In some cases, you can tell immediately.
When the Ottawa Senators' David Dziurzynski was felled by Toronto Maple Leafs enforcer Frazer McLaren last week, it was apparent to all he was concussed.
But more often than not, a diagnosis requires guesswork, as in the case of Montreal Canadiens' forward Rene Bourque, who first reported what he thought were flu symptoms only to find out he had suffered a brain injury in the recent past.
Was it in a fight? Could it have been a succession of small impacts? No one knows for certain.
The injury is mysterious, and hard to detect even for doctors, which is why an international panel of medical experts has crafted an improved test to help lay people – parents, coaches, teachers – spot concussions in kids.
It's called the "pocket concussion recognition tool," a checklist of questions designed for people who aren't versed in the ins and outs of head injuries.
"More than anything it's to tell people what to look out for – red flags – and help them determine when they need to seek medical attention immediately," said James Kissick, a doctor and Ottawa-based concussion specialist who works with Canada's national sledge hockey team.
The diagnostic aid is among the fruits of a global summit on concussions in sport held last November in Zurich – the meetings are every four years and serve as a forum to update evaluation and treatment methods based on the latest research and scholarship.
A newly released consensus report to be published in the April issues of the British Journal of Sports Medicine and the Clinical Journal of Sports Medicine – it was co-authored by a number of leading Canadian concussion experts including Kissick – also includes an updated version of existing diagnostic aids for medical professionals.
For the first time there is also one aimed specifically at children aged 5 to 13.
The 30-member panel is also calling into question a long-standing concussion treatment: it turns out that for the 20 per cent or so of concussion sufferers who experience long-term symptoms, rest may not be the best solution.
It isn't to say that people with concussions should immediately start jogging – although research now shows pretty conclusively that concussion symptoms are resolved in 7-10 days in about 80 per cent of cases – just that too much rest can be just as harmful as not enough.
Light activity is the new prescription for people whose symptoms drag on for weeks and months.
"The main point is that we need a multidisciplinary approach, more of an active approach ... as opposed to just rest and more rest," said Willem Meeuwisse, a University of Calgary physician and researcher who is one of the international report's lead authors.
The panel also issued a series of guidelines regarding the return to play – Meeuwisse said the best advice, especially for kids, is still "when in doubt, keep them out" – and agreed that under no circumstances should any player be allowed to continue if a concussion is suspected.
The experts also reviewed the relevant studies and concluded there is no such thing as a concussion-proof helmet, and that while things like mouthguards and neck-strengthening exercises may reduce the impact of concussions, there's no firm evidence they do, and that rule changes are usually the most effective way to cut down on the incidence.
The report also finds that while baseline tests – often administered through a computer program and increasingly popular in minor hockey – can be helpful in managing concussions "there is insufficient evidence to recommend the widespread routine use of baseline neuropsychological testing."
Because the panel strives to reach decisions and recommendations based on consensus, not unanimity, it scrupulously avoided taking a strong position on the most contentious issue it addressed.
Though researchers at Boston University have published studies suggesting a causal relationship between concussions and a degenerative brain condition called Chronic Traumatic Encephalopathy (or CTE) in the cadavers of more than 60 former football players and other professional athletes – including at least three former NHLers – the panel's report says the scientific jury is still out.
"A cause-and-effect relationship has not as yet been demonstrated between CTE and concussions or exposure to contact sports ... it was also recognized that it is important to address the fears of parents/athletes from media pressure related to the possibility of CTE," it reads.
The topic generated a great deal of discussion at the conference – "it was spirited," is how Meeuwisse summed it up.
One of the Boston University experts, Dr. Robert Cantu, sat on the expert panel, and while it can be painted as a disagreement among scientists over epidemiological questions and methodology – such as sample size and genetic factors – cynics will note three other doctors with links to the NFL, which has criticized the research in the past, were also among the report's authors.
The NFL is facing a billion-dollar lawsuit from a raft of former players who allege the league covered up what it knew about the long-term impact of concussions. The NFL denies the claims.