Mild Traumatic Brain Injury as a Heterogeneous Neurological Disorder: Definition, Epidemiology, Neurobiology, and Clinical Course

Authors: Youngbo Shim, Minwoo Kim, Hong Gyu Yoon, Jong Joo Lee, Je Beom Hong, Yeongu Chung, Jae Keun Oh, Yu Sam Won

Abstract

Mild traumatic brain injury (mTBI) is the most common form of traumatic brain injury (TBI) and represents a major yet frequently underrecognized public health concern. Although traditionally considered a transient and benign condition, mTBI is not recognized as a biologically complex and clinically heterogeneous disorder, with a substantial proportion of patients experiencing prolonged cognitive, emotional, and somatic symptoms. Current diagnostic frameworks remain largely symptom-based; however, recent consensus efforts have aimed to improve definitional precision and applicability across diverse clinical settings. In Korea, nationwide data indicate that mTBI constitutes a substantial share of the overall TBI burden, with increasing relevance in older adults and notable variation according to occupational and regional context. Mechanistically, mTBI involves not only primary biomechanical injury but also downstream neurometabolic disturbance, blood–brain barrier dysfunction, neuroinflammation, and axonal and network-level alterations. Clinically, recovery is often favorable but highly variable, with distinct trajectories shaped by acute symptom burden, premorbid vulnerability, and environmental factors. Future progress will depend on more biologically informed classification systems, validated biomarkers, and longitudinal studies that better capture heterogeneity in mechanisms and outcomes.

INTRODUCTION

Mild traumatic brain injury (mTBI) accounts for the majority of traumatic brain injuries worldwide and represents an important, yet often underestimated, public health concern.415) Despite the term “mild,” its clinical impact is not always minor. A substantial subset of patients experience persistent cognitive, emotional, and physical symptoms beyond the acute phase, challenging the traditional assumption that mTBI is uniformly transient and clinically inconsequential.1623) Recent advances in neuroimaging and translational neurobiology have further reshaped the understanding of mTBI, demonstrating that even injuries without overt abnormalities on conventional imaging may involve meaningful disruptions in brain structure and function.3825)

At the same time, several challenges continue to limit progress in both research and clinical care. Diagnostic criteria remain partly inconsistent across studies and clinical settings, symptom presentations are heterogeneous, and recovery trajectories vary considerably among individuals.417) These factors complicate classification, hinder comparability across studies, and pose challenges for standardized management. In this review, we summarize current perspectives on mTBI, with a focus on its definition, epidemiology in Korea, neurobiological mechanisms, and clinical course.

DEFINITION OF mTBI

mTBI is defined mainly by clinical features rather than by abnormalities on conventional structural imaging. This reflects the fact that mTBI often produces functional and microscopic disturbances that are not readily captured by routine computed tomography (CT) or standard magnetic resonance imaging (MRI). Although multiple definitions have been proposed over time, the most widely adopted framework originated from the American Congress of Rehabilitation Medicine (ACRM) Mild Traumatic Brain Injury Committee in 1993 and was later supported and refined by the World Health Organization Collaborating Centre Task Force.14) These criteria have served as the principal foundation for clinical diagnosis and research enrollment.

Under the traditional ACRM framework, mTBI is identified by a Glasgow Coma Scale score of 13–15 after injury together with at least one indicator of acute brain dysfunction, such as loss of consciousness lasting less than 30 minutes, post-traumatic amnesia lasting less than 24 hours, altered mental status at the time of injury, or transient neurological abnormalities.14) This definition distinguishes mTBI from moderate and severe traumatic brain injury (TBI) while preserving a clinically practical threshold for diagnosis. However, the category remains broad and includes patients with markedly different symptom severity, recovery patterns, and biological profiles.

To address these limitations, the ACRM published updated diagnostic criteria in 2023 following a structured Delphi consensus process.2224) The revised framework places greater emphasis to establishing a plausible mechanism of injury and integrating clinical signs, symptom clusters, examination findings, laboratory evidence, and neuroimaging abnormalities when available. It also formalizes the assessment of confounding factors and introduces the concept of “suspected mTBI” when diagnostic certainty is limited (FIGURE 1). Although these updates improve conceptual clarity, they do not eliminate the fundamental issue that mTBI encompasses a broad and biologically diverse spectrum. This underscores the need for future classification systems that move beyond symptom-based criteria and incorporate mechanistic and prognostic information.

