Hayman played 45 tests for the All Blacks between 2001-07, during which time he was considered the best tighthead prop in the game, before becoming one of the world's highest-paid players when he joined Newcastle and Toulon, winning three European titles with the latter French team.
As Hayman revealed to Dylan Cleaver's The Bounce, however, the close to 450 professional games he played took an irreversible toll on his brain.
"I spent several years thinking I was going crazy. At one stage that's genuinely what I thought. It was the constant headaches and all these things going on that I couldn't understand," Hayman said.
"At the time I felt indestructible. I never got injured, I trained bloody hard... but if I knew then what I know now, I don't think I would have played post the [2007] World Cup. I think I would have stopped playing."
Hayman's harrowing account of the effects of his early-onset dementia detailed alcohol abuse, suicidal thoughts and erratic behaviour; the latter leading to a suspended prison sentence in France after he admitted to charges of domestic violence.
Since extensive testing in England revealed his diagnosis, Hayman has joined the landmark lawsuit being prepared on behalf of 150 former professional rugby players that claims rugby's governing bodies, including World Rugby, failed to protect players from the risks caused by concussions and sub-concussions.
All Blacks assistant coach John Plumtree, speaking from Rome ahead of the test against Italy on Sunday morning (NZT), expressed his regret at Hayman's situation.
"It's really sad," Plumtree said. "Carl is a 45-test All Black and he's done a lot for New Zealand rugby. It's a real sad situation if he's struggling with dementia at such an early age.
"We've got a lot of empathy for that. He's been a popular person in this environment so it's not nice to hear about those stories."
Plumtree began his playing career with Taranaki in 1985 and has been coaching professionally since joining Swansea in 1997.
Despite Hayman being the latest modern-day athlete to reveal extensive brain damage from contact sport, Plumtree believes rugby, and the All Blacks, have taken major strides in attempting to mitigate the effects of repeated head knocks.
"We look at our laws now, they really protect the head," Plumtree said.
"It has changed a lot. There's a greater awareness around it in all parts of the organisation from the top level down, to us as coaches. We're trying to minimise the accidents around the head as much as we can and we know the game is under pressure to do that.
"We've got a responsibility as coaches to make sure the game is safe, so parents want their kids growing up to want to play.
"If you look at the forwards, defence, backs, attack coach we all try and coach perfect technique; whether it's the high ball, tackle situation or the breakdown. The head is a protected area.
"We know it's not good for our game and we know we have to protect our players whether that's at training or during a game.
"If anything happens out there it has to be an accident. I hate seeing the head injuries. If you look at this tour we've been on we've had a couple of injuries but we haven't had a concussion.
"I think we're going on the right path with the way we're looking after our athletes. If you look at the stats now, hopefully, they'd say there's less of it.
"I feel sorry for Carl or any other players that are affected in this regard. I know he's a good Naki farming boy, and that's my roots as well, so it wasn't a nice article to read."
More studies needed: former All Blacks doctor
Former All Blacks doctor John Mayhew says more studies need to be done to determine a direct link between concussions and the risk of dementia, but it's a potential problem rugby bodies must be aware of.
In the wake of ex-All Black Carl Hayman's early-onset dementia and probable CTE diagnosis, Mayhew told Newstalk ZB's Kerre McIvor that people shouldn't jump to conclusions about the causes.
"Obviously, we don't know the cause of his dementia and just because he's a prominent ex-All Black and played rugby, there's not always a definite cause or link between the two," Mayhew said.
"But it's certainly very sad news and certainly rugby is aware of the risks of head injury and the chronic effects. We still don't know if it's a direct link between several concussions and the risk of dementia in later life, so it's unfounded."
"But certainly, in this case, it needs to be taken seriously and unfortunately chronic traumatic encephalopathy is usually a diagnosis made post-mortem, which obviously we don't want that to be the case here."
Mayhew said that while rugby had "to be aware there's a potential problem and follow the evidence-based medicine approach to it" there could be other contributing factors to early onset dementia such as alcohol and drug use.
"We still don't know, directly, the link between recurrent head injuries and the subsequent brain damage," Mayhew said.
"It is very, very young to have dementia - if that's what he's got. Some of the other cases I've been involved with in the past, the diagnosis hasn't been dementia and so we need to be aware that there can be other factors - alcohol, drugs and things like that – and there are some conditions which cause pre-senile dementia, so to speak.
"We need to be aware of that. Each case has to be analysed on its merits, and it's hard to look back on someone's career and say 'well, what dose of head injury did you have to cause this? Did you have enough, or was it just the result of one blow, or multiple blows?'"
In recent times, rugby has made changes to lower the risk of head injuries, particularly with rules around the tackle and the breakdown. Players are now also taken from the field to be assessed for concussion if they suffer a knock to the head, and can return to a doctor rules them fit to continue.
"It's a concern, but the right things are being done," Mayhew said. "I think we've got to make sure that we don't scare people away from our national game and unnecessarily. Most head injuries that we see in hospital and things like that are not related to rugby; they're due to car accidents or other types of trauma.
"That macho attitude has gone away and you see today the guys come off which what seem relatively trivial injuries and they're assessed properly by independent doctors and taken off the field, and I think the players accept that, the coaching