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Traumatic brain injury (TBI) damages the brain's structure and normal function caused by a head impact or external force. Based on the neurobehavioral deficits after the injury and Glasgow Coma Scale (GCS) score, TBI can be classified as mild, moderate, or severe. All blows or hits to the brain do not cause TBI. Penetrating injuries, blunt trauma, and blast injuries may all cause TBI.

Classification

TBI can be classified in many ways, including location, type, severity, mechanism of injury, and physiological response to injury.

TBI is highly heterogeneous, with short and long-term outcomes affected by age, the specific intracranial injury, concomitant extracranial injury, and pre-existing comorbid Conditions. 

Most classification systems are based on clinical exam, patient symptomatology, or diagnostic details during the early phase and do not account for the TBI process later on.

Classification based on clinical seriousness

GCS is used to classify TBI into levels of severity and prognosis. After TBI, there is an opposite relationship between the GCS score and the rate of intracranial injury (ICI), the incidence of positive findings on computed tomography (CT), and the need for neurosurgical procedures(craniotomy, elevation of skull fracture, increased intracranial pressure monitor) doubles when the GCS drops from 15 to 14.

• Mild TBI:  mortality 0.1%, GCS 13-15
• Moderate TBI: mortality 10%, GCS 9-12 
• Severe TBI: mortality 40%, GCS <9

Classification by broad etiology

It may be non-invasive, piercing, and blast-related.

Blunt or non-invasive: TBI occurs when external mechanical force leads to rapid acceleration or deceleration with brain impact. It is typically found in motor vehicle-related injuries, physical altercations- falls, or crush injuries.

Penetrating TBI: Occurs when an object penetrates the skull and ruptures the dura mater, seen commonly in stab wounds and gunshots. 

Blast TBI: commonly occurs after warfare and bombings due to a combination of overpressure, contact and inertial forces, and acoustic waves.

Classification by the involved area

TBI can be classified by area of involvement, as focal or diffuse, although the two types usually exist together.

Diffuse brain injury encompasses

• diffuse axonal injury (DAI), 
• hypoxic brain injury, 
• Diffuse cerebral edema, or
• Diffuse vascular injury.

Focal injury includes specific lesions such as intracranial hematomas, contusions, infarctions, cranial nerve avulsions, axonal tears, and skull fractures.

Classification by progress of injury: Primary vs. Secondary

Primary injury is due to abrupt mechanical force, whether blunt, penetrating, or blast, and may include the following:

• Concussion
• Hematoma
• Skull fracture
• Contusion
• Axonal shear or laceration
• Subarachnoid or focal hemorrhage.

Secondary injury refers to pathophysiological results of the primary injury and includes complex neurobiological events initiated or altered at a cellular level following the primary injury, and may include the following:

• Hemorrhage
• Cerebral edema
• Seizures
• Infection
• Increased intracranial pressure
• Ischemia

Epidemiology

TBI is the primary cause of mortality and morbidity accounting for approximately 2.5 million emergency department visits yearly in the US. The probability of disability and death due to TBI is higher in low- and middle-income countries compared with high-income countries.

90% of patients with TBI approx. suffer a minor head injury and are treated and sent home without hospital admission or intervention. 

Some 10% have injuries that require hospitalization, and about 2% die.

TBI occurs most frequently in very young children (ages 0 to 4 years) and young adulthood and adolescents (ages 15 to 24 years), with a visible peak in incidence in older adults (over age 65). 

Older adults comprise the group with the highest TBI-related hospitalizations and deaths rates. Estimated average yearly rates of TBI are higher for males than for females across all age groups.