In the literature, the terms “mTBI,” “concussion,” and “concussion-type injury” are often used interchangeably; however, they may carry distinct clinical or contextual implications. In this review, “mTBI” is used to denote the formal clinical diagnostic category, whereas “concussion” is used in a more traditional or symptom-based context, and “concussion-type injury” is applied when referring to administrative or epidemiological datasets. These distinctions are maintained consistently throughout the manuscript.

EPIDEMIOLOGY OF mTBI IN KOREA

In Korea, nationwide administrative data suggest that mTBI, often approximated by concussion-type diagnoses in claims databases, accounts for a substantial proportion of the overall TBI burden. In a population-based longitudinal analysis using the National Health Insurance database from 2008 to 2017, Kim et al.12) reported that the age-adjusted incidence of overall TBI declined over time, whereas concussion remained considerably more common than intracranial injury, reaching 415.2 per 100,000 and 72.0 per 100,000, respectively, in 2017. Notably, this decline was not uniform across age groups, as the burden of TBI and concussion increased among older adults, particularly those aged 70 years or older. These findings indicate that mTBI constitutes a major component of the national TBI burden in Korea and is becoming increasingly relevant in the context of population aging.

A complementary perspective is provided by occupational data. Using Workers’ Compensation Insurance records from 2010 to 2019, Bae and Lee2) reported a mean incidence of work-related TBI of 28.4 per 100,000 workers, including 12.9 per 100,000 for mTBI and 15.5 per 100,000 for moderate-to-severe TBI. Although mTBI incidence exceeded that of moderate-to-severe TBI in 2010, this relationship reversed after 2011. Because these estimates were derived from compensation-based data, they should be interpreted with caution, as observed incidence may reflect not only the occurrence of workplace brain injury but also reporting behavior, claim submission patterns, approval processes, and diagnostic coding practices. Nevertheless, these findings suggest that mTBI represents an important component of occupational injury in Korea and warrants consideration in workplace surveillance and prevention strategies.

More recent nationwide data further highlight substantial age-, sex-, and region-related variation in the overall burden and management of TBI in Korea. Using the National Emergency Department Information System from 2016 to 2018, Jwa et al.10) analyzed 117,830 patients with TBI managed by neurosurgery in emergency medical centers and reported an annual crude incidence of 79.4 per 100,000 persons, with peak incidence in children aged 0–4 years and adults aged 75–79 years. The study also demonstrated marked regional differences in ambulance use, trauma center utilization, neurosurgical procedures, and in-hospital mortality. Although this cohort was not limited to mTBI, it provides important healthcare-system context for understanding the care burden of head injury in Korea.

Compared with international reports, the epidemiology of mTBI in Korean shares the increasing burden of head injury in older adults while exhibiting a somewhat mixed pattern of injury mechanisms. Korean nationwide data indicate that concussion-type injury remains a major component of the overall TBI burden, with increasing impact among older adults despite an overall decline in TBI incidence. This pattern is consistent with international observations showing a growing burden of mTBI in elderly populations, particularly in relation to falls.1320) However, unlike the predominantly fall-related pattern often reported in older populations in high-income countries, Korean data suggest that traffic-related injuries continue to contribute substantially at the population level, whereas falls become more prominent with advancing age. In addition, occupational estimates should be interpreted cautiously, as compensation-based datasets may be influenced by underreporting of milder injuries, claim behavior, and administrative processes.

NEUROBIOLOGICAL MECHANISMS OF mTBI

mTBI is increasingly recognized as an evolving pathological process rather than a purely transient functional disturbance. Even when conventional imaging appears normal, mTBI can trigger a cascade of interacting molecular, vascular, inflammatory, and network-level changes that contribute to both acute symptoms and prolonged sequelae. This broader conceptual framework helps explain why patients with apparently minor injury may develop persistent clinical impairment despite the absence of overt structural lesions on routine imaging.

The earliest stage of injury reflects rapid acceleration–deceleration and rotational forces applied to the brain. Unlike severe TBI, which is more commonly associated with focal contusions, hematomas, or overt tissue destruction, mTBI more often involves diffuse biomechanical strain affecting axons, neuronal membranes, synapses, and microvasculature. These forces can produce transient membrane disruption and subtle axolemmal injury, especially in vulnerable white matter pathways. Although such changes are typically microscopic and spatially diffuse, they can initiate downstream disturbances in ionic homeostasis and axonal transport that are clinically meaningful even if they remain undetectable on standard CT or MRI.1927)

