It depends not only on the severity of the blow, they say, but also on how many previous concussions a person has suffered.

Coupled with a previous concussion, the move resulted in Ong's death a few days later.

And for Eric Lindros, who had a previous concussion some years ago, there's a family factor.

The hit did not receive suspension and did not spark controversy as Booth's previous concussion had.

He has had one previous concussion, against the Houston Oilers in November, but returned for the next game.

We think sending someone back in when they still have symptoms from a previous concussion puts them at greater risk for another concussion.

Warner had a previous concussion, in the 2000 season, but he insisted both were mild concussions.

Looking at his unfocused eyes and bruised head, I was sure that this was only the latest in a long series of previous concussions.

His four previous concussions are documented.

After each of his two previous concussions this year, he started the Rams' next game.