JEFFREY BROWN: How tough is too tough when it comes to sports and brain injuries? It’s an issue we have followed over a number of years. Today, there was new data to chew on.

Week after week, the big hits keep attracting big TV audiences to professional and college football. But concerns over head injuries in football and other sports have also continued about a connection between repeated blows and a degenerative brain disease known as chronic traumatic encephalopathy, or CTE.

They have found evidence of CTE in 68 cases, almost all of them athletes. The football players included linemen, running backs and tight ends who had received repeated hits throughout their careers. One was the late John Mackey, profiled with his wife, Sylvia, in 2009 by Ray Suarez.

RAY SUAREZ: In the good days, how is it different from what we’re seeing now for Mr. Mackey?

SYLVIA MACKEY, wife of John Mackey: He will get up and walk up and down. He can — he will throw and catch the ball. Actually, today would be a good day if it weren’t for the myoclonic twitching, they call it, or myoclonic jerks.

RAY SUAREZ: And speech?

SYLVIA MACKEY: He doesn’t talk anymore, very rarely.

JEFFREY BROWN: Mackey passed away in July of last year.

Others in the study who show signs of CTE were Derek Boogaard, a former hockey enforcer who died of an accidental overdose in May of last year, and former NFL safety Dave Duerson, who took his own life in February of 2011 after complaining of headaches and a deteriorating memory.

The NFL faces a class-action lawsuit filed last July by thousands of former NFL players and their families, citing lack of disclosure about potential dangers.

For more, we’re joined by one of the lead researchers, Dr. Ann McKee, a neurologist and co-director of the center for the Study of Traumatic Encephalopathy at BostonUniversity, and Mark Fainaru-Wada, an investigative reporter with ESPN who is working on a documentary about this subject for “Frontline.”

Well, Ann McKee, let me start with you.

What do you see as the key finding from this study that perhaps we didn’t know before?

DR. ANN MCKEE, BostonUniversity: Well, this study, this disease, chronic traumatic encephalopathy, has been around since the 1920s.

But there have really only been a smattering of reports.

In this paper, we more than doubled the world’s experience with this disorder and take it from the very beginning, where it first affects the nervous system, where it affects the nervous system, and then we see it expand progressively in older and older individuals, until it really is a destructive disease that affects most of the brain.

JEFFREY BROWN: And just to be clear here, the focus is less on the — I guess, the major hits or major concussions, and more on sort of repetition over time?

DR. ANN MCKEE: Right.

This is exposure to what we call mild traumatic brain injury usually considered almost insignificant hits, not — they don’t even have to rise to the level of concussion. They can be sub-concussion. But when you’re exposed to these hits over a very long period of time, usually many years, you can set yourself up for some long-term consequences.

JEFFREY BROWN: All right, Mark Fainaru-Wada, you have been reporting on some of the — well, there was pushback from some of the experts to these findings.

Tell us what you have been hearing from them and from the NFL, for example.

MARK FAINARU-WADA, ESPN/”Frontline”: Right. Well, my brother and colleague Steve Fainaru and I have been reporting on this for a while for a book and a documentary and for ESPN.

And, as Dr. McKee stated, the findings on this are substantial in terms of the numbers. They’re the largest number ever reported.

And what we did find, though, was there’s a lot of pushback in the scientific community. Dr. McKee told us about her recent experience at a conference in Zurich where there were various folks who criticized the work. We talked to some of those people, including some of NFL doctors who were there.

One NFL member of their brain committee said, you know, relating these cases, creating a causal link based on these case studies to football is akin to saying basically all the ankle injuries suffered by football players wearing Nike shoes were because of the shoes.

The argument there is that — and, as Dr. McKee has acknowledged — the data set they’re working from is skewed because it’s based on a series of brains from athletes and players who were showing signs before death in large cases of having mental issues.

And so the question that everybody is trying to figure out is, what is the actual incidence of this disease in looking at brains that were healthy vs. players who actually were exposed to football?

JEFFREY BROWN: Well, let me ask you, Dr. McKee, to comment on that. How definitive is this? What else do you need to look at to know especially things like how to treat people?

DR. ANN MCKEE: Well, this study is definitive in describing and defining what the disease is and how it affects the nervous system.

But a key question remains, what’s the incidence and prevalence? How common is this disorder? And that, we will never establish from an autopsy study.

For that, we really need to be able to identify this disease in living individuals. And that’s a huge focus of our more recent research. How can we identify this in people that are living?

And that might be through MRI scans or PET scans and especially the ones that might peg the TAL protein that develops. It might be seen through blood tests or tests of your urine or CSF.

But we are really going to need those tests to be able to determine if a living person has this disease and then be able to measure that person’s exposure to head trauma. And that will be the defining moment. And that will probably take a longitudinal prospective study involving probably thousands of subjects.

JEFFREY BROWN: Well, so, Mark Fainaru-Wada, how are professional leagues, people who work with students, student athletes, how are they taking this? How should parents take this in terms of the stakes here, the implications for when people should be in contact sports?

MARK FAINARU-WADA: Well, I think the numbers what are strike most people.

And I think those are the questions that raise issues for some of the leagues and how they deal with it and how significant this is.

The study talks about, out of 34 NFL players studied, 33 of them ended up having CTE. And so I think that obviously creates questions. Some people have suggested that the numbers raise more alarming questions than are real.

But the league for its part, the NFL, which faces the most scrutiny on this issue, with, as your piece noted, 4,000 former players suing the league right now, the league has looked at the changed rules to address the issue of these sort of larger hits that we have seen, these sort of defining hits.

I think the larger question, though, as Dr. McKee touched on earlier, is the subconcussive blows, there’s increasing research around that; 40 percent of the cases of CTE they identified in NFL players were lineman.

And so that raises questions about, is just the definitive — the nature of repetitive hits in the game something that exposes a player?

And I think that’s what the leagues and parents are trying to figure out as they try to find out how significant an issue is this and what the risk is for their kids.

JEFFREY BROWN: Well, Dr. McKee, how far would you go at this point in terms of what you say to parents or athletic directors or professional teams about taking the findings and using them somehow?

DR. ANN MCKEE: Well, I think the major caution I would have is that all parents, coaches should take every head injury seriously and try to eliminate the minor hits or the hits to the head as much as possible in any sport.

We have already seen in the last four or five years a tremendous focus on concussions. It used to be considered a fairly trivial injury that no one needed to sit out for.

And now we’re taking it seriously and we’re resting those athletes and making sure they’re completely asymptomatic until they return to play.

And I think this has been a tremendous development. And it will probably go a long way in terms of preventing the development of these long-term consequences in these individuals.

At this point, we don’t have any evidence, there’s no available evidence that a single isolated or a few isolated concussions that are well-managed — that means well-rested, the individual is asymptomatic before he goes back to play — there’s no evidence that those injuries lead to this disease.

The injuries that seem to lead to this disease are years and years of exposure to many hits. And those hits may be relatively mild.

JEFFREY BROWN: And, Mark, let me just ask you very briefly. You mentioned that lawsuit. Just tell us briefly where do things stand? That goes on, right?

MARK FAINARU-WADA: Yes, it’s ongoing.

And the current state is that both sides are arguing. The NFL has argued for dismissal of the case. It’s before a federal judge in Philadelphia.

I think both sides expect some ruling on whether the case will get tossed out or not in the spring. And, surely, there will be an appeal, one would expect, either way. So I think one expects the lawsuits are going to drag on well into next year at least.

JEFFREY BROWN: Mark Fainaru-Wada and Dr. Ann McKee, thank you both very much.

DR. ANN MCKEE: Thank you.