Following this primary insult, the injured brain enters a neurometabolic cascade characterized by potassium efflux, calcium influx, and excessive glutamate release. The result is increased metabolic demand in the setting of relatively reduced or regionally dysregulated cerebral blood flow, leading to a mismatch between energy supply and demand. Mitochondrial dysfunction, oxidative stress, lactate accumulation, and impaired oxidative metabolism further amplify cellular stress. Importantly, these metabolic abnormalities may persist beyond the resolution of overt clinical symptoms, providing a biological basis for the period of increased vulnerability after concussion and the risks associated with premature return to cognitively or physically demanding activity.1927)

Accumulating evidence also implicates blood–brain barrier (BBB) disruption as a key feature of mTBI pathobiology. Even in the absence of overt hemorrhagic lesions, subtle BBB dysfunction may permit blood-borne factors to enter the brain parenchyma and alter neuronal and glial function. Experimental and translational studies have shown that regions of BBB leakage after mTBI exhibit marked reductions in neuronal markers such as NeuN, parvalbumin, and CaMKII despite preserved neuronal survival, suggesting the emergence of an altered but viable neuronal state rather than frank neuronal loss. These findings, together with evidence of synaptic spine alterations, support the hypothesis that BBB dysfunction contribute to persistent abnormalities at both cellular and circuit levels.918)

Neuroinflammation further shapes this secondary injury response. Activated microglia and astrocytes release cytokines, chemokines, and reactive oxygen species that may facilitate repair under certain conditions but can also promote synaptic dysfunction, altered plasticity, and persistent symptoms when prolonged or dysregulated. Previous studies have highlighted inflammatory mediators such as interleukin (IL)-1β and IL-6, as well as impaired glutamate transporter-1–mediated glutamate regulation, as contributors to sustained excitotoxic and inflammatory stress following mTBI.19) These findings support an integrated model in which vascular disruption, glial activation, excitotoxicity, and oxidative stress reinforce one another over time.

Although diffuse axonal injury is classically associated with more severe trauma, mild TBI can also produce microstructural axonal damage. Axolemmal permeability changes, intracellular calcium accumulation, cytoskeletal disruption, and impaired axonal transport contribute to abnormalities in white matter integrity. Advanced neuroimaging studies have identified corresponding alterations in diffusion-based metrics, as well as structural and functional connectivity disturbances in patients with mTBI. These large-scale network abnormalities provide a mechanistic framework for understanding persistent cognitive, emotional, and behavioral symptoms after concussion.714)

Taken together, the neurobiology of mTBI is best conceptualized as a multiscale and temporally evolving process. Primary biomechanical stress is followed by neurometabolic disturbance, BBB dysfunction, inflammatory and glial responses, and subsequent network-level dysregulation. This framework helps explain both the heterogeneity of clinical trajectories and the frequent mismatch between conventional imaging findings and real-world functional impairment (FIGURE 2).

CLINICAL COURSE AND NATURAL HISTORY

Most patients with mTBI show clinical improvement within days to weeks after injury; however, recovery is neither uniform nor invariably complete. In non-athletic adult populations, a recent systematic review reported symptom resolution in only 49.0%–69.5% of patients at 1 month, 40.8%–84.4% at 3 months, 38.3%–72.2% at 6 months, and 58.1%–68.3% at 12 months.6) These figures indicate that a substantial proportion of patients remain symptomatic well beyond the acute phase. Earlier reviews similarly concluded that, although many adults recover within several months, persistent post-concussive symptoms are not uncommon and reported prognosis varies considerably depending on definitions, outcome measures, and follow-up methods.4) Taking together, current evidence supports a more nuanced view of mTBI recovery, in which early improvement is common but prolonged symptoms remain within the expected spectrum of outcomes.

Recovery after mTBI is also highly heterogeneous. In a prospective adult cohort analyzed using group-based trajectory modeling, Keatley et al.11) identified four distinct symptom trajectories over the first 6 9% of patients had minimal acute symptoms that improved over time, 45% had mild symptoms that decreased, 33% had relatively higher symptoms that improved, and 13% had relatively higher symptoms that worsened over time. These findings show that average group-level improvement may obscure clinically important subgroups with persistent or progressive symptom burden. Consistent with this, Rabinowitz et al.21) reported that 52% of adolescents and young adults with mTBI continued to show elevated symptom levels at 3 months compared with orthopedic injury controls, further underscoring that a substantial subset does not follow a simple rapid-recovery pattern.

