Second Impact Syndrome is the fatal result of receiving a second traumatic brain injury (concussion) before a prior traumatic brain injury has healed. After a concussion, a cerebral edema (swelling of the brain) can occur. Fortunately, the brain has auto-corrective mechanisms to compensate and protect against massive swelling. If a second injury occurs while the brain is healing, the brain loses its ability to auto-regulate. Therefore, even a relatively minor second injury can result in excessive swelling of the brain and death.
Due to the deadliness of Second Impact Syndrome, healthcare professionals strongly urge anyone who might have suffered a concussion, especially young athletes, to be examined for symptoms, then rest and confirm that the brain has healed prior to resuming any strenuous activity.