Nearly two-thirds of players who were involved in head collisions during the 2014 World Cup weren't adequately monitored by qualified health professionals to determine if they should have continued playing, according to an extensive video study published today in the Journal of the American Medical Association.

Why it matters: Concussions — from colliding with another player or repeatedly heading the ball — may structurally change the brain and have long-term effects on brain health and cognition.

The study: Trained reviewers looked at every, single event that involved head contact during all 64 World Cup matches for obvious signs of concussion like disorientation, temporary unconsciousness, or seizure-like movements.

During the 64 World Cup games, 61 players had 81 head collisions in 72 separate events. Health care personnel on the sideline assessed the player in only 15% of the cases. In the remaining instances, 45 players were assessed on the field by other players, referee or personnel on the field, while 21 players received no assessment at all. Shockingly, for players who had three or more signs of an obvious concussion, 86% returned to play during the same game.

The rules: International protocols from the sport's governing bodies say that players who show any features of a head concussion during a World Cup match should be immediately withdrawn and assessed by sideline health care personnel. The study shows that these protocols were largely ignored during the 2014 World Cup.