It's something of a subjective injury, therefore unpredictable, and it's also mysterious, which can make it scary.
Despite technological marvels and the best efforts of modern neuroscience, more is unknown than known when it comes to concussion.
There are a finite number of things that can go wrong with a knee or a shoulder, but it's not clear, for example, exactly why one person's brain can need months or years to bounce back whereas another will make what seems like a miracle recovery in a week or two.
"I'm one of the lucky few," said Montreal Canadiens forward Max Pacioretty, who lay frighteningly still for nearly five unconscious minutes after Boston Bruins defenceman Zdeno Chara guided his head into a rink-side stanchion at the Bell Centre last March 8.
In the end, it took Pacioretty longer to recover from a neck fracture suffered in the same incident than it did to rebound from his brain trauma – which was diagnosed at the severe end of the scale.
"As soon as I got out of the hospital [after two days]I was looking every day to try and find symptoms. One thing I know for sure I was always tired and lethargic, and obviously there was the memory loss leading to the event. I found myself sleeping a lot more, and the doctor said that was probably the best thing for it," Pacioretty said Tuesday. "Everyone's brain is so different, you see incidents where someone has a not-so-severe blow to the head and they end up with headaches for months."
That latter category has come to include Pittsburgh Penguins centre Sidney Crosby, who sat out nearly 11 months even though he never lost consciousness after suffering a pair of hits in early January (it's a popular misconception that one has to be knocked out to suffer a concussion).
There is a broad scientific consensus that repeated concussions can increase susceptibility to greater and longer-lasting injury, but that in most cases the brain is able to recover more or less completely.
Statistics indicate about 85 per cent of concussion victims see their brain recuperate in seven to 10 days.
"I tend to see the other 15 per cent," quipped James Kissick, an Ottawa-based sports-medicine specialist who is a member of an international advisory panel on concussions.
Kissick, a former Ottawa Senators team doctor, added, "There are a lot of mysteries. ... There are a lot of things about the brain that aren't well understood. Most of the research is relatively new by medical standards."
In addition to establishing the long-term consequences of concussion – several researchers are investigating links with chronic traumatic encephalopathy, a neurodegenerative disease – scientists are trying to figure out whether there are genetic, hereditary and other factors at work.
There's no clinically tested treatment for the injury (rest is the universal prescription), and it is also not well understood why some people feel some symptoms – headaches, nausea, dizziness, sensitivity to light – and others have a different experience that involves, say, the sudden onset of depression.
"We tend to lump them all together, but every concussion is different," Kissick said.
There is no way to say for certain what happens to a concussed brain – it doesn't appear to suffer any evident structural damage, it would seem the effects occur at a cellular level.
That's partly why Pacioretty decided to launch a foundation in aid of a brain trauma research centre at the Montreal General Hospital – its main aim is to raise funds for a functional magnetic resonance imaging machine (fMRI) that would provide a sharper picture of what's happening.
For the 22-year-old Connecticut native, a healthy element of mystery remains.
"I'm still searching for symptoms, I'm still maybe thinking a little bit too much about whether I'm going to feel a symptom here or doing this or that," conceded Pacioretty, who hasn't missed a beat on the ice this season. "Hopefully that machine can build an answer for everyone."