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

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.