This is the third instalment in Speak Out On Suicide series running on Canada AM and on canadaam.ctv.ca. Click here for Monday's feature on suicide and youth. And click here for Tuesday's story on suicide in the military.

It's not even yet over, but 2011 is already being called hockey's "annus horribilis." This year has seen Sidney Crosby's devastating concussion; a plane crash that killed 35 members of a Russian hockey team; and the one-two-three punch of the deaths of Rick Rypien, Derek Boogaard, and Wade Belak this past summer.

The passings of each of these three would have been tragic on their own. But when they came within four months of each other, it was hard not to wonder whether there might be a pattern.

All three died young. All three had served as so-called "hockey enforcers." And two had died of suicide, the third of an overdose of alcohol and prescription pain relievers.

Sports columnists asked about whether it was the stress of the enforcer job that led to these deaths. Indeed, many fans were forced to think about what might be one of the toughest positions in hockey: the equalizer, whose job is not to score, but to target other players for payback and try to knock their lights out.

Some asked much-needed questions what role punches to the head and concussions might have contributed to the deaths. And then there were the musings about whether the over-celebrated culture of professional sport makes it too hard for some players to walk away and return to "regular" life.

Dr. David Goldbloom, the senior medical adviser, at the Centre for Addiction and Mental Health in Toronto, says it might be easy to draw parallels between the three, but it's also a bit misguided.

"We always look for patterns to try to explain human behaviour and particularly behaviour that on the surface, appears inexplicable. But the reality is that in Canada, every year, 4,000 people kill themselves. And they come from every conceivable walk of life," Goldbloom told CTVNews.ca.

"So to take this issue and to try to make linkages is a very risky business."

Goldbloom points out that Boogaard, Rypien and Belak were separate individuals, each with his own mental health history. Rypien and Belak had both battled depression, and Boogard had been trying to recover from concussions and other injuries. Lumping their deaths together does a disservice to the complex reasons why anyone falls into depression or chooses to end their lives.

The simple fact that all three were men already made it more likely they would kill themselves, he says. Men die by suicide at a rate three to four times that of women. Though women make attempts at suicide much more often, men tend to choose more lethal means and thus complete suicide at a higher rate.

Two of the men struggled with depression – which is also a strong risk factor for suicide, not surprisingly.

And no matter how hard anyone tries to understand why these players died, none of us ever will, because we were neither there with them on their final night, nor knew what was going on in their heads.

"We live in a culture of celebrity and so we think we know famous people," Goldbloom says. "But we only have a perception of them; we don't know them. So when terrible things happen, there is a rush to explain it in ways that we wouldn't do for non-famous people."

Brain disease linked to depression

That's not to say it's not worth asking whether something about their jobs contributed to these men's deaths -- especially since there is currently such intense focus on the brain-altering, life-changing effects of concussions.

For that, hockey has professional football to thank. It's the NFL that helped move chronic traumatic encephalopathy -- or  CTE, as it's better knwon – from the dusty world of medical dictionaries to the popular lexicon.

CTE is a progressive brain disease that appears to be caused by repeated head trauma. The disease, once called punch-drunk syndrome, can bring on dementia in people in their 40s or 50s, as well as intense depression and bizarre behaviour changes, some of which lead to suicide.

CTE first made headlines when it was identified in 2006 in former Philadelphia Eagles player Andre Waters, who had died of suicide at the age of 44. It's since been found in about 20 NFL vets, several of whom also took their own lives.

The frightening power of the disease hit home for many in 2010, when the early signs of it were discovered in University of Pennsylvania co-captain Owen Thomas, who hung himself in April, 2010. Thomas was just 21, but already had early but clear signs of CTE, suggesting it doesn't take decades for the disease to develop.

Former NFL star Dave Duerson's February, 2011 death was another eye-opener. Duerson decided to shoot himself in the heart, not the head, so his brain could be examined for CTE. Duerson knew exactly what was destroying his memory, and indeed, an autopsy proved he had a moderately advanced case of CTE, likely brought on by years of concussions.

Concussion's link to depression

Even when concussions don't lead to CTE, they can cause depression, years of research has shown. The rate of depression in the general population is estimated to be around 5 to 10 per cent; in head trauma patients, it can reach 40 per cent.

Some have wondered whether the concussions actually cause the depression, or whether it's being sidelined that brings on the blues. Researchers at the Montreal Neurological Institute of McGill University have tried to answer that one by taking a look at the neurological basis of depression in athletes.

Since brain scans can't spot the microscopic damage caused by concussions, Dr. Alain Ptito and a team recently used functional MRI (fMRI) to examine the brains of athletes recovering from concussions, who also had symptoms of depression.

Ptito tells CTVNews.ca that the concussed and depressed athletes showed the same pattern of brain activation and inactivation as patients with major clinical depression.

"What it means is that the symptoms of depression after concussions look almost exactly the same as those that we find in major depression – meaning that there's something going on that causes the depression; (the depression) is not simply a reaction to not being able to play, for instance," Ptito said.

Importantly, the key area of the brain area that was inactivated, or "turned down," in both people with depression and those with concussion-induced depression, is located right behind the forehead -- an area that is often the target for boxers, football players and NHL enforcers.

CAMH's Dr. Goldbloom says while he supports and encourages research into how head injuries might cause depression, he's not convinced it's fair to link head injuries to suicide.

"Yes, there's a growing awareness of the broad impact of concussion, but I don't think the research at this point is at a stage where it allows us to draw a direct line from concussion to suicide," he says.

Not all cases of depressions lead to suicide – not even close, Goldbloom notes. One in 5 women will have an episode of major depression in her life, as will one in 10 men, yet only about 10 to 15 per cent of cases of clinical depression will lead to a suicide attempt.

There are many factors that drive someone into the kind of depression that can lead to suicide: trauma, substance abuse, a lack of support from family and former friends. And hard as it is for most of us to comtemplate, for retired and fading athletes, leaving pro sports can be its own form of trauma.

Players are forced to say goodbye to the constant hero worship and the excitement of a game they loved, and then decide what to do with a life that, until that point, had been solely focused on sport. It's not surprising that many find themselves depressed and adrift.

If there's a silver lining to be found in the "annus horribilis" that was 2011, it's that these deaths have forced us to think about depression and suicide, says Goldbloom. They've also shown us that all the athleticism, fame and money in the world doesn't make anyone immune to depression or mental illness.

"If you're looking to increase public understanding and awareness (about suicide), the reality is when a famous person kills themselves, it brings the issue to the surface for everybody," he says.