Concussions generally do not have precursors, as they occur immediately after the brain is injured and certain clinical symptoms manifest. There is no concept of a precursor with concussions. The clinical symptoms of a concussion primarily appear after a trauma and include headaches, dizziness, nausea, vomiting, retrograde amnesia, and brief periods of unconsciousness. These symptoms must all be present to classify as related to a concussion. If symptoms are present and a head CT scan shows no significant bleeding, a concussion can generally be diagnosed. Therefore, there is no such thing as precursors, only related clinical manifestations.