I am a Neuroradiologist. You are being a derelict parent if you allow your kids to play football or any sport that can result in a concussion/being knocked out. There is a lot of research coming out right now indicating that brains are much more easily injured than previously thought. A concussion was previously considered a reversible injury. It is now considered to be an indicator that at least some neurologic tissue has been damaged. Advanced imaging studies show changes in white matter volume, metabolic changes, and even microhemorrhages. Neurologic tissue does not regenerate the way an injured muscle or a broken bone does. Most neurologic tissue damage is irreversible. This is very unfortunate, because I, like many people really enjoy watching American Football, boxing, and MMA.
