What is a Brain Injury? A brain injury is a disruption of normal brain function. A traumatic brain injury (TBI) is categorized as mild or moderate, both often called concussion, or severe. But no matter the severity, all traumatic brain injuries are serious and even a mild brain injury may need medical attention. With mild traumatic brain injury, symptoms may develop over the course of several days. If symptoms last more than two weeks you should be seen by someone who is an expert in treating brain injuries. Traumatic brain injuries are caused by a blow or jolt to the head or when something penetrates the brain. The leading cause of TBI is falls, followed by being struck by an object (pedestrian hit by a car), motor vehicle crashes (car, motorcycle, ATV, etc.), being struck by or falling against an object, and assault.