Poor recovery appears to be influenced by a combination of acute injury-related characteristics and premorbid vulnerability factors. In a recent systematic review of non-athletic adults, the most consistent predictors of prolonged symptoms were greater initial symptom burden and pre-existing psychiatric conditions such as anxiety or depression,6) while several studies also identified early post-injury symptoms such as headache, nausea, and dizziness as markers of less favorable recovery.526) In the trajectory study by Keatley et al.,11) female sex, prior depression, and other psychiatric history were associated with a subgroup characterized by relatively high acute symptoms with subsequent worsening, whereas higher education and the absence of loss of consciousness were associated with a minimal-symptom trajectory. In the same cohort, depression, anxiety, sleep disturbance, and pain at 1 month differed significantly across trajectory groups, suggesting that early post-injury affective and somatic symptoms are closely linked to subsequent recovery patterns. Rabinowitz et al.21) likewise found that older age, female sex, and greater acute symptom severity predicted poorer symptomatic recovery at 3 months, while lower socioeconomic status was associated with worse cognitive outcome. Overall, the natural history of mTBI is better conceptualized as a spectrum of recovery trajectories than as a single stereotyped course, with acute symptom burden, psychiatric vulnerability, and post-injury mood, sleep, and pain disturbances contributing to the risk of persistent symptoms.

CLINICAL AND HEALTHCARE SYSTEM IMPLICATIONS IN KOREA

Recent refinements in diagnostic frameworks and advances in the biological understanding of mTBI have important implications for both clinical practice and healthcare systems in Korea. In emergency care settings, improved diagnostic clarity may facilitate more accurate identification of patients with suspected mTBI, including those without overt imaging abnormalities. In rehabilitation, increasing recognition of the heterogeneous and multi-domain nature of mTBI may support more individualized treatment approaches addressing cognitive, vestibular, and psychological components.

At the healthcare system level, evolving diagnostic criteria may also influence insurance coverage, healthcare utilization, and industrial accident compensation. Broader and more biologically informed definitions may increase the number of patients classified as mTBI, potentially affecting claim patterns and resource allocation. In occupational settings, changes in diagnostic thresholds may alter reporting rates and eligibility for compensation, underscoring the need for alignment between clinical definitions and administrative frameworks. These considerations highlight the importance of integrating emerging scientific insights into both clinical decision-making and policy development.

FUTURE DIRECTIONS AND UNRESOLVED QUESTIONS

Despite substantial progress in recent years, several fundamental questions remain unresolved in mTBI. One major challenge is the absence of a unified framework for defining and classifying the condition across clinical, research, imaging, and mechanistic contexts. Current diagnostic approaches continue to rely heavily on symptom-based criteria and broad severity thresholds, whereas accumulating evidence indicates that mTBI is highly heterogeneous with respect to biomechanics, cellular injury patterns, recovery trajectories, and long-term outcomes. This heterogeneity complicates comparisons across studies, limits reproducibility, and hinders the development of targeted interventions. At the same time, there is a critical need for objective biomarkers to improve diagnosis, risk stratification, and prognostication. Although blood-based biomarkers, advanced imaging techniques, and multimodal prediction models have shown promise, they have not yet reached a level of standardization or clinical utility sufficient for routine implementation.619)

Another key priority is to better define the long-term consequences of mTBI and the factors that determine vulnerability to persistent dysfunction. While many patients recover within weeks to months, a substantial subset experience prolonged symptoms, persistent cognitive difficulties, or enduring neurobiological abnormalities even after a single injury. Experimental evidence further suggests that processes such as BBB disruption and atypical neuronal responses may persist for months, raising important questions about how early pathophysiological changes translates into chronic clinical impairment.1518) Future research should therefore move beyond group averages and focus on biologically informed subtyping, longitudinal recovery trajectories, and the interaction between injury-related factors and environmental context, including psychiatric comorbidity, social determinants, occupational demands, and repeated exposure risk. Resolving these questions will be essential for advancing both mechanistic understanding and precision clinical care in this prevalent yet complex condition.

CONCLUSIONS

mTBI should no longer be regarded as a uniformly transient or biologically insignificant condition, but rather as a complex and evolving disorder. It represents a heterogeneous clinical and pathobiological entity encompassing diverse mechanisms of injury, variable recovery trajectories, and a meaningful risk of persistent impairment. Contemporary evidence indicates that mTBI is common, increasingly relevant in the context of population aging, and associated with complex neurobiological processes involving metabolic disturbance, vascular dysfunction, inflammation, and network-level alterations. Although many patients recover favorably, a substantial subset experiences prolonged symptoms shaped by interactions among injury-related factors, premorbid vulnerability, and post-injury psychosocial and biological influences. A more precise and clinically useful understanding of mTBI will require improved classification systems, validated biomarkers, and longitudinal multidimensional research capable of capturing the true heterogeneity of this disorder.