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Here's what we know about football, concussions, and the brain

Watch: Can the NFL survive its concussion crisis?

The NFL's concussion crisis has now been going on long enough that it's easy to get lost in it.

But though the science may be complex, the basic picture is relatively simple.

In recent years, scientists have discovered that the repeated blows to the head suffered by many football players can cause long-term behavioral changes, dementia, and depression, as part of a disease called CTE. At least 50 deceased players have been diagnosed with CTE so far, and an unknown number are living with it.

After years of ignoring the issue, the NFL is now trying to do something about it, changing the rules to minimize the most violent blows to the head. But it's unclear whether they'll be able to eliminate CTE from a fundamentally violent game.

Here's everything you need to know to get up to speed.

Repeated concussions lead to long-term brain problems — a disease called CTE

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Junior Seau, a linebacker who committed suicide in 2012, was later diagnosed with CTE. Photo by David Madison/Getty Images

Over the past decade or so, several prominent former NFL players have committed suicide after suffering from years of emotional and memory problems. When doctors examined their brains afterward, they found evidence of a long-term disease called chronic traumatic encephalopathy, or CTE.

For nearly a century, doctors knew the disease could occur in boxers who were repeatedly punched in the head (they called it "punch drunk"), but awareness that football players were also susceptible came slowly. In 2002, Mike Webster — a former Pittsburgh Steelers player who suffered from depression and dementia before dying from a heart attack at age 50 — was diagnosed, becoming the first former NFL player confirmed to have it.

Although the symptoms may appear during a patient’s lifetime, a definitive diagnosis can only be made after death, when doctors have the chance to examine the brain and look for telltale signs of the disease. In postmortem exams, heavy depositions of a protein called tau in the brain are one sign of CTE.

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Left: a normal brain (slice at top, microscope slide at bottom). Right: a brain with high amounts of tau. Boston University

At least 50 retired NFL players have been diagnosed with CTE so far, but hundreds more are probably living with it undiagnosed. Scientists have found it can cause a range of problems — headaches, difficulty concentrating, erectile dysfunction, depression, increased anger and aggression, reduced impulse control, and eventually, severe dementia — which often don't appear until years after a player has retired.

Most players diagnosed with CTE played for many years and suffered multiple concussions, but that's not always the case: in 2010, a 21 ­year-­old University of Pennsylvania football player named Owen Thomas killed himself and was found to have early stages of CTE, making him the one of the youngest cases diagnosed so far.

Athletes in other sports have also been found to have CTE, including Derek Boogaard, an NHL hockey player with a particularly violent playing style who died from a drug overdose at the age of 28. Some soldiers who’ve been exposed to explosions and suffered brain trauma have also been found to suffer from it.

Scientists are still learning about the disease

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Ann McKee, one of the leading neurologists looking into CTE, examines a brain at Boston University's brain bank. (Stan Grossfeld/The Boston Globe via Getty Images)

As new findings are made about CTE, the news just keeps getting bleaker. What might be most disturbing is the mounting evidence that mild, routine hits — which present no immediate symptoms and are generally categorized as sub-­concussions, rather than concussions — might also lead to CTE. Last spring, new research came out showing that college football players have detectable changes in their brains after spending just a few years on the field, and the degree of change correlated with the number of mild hits to the head, in addition to concussions.

Scientists still don’t really understand how repetitive brain trauma causes CTE. Some hypothesize that milder hits contribute to it by disrupting the blood-­brain barrier (the lining that prevents molecules in your bloodstream from entering your brain), potentially causing undetectable long-­term damage.

This could be a big problem because a lot of football players (especially offensive and defensive lineman) experience these sorts of hits over and over throughout a game as a matter of routine. If CTE is as common as some scientists believe, the NFL could have an epidemic on its hands.

But not everyone who experiences brain trauma gets the disease. Researchers are currently trying to identify other risk factors (that is, factors that put people at greater risk of getting CTE, such as genetics), but they’re in the early stages. As part of the effort, a number of current and retired players in football and other contact sports have pledged to donate their brains to Boston University's CTE Center after their deaths.

The NFL is finally trying to do something — but it might not be enough

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NFL commissioner Roger Goodell dodged the issue for years before acknowledging the problem. Photo by Chip Somodevilla/Getty Images

For years, the NFL tried its best to cover up evidence about football and brain trauma. In 1994, the league established a committee to examine the long­-term effects of concussions on players’ health, but it released findings that were deeply inconsistent with those of other neurologists. Among other things, the committee called concussions "minor injuries," told players that there was no problem with a concussed player returning to a game, and declared there were no long-­term health issues associated with the injuries. Independent researchers sharply criticized these findings.

Then, in 2009 — after Congress grilled NFL commissioner Roger Goodell during hearings — the league abruptly changed course, finally acknowledging the problem.

It's taken a number of actions to begin addressing it. To start, it's donated money for concussion and CTE research, and offered money for improved helmets and sensors that can detect the severity of hits.

It's also made some rule changes to reduce the number of concussions suffered by players — outlawing helmet-to-helmet hits, altering kickoffs (one of the most dangerous plays) so that more result in harmless touchbacks, and limiting the amount of hitting during offseason practices. Importantly, it's put new protocols in place to ensure that concussed players are properly diagnosed and stay away from the game longer, to reduce their risk of second concussion. All these rules certainly won't eliminate concussions, but they might help.

The league also settled a class-action suit filed by thousands of former players, offering $765 million to compensate those suffering from dementia or depression related to blows to the head. But some players are appealing the settlement, saying they weren't properly represented and there are undue restrictions on retired players seeking compensation under its terms.

So what happens next?

There's one big question going forward: can football really be safe? Or is violence too deeply embedded in the game's nature?

It's difficult to tell if the rule changes have reduced concussions so far. This is partly because both teams and players long avoided reporting concussions to the media, so the historical data is artificially low.

concussion chart

But even if concussions could be entirely eliminated, what's particularly troubling is the possibility that mild, sub-concussive hits can cause CTE over time. If that's the case, it seems that tackle football will inextricably lead to long-term brain problems.

Whether people would play and watch it with that knowledge is unclear. Right now, the NFL's popularity continues to climb to record heights: its games, on average, get ten times more viewers than any other sport in the US. At the same time, there's also evidence that fewer parents are letting their children play tackle football.

Perhaps football will remain the most popular sport for decades, regardless of the health risks. Perhaps, someday, it will be seen as a fringe, barbaric sport — like boxing.

Further viewingWatch the outstanding PBS Frontline documentary "League of Denial"

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