Clinical Context and Diagnostic Criteria

TBI is a major cause of morbidity and mortality worldwide. Each year, in the United States alone, more than 2.5 million TBIs occur; over 55,000 individuals die, over 300,000 are hospitalized, and more than 2 million are treated and released from emergency departments.² An unknown but likely very large number of TBIs are unrecognized or dismissed.³

TBI is commonly classified as to its severity based on clinical examination at the time of injury. This classification, based largely on the Glasgow Coma Scale (GCS; scale range 3-15), delineates severe (GCS 3-8), moderate (GCS 9-12), and mild (GCS 13-15) TBI. This GCS-based classification is an excellent predictor of survival to hospital discharge, but does not effectively predict long-term outcome, especially with more mild degrees of injury.⁴ Additional factors used for classification include the duration of unconsciousness, the duration of posttraumatic amnesia, and the presence of focal neurological deficits and imaging ab- normalities.⁵⁶

Mild TBI (mTBI, also termed concussion) is the most common form of TBI.² Patients with mTBI experience a disturbance of neurocognitive function following the injury, which may include confusion, disorientation, imbalance, and other features. Some, but by no means all, mTBI patients experience actual loss of consciousness.⁷ Diagnostic criteria for mTBI include no more than 30 minutes of unconsciousness, no more than 24 hours of amnesia, and absence of focal neurological deficits.⁸ mTBI is typically not associated with abnormalities on conventional computed tomography (CT) and magnetic resonance imaging (MRI).¹ Notwithstanding the relatively mild initial clinical features and absence of conventional imaging abnormalities, clinical manifestations of mTBI do arise from brain pathology, particularly traumatic axonal injury (TAI).⁹

Most patients with mTBI will recover, but a significant minority (15-30%,) will sustain long-term adverse consequences, including persistent post- concussive symptoms, cognitive impairment, and behavioral dysfunction.⁷ Thus the consequences of “mild” TBI are by no means necessarily mild. High- profile coverage of sports and military-related mTBI and its long-term adverse effects, including delayed neurodegenerative disorders, such as chronic traumatic encephalopathy (CTE),¹⁰ have driven increased awareness of mTBI. Patients with GCS scores in the mTBI range (13-15) who do have gross imaging abnormalities are often classified as mild-complicated TBI. These individuals may have worse prognosis for recovery.¹¹