<unk>  grading systems are sets of<unk>  used in sports medicine to determine the severity, or grade, of a<unk> , the mildest form of traumatic brain injury. At least 16 such systems exist, and there is little agreement among professionals about which is the best to use. Several of the systems use loss of consciousness and amnesia as the primary determinants of the severity of the<unk> .
The systems are widely used to determine when it is safe to allow an athlete to return to competition.  Concern exists that multiple<unk> s received in a short time may present an added danger, since an initial<unk>  may leave the brain in a vulnerable state for a time.  Injured athletes are prohibited from returning to play before they are symptom-free during rest and exertion and their neuropsychological tests are normal again, in order to avoid a risk of cumulative effects such as decline in mental function and second-impact syndrome, which may occur on very rare occasions after a<unk>  that occurs before the symptoms from another<unk>  have resolved.
Three grading systems are followed most widely: one was developed by Robert Cantu, one by the Colorado Medical Society, and a third by the  American Academy of Neurology.
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