 0.0001).

In summary, there was comparable incidence of future concussion among the two cohorts, but greater increase in deviations from baseline to post-injury in symptom burden among those with chronic headaches. Multiple plausible explanations could unify these seemingly disparate findings. First, clinicians may simply have a greater symptom threshold to make the clinical diagnosis of concussion among those with premorbid headaches. Post-concussion symptoms are difficult to distinguish from the day-to-day symptoms those with chronic headaches experience, creating a potential for missed diagnoses and mismanagement of head trauma. In fact, among our patient cohort all 22 classic post-concussive symptoms were elevated at baseline in athletes with chronic headaches as compared to their peers. The challenge in distinguishing post-concussive sequelae from symptoms of premorbid conditions, such as depression and anxiety, has been described before [29, 30]. So long as post-traumatic symptomatic evaluation remains the gold-standard for concussion diagnosis, those with a history of headaches or other neuropsychiatric comorbidities may be prone to clinical anchoring bias of their baseline morbidities as the most plausible explanation of their post-traumatic symptoms [31, 32], leading to underdiagnosis of concussion and therapeutic mismanagement. Second, coaches and athletic staff may sideline those with a history of chronic headaches during intense, high stakes play, resulting in fewer minutes per season for those athletes to be at risk for head trauma [33]. Indeed, chronic headaches were independently associated with more games missed throughout the season (OR per game missed 1.06 [1.03–1.09], P < 0.0001), and when the number of games missed was controlled in the multivariate analysis for future concussion incidence, chronic headaches were no longer associated with an increased risk for future concussion. However, as cautious coaches manage this population of students conservatively, one would expect lower symptomatic and neurocognitive thresholds to concussion diagnosis. Greater, rather than lower, deviations from baseline to post-injury symptomology in this study’s cohort of concussed athletes with chronic headaches makes this explanation less likely.

The most salient positive risk factors for future concussions included participation in contact sports, fewer years of experience in the sport, and a history of previous concussions [Supplementary Table 2]. Previous concussions could put athletes at a greater risk for future concussion because of incomplete recovery, the tendency to be careless on the field, and a lack of situational awareness. This finding could also explain why multivariate models found chronic headaches were not independently associated with future concussions, in that having a history of past concussions more specifically predicts for future concussions. However, supplemental analysis showed a history of past concussions was independently associated with fewer rather than greater deviations from baseline to post-injury Symptoms scores, Processing Speeds, and Reaction Times [Supplementary Table 4]. Less rather than more severe concussions among previously concussed athletes make explanations of past concussions modulating future concussion risk less likely in our cohort. Rather, there may once again be a differential threshold to concussion diagnosis. Specifically, providers may be more cautious in the medical management of those with a history of diagnosed concussions, having a lower threshold to concussion diagnosis in this patient population, secondary to confirmation and anchoring biases of their medical history [31, 32].

In the end, it seems that though concussions may be significant contributors to the national adolescent headache burden, premorbid headaches themselves may not be an independent risk factor for concussion, potentially because of greater clinician threshold to diagnosis, especially considering premorbid headaches were associated with more severe concussions. Though the effect of premorbid headaches on concussion trends has not been studied extensively, preliminary evidence among Canadian ice hockey players in early adolescence suggested that preseason reports of dizziness, headaches, and neck pain were associated with increased concussion incidence [34]. Importantly, patients were asked about these symptoms at one point at the beginning of the season. Subjects were not questioned on the chronicity or severity of these symptoms. The patients in the present study were specifically asked about headaches diagnosed and treated by a physician and were asked to quantify their chronic symptom burden. Furthermore, the Canadian study was limited to ice hockey and only controlled for six demographic and sports-related variables in the multivariable analysis. Our multivariable analysis accounted for 15 of more than 50 demographic, clinical, academic, and sports-related variables that demonstrated significance in univariate analysis and included athletes from a wide range of youth sports.

Those with chronic headaches had greater deviations in Symptom Score from baseline to FU (OR 1.11 [1.00-1.22], P = 0.04), suggesting impaired symptomatic recovery as compared to those without chronic headaches. Specifically, females (OR 1.13 [1.01–1.27], P = 0.04) and those with headaches characterized by dizziness at baseline (OR 1.08 [1.00-1.17,] P = 0.048) were more likely to have severe concussions and impaired recovery. Moreover, those with no history of chronic headaches, reported certain symptoms at follow-up that were even better than at baseline, including vomiting (-0.02 ± 0.006), insomnia (-0.09 ± 0.05), nervousness (-0.04 ± 0.02), and sleeping less than usual (-0.17 ± 0.02). However, those with a history of chronic headaches had no such improvements.

Given significant symptom overlap between chronic headaches and post-concussive sequela, recovery among this patient cohort presents new challenges. School aged girls with premorbid headaches have shown to take significantly longer to recover from a concussion [11]. We confirm such trends in the adolescent and young adult population. Those with a history of premorbid headaches, especially females and those whose headaches were characterized by dizziness at baseline, were less likely to recover to baseline symptomatology at a median of 7 days post-concussion than those with no such history. The greater symptomatic severity of concussions at post-injury among those with a history of premorbid concussions may also play a role in protracted recovery, however this variable was controlled in the multivariable analysis. Multiple premorbid conditions, including depression/ anxiety [23] and a history of prior concussions [35], have also shown to impede recovery, however both comorbidities were also controlled in the adjusted analysis. It seems head trauma increases the severity of chronic headaches longitudinally, impeding recovery to baseline levels.

