A traumatic brain injury (TBI) is caused by a bump or impact to the head that disrupts the brain's normal function. According to the CDC, most TBIs that occur each year are mild and sometimes called concussions. Mild traumatic brain injuries (mTBI) can cause changes in brain activity, leading to headaches, fatigue, disorientation, and irritability. A concussion is medically defined as a clinical syndrome characterized by immediate and transient alteration in brain function.