It was the way Sidney Crosby struggled to his feet after being blindsided in the Winter Classic game that convinced neurologist Richard Wennberg the star centre had sustained a concussion.
"To see it was painfully obvious. The telling sign was when he tried to get up," said Dr. Wennberg, a University of Toronto concussion expert who also acts as a consultant to the National Hockey League's Players' Association.
Mr. Crosby's right foot slipped behind him as he got back on his feet, Dr. Wennberg said, and his mouth guard slipped out of his mouth - subtle signs that that the hit from Washington Capitals winger David Steckel jarred Mr. Crosby's brain.
So how could Mr. Crosby have been allowed to finish the Winter Classic and continue playing the next week?
The short answer is that he displayed none of the classic symptoms of concussion, which would have caused officials to pull him from the game. If trainers or doctors see one or more of those symptoms, they'll typically withdraw the player and ask him questions to determine whether he's okay.
Mr. Crosby had a sore neck, but experts say that alone is not usually a sign of a concussion. Concussions are not visible injuries and athletes are notoriously loath to admit to weakness or be pulled out of games.
The danger is that when the NHL's most famous player was hit a few days later, making his return to the game uncertain, his risk of more damage would have been increased if he had already suffered a concussion on Jan. 1, according to neuroscientists.
Critics are growing increasingly vocal in saying that if the rules for when a player is pulled out of a game after a hit to the head were followed in Mr. Crosby's case, perhaps the rules need to be changed. And this week, even Mr. Crosby's agent called on the league to consider making all contact with the head illegal.
Dr. Wennberg said he understands the pressure on the team trainer not to remove a star player from such a high-profile game, especially since Mr. Crosby didn't report of any of the constellation of symptoms used to assess whether a player has a concussion, including dizziness, headache, nausea and vomiting, blurred vision and confusion.
As such, Mr. Crosby's case illustrates the difficulties - and broad array of occasionally contradictory expert medical opinion - in assessing what is an uncommonly tricky and unpredictable injury.
Charles Tator, an eminent University Health Network neurosurgeon and leading authority on concussions, reviewed a video of the Jan. 1 hit, but couldn't tell whether Mr. Crosby sustained a concussion from what he nonetheless described as a "highly offensive, vicious hit."
It takes an experienced professional about five minutes to assess whether a player likely suffered a concussion, he said. They have to be inquisitive and ferret out symptoms.
"You need to ask some searching questions about how a player is functioning," said Dr. Tator, who is also based in Toronto.
As it happens, several people in Pittsburgh are well equipped to do that.
The Penguins' team doctor, Charles (Chip) Burke, is a highly regarded sports physician with extensive expertise in concussions (he once led the NHL's Injury Panel and helped author a major head-injury study commissioned by the league).
"It goes to symptoms. When someone separates a shoulder you can see it . . . with a concussion it's much trickier," said Penguins general manager Ray Shero, adding that he has "complete confidence in our medical and training staff," and that any contention that the team would trifle with Mr. Crosby's or any player's health is "completely ludicrous."
"These are delicate, complicated situations, as we're seeing in Edmonton with [Oilers winger]Ales Hemsky, it can happen because of a series of incidents," Mr. Shero said.
Mr. Hemsky was pulled from a game on Tuesday with concussion symptoms, but they don't appear to have been brought on by a single event.
But Mr. Shero is firmly among those who believe the league should work harder to prevent head injuries.
"We need to do more to get head shots out of our game," he said.
Mr. Crosby's agent, Pat Brisson, said on Tuesday that the league should consider a rule that makes all contact with the head illegal (the Ontario Hockey League, among others, has such a rule).
"Almost in the same way as with a high stick, even if someone hits someone accidentally, he could be automatically suspended," he said. "That's my opinion."
Mr. Brisson also believes that the discussion over the past few days about concussion diagnosis and treatment has been helpful and necessary.
"We're building traction with this topic," said Mr. Brisson. "It's imperative that we keep this going forward."
Mr. Crosby made it through two days of practice without experiencing further symptoms, and played in a game on Jan. 5 - Mr. Shero spoke with him for 10 minutes that evening and said he saw nothing amiss - in which he was crushed against the glass by Tampa Bay Lightning defenceman Victor Hedman.
The 23-year-old superstar later said he felt "a little off" in that game, but after examination by the team's medical staff, he opted to take a team flight to Montreal.
When he woke up the next day, Mr. Crosby had headaches and nausea and reported the symptoms to the team. Winger Matt Cooke had already chartered a plane back to Pittsburgh to attend to a family matter, so Mr. Crosby hopped on - most experts agree flying isn't an issue with concussion sufferers - and went to meet the Pens' doctors.
That night, it was announced he had a "mild concussion" - a term that irritates some experts, who feel there is no such thing.
But professional athletes can use a much higher bar than most people to determine if they have a concussion, said Robin Green, a neuroscientist and clinical neurophysiologist at Toronto Rehab hospital.
"What for them constitutes a concussion is a very significant event that stops them from playing," she said.
James Kissick, a concussion expert and former Ottawa Senators team doctor, identified another thorny aspect of the question: "We're dependent on what the athlete or patient tells us."
Dr. Green and her colleagues are working on new neuroimaging techniques to diagnose a concussion and take the guesswork out of it.
In recent years, researchers have gained crucial new insights into how the injuries affect athletes' brains, both in the immediate aftermath of a hit - and possibly long after they've retired from the sport.
They are investigating the long-term impact of repeated concussions amid growing evidence they may be linked to an Alzheimer's-like condition associated with personality changes and dementia.
Dr. Green said that there is evidence that even two or three concussions can have a cumulative impact and affect attention and processing speed.
As well, someone who has had a concussion is more at risk of having a second.
Concussions can affect the ability to multi-task and to concentrate. There is no medical treatment per se; people need physical and mental rest to recover.
Mr. Crosby has now missed six games, and though Mr. Shero said he is feeling better, there is no timetable for his return.
With a report from Eric Duhatschek
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