Return to play should be prohibited if there is loss of consciousness, symptoms lasting more than 15 minutes, recurrence of symptoms with exertion, amnesia, or a history of prior concussion. Classification systems for concussions are not based on scientific evidence and advances have included neuropsychological or balance evaluations. These tests are more helpful if baseline data is collected on athletes before they are injured for a set of normative data for each individual. Therefore, a low baseline neuropsychological test would be the athletes standard for comparison and not solely be a reason to disallow return to competition.
The review article by Wojtys et al is a special report of the findings of the 1997 Concussion Workshop, sponsored by the AOSSM, and gave 9 recommendations for improving diagnosis, treatment, and return to play guidelines for athletes with head injuries.
An additional reference by McCrory et al from 2008 is added which describes more recent stricter recommendations regarding return to play. In general, athletes should not return to play the same day after any concussion.
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Indeed, the man whom ESPN would later find a "violent, combative player known for his short temper" inspired the league rule against throwing a helmet after having done so himself to an opponent's helmet.  Peter Alzado, Lyle's brother, later identified the years of their youth—marked by an absent, alcoholic father and an over-worked mother—as the crucible for Alzado's unremittingly fierce style of play. "That violence that you saw on the field was not real stuff," his brother held. "Lyle used football as a way of expressing his anger at the world and at the way he grew up."  Defensive end Greg Townsend , a teammate on the Raiders, contended that the savagery for which Alzado became noted represented only part of a "split personality." "Off the field," remembered Townsend, "he was the gentle giant. So caring, so warm, so giving."