Traumatic brain injury (TBI) is defined as damage to the brain resulting from an external force, such as impact in a car accident, blast waves from an explosion, or a penetrating projectile. Even in the absence of an impact, significant acceleration or deceleration of the head can cause TBI; for example, violently shaking an infant causes shaken baby syndrome.
With a traumatic brain injury, damage may occur directly at the site of impact (coup injury), or it may occur on the side opposite the impact (contrecoup injury). When a moving object impacts the stationary head, coup injuries are typical, while contrecoup injuries are usually produced when the moving head strikes a stationary object.
Primary and Secondary Injury
Primary brain injury is the damage that occurs at the moment of trauma (e.g., a blow to the head) when tissues and blood vessels are stretched, compressed, and torn. Secondary injury is the damage that occurs in the minutes to days following the initial trauma. Secondary injury can dramatically worsen the damage caused by primary injury; problems such as ischemia (insufficient blood flow); cerebral hypoxia (insufficient oxygen in the brain); cerebral edema (swelling of the brain); and raised intracranial pressure (the pressure within the skull) account for a large percentage of TBI deaths.
How does the Brain Get Injured?
The brain is a soft organ (like gelatin) that is surrounded-by and floats-in cerebrospinal fluid within the skull. Normally, the fluid around the brain acts like a cushion that keeps the brain from colliding with the walls of the skull; but, if the head is struck or jolted with enough force, the brain can crash into the inner walls of the skull causing the brain to bruise, bleed, tear, or swell.