Our findings suggest sports-related concussions have likely become a significant contributor to the national adolescent headache burden. Now more than ever post-concussion syndrome ought to be on the differential when assessing chronic headaches in both male and female youth, especially given its specific management paradigms, ranging from counseling return to athletic participation, especially at high-risk on-field positions, to management of its neuropsychiatric manifestations, such as depression and anxiety [36]. It does not help that post-concussion syndrome has been reported as prevalent and vastly underdiagnosed in the pediatric population, and that it seldom resolves spontaneously, remaining persistent even 60 months after concussion [37]. Moreover, with promising therapies for post-concussion syndrome such as hyperbaric oxygen in the clinical trial pipeline [38], recognizing the role of concussions on the national adolescent headache burden will be vital for proper management of pediatric headaches into the future.

Next, though we found athletes with chronic headaches were more likely to be sidelined during the season, recognizing that head trauma confers more severe concussions with protracted recovery in this patient population should encourage even more conservative counseling on return to play, especially among females and those with headaches characterized with dizziness.

Finally, we hypothesized that concussions may be underdiagnosed among youth with chronic headaches, given significant symptom overlap between the two conditions. Specifically, we found clinicians referred patients with chronic headaches to post-injury testing when they had significantly greater deviations in symptoms from their baseline as compared to their peers. Clinical underdiagnosis of concussion can lead to mismanagement of head trauma, with continued engagement in scholarly and athletic responsibilities, loss of critical periods of rest, and earlier return to play [39]. Mismanagement of head trauma, in turn, can lead to significant long-term complications, including increased risk for further head trauma, second impact syndrome, and post-concussion syndrome [40]. Of note, unlike the Symptom composite, the four neurocognitive ImPACT composite scores were either impaired to a similar extent at post-injury or marginally elevated in those with premorbid headaches as compared to those with no such history, suggesting similar neurocognitive concussion severity and/ or a comparable neurocognitive threshold to concussion diagnosis. Simply put, physicians may not be taking changes in verbal memory, visual memory, reaction time, or processing speed into consideration when making a clinical concussion diagnosis. As previously suggested [40,41,42,43], clinical concussion diagnosis supplemented with tests tracking changes in neurocognitive ability may help distinguish post-concussive symptom sequelae from chronic headache symptomology, potentially addressing some extent of underdiagnosis in this patient population. Interestingly, Verbal Memory and Reaction Time were not elevated among those with chronic headaches at baseline, meaning post-injury neurocognitive testing may be beneficial for diagnosis even with a lack of baseline testing to normalize the outcomes. Not only would appropriate diagnosis lead to better short-term and long-term clinical outcomes, it may also have implications for the health-care industry at large as well as insurance providers.

Of note, the CDC currently lists computerized cognitive testing as a moderate, Level C recommendation for mild traumatic brain injury among children [16]. Additionally, only 20% of American and a mere 1% of Canadian family practitioners have been reported using neurocognitive testing [44]. Our findings may support the increased use of validated, age-appropriate computerized cognitive testing among student-athletes with premorbid neuropsychiatric comorbidities such as chronic headaches.

This study included a large sample of adolescent and youth athletes from multiple sporting backgrounds accounting for over a decade of data. The data acquired was extensive with over 50 demographic and clinical variables, as well as more than 25,000 ImPACT assessments. The data was acquired prospectively, and data collection was supervised by a national research-oriented organization. Of note, in those that endorsed or denied chronic headaches at baseline, only 14 patients had missing/ incomplete data in any of the 50 + variables at baseline, meaning 99.8% of baseline tests had complete data, suggesting rigorous quality assurance. To our knowledge, this is the first study to examine the role of historic concussions, as well as the incidence of future concussions and concussion severity among adolescents and young adults with premorbid chronic headaches.

There are multiple limitations worthy of acknowledgement. First, concussions were not always initially examined by physicians. Other members of the athletic staff, including physiotherapists and athletic trainers also evaluated head injury. Though the gold-standard remains physician diagnosis of concussion, initial examination of head trauma on the field is typically performed by a variety of athletic staff and midlevel providers who are trained in concussion management, bolstering the external validity of our study. If students are to continue to be examined by non-physicians, then it is imperative that all members of the adolescent health care team understand that those with a history of chronic headaches may require more conservative management and may potentially be underdiagnosed. Second, though the data was prospectively collected in a well-organized electronic format and supervised by a research-oriented organization, the analysis was completed retrospectively. This means the database was not specifically curated for the question at hand. We could not include variables such as the months or years experiencing chronic headaches in our analysis. Finally, though a decade worth of data is an apparent strength of the analysis, we must acknowledge the significant changes in concussion diagnosis and management guidelines in this timeframe, including increased outpatient follow-up and return to play time [16]. Even so, the recent endpoint of the study (2019) bolsters its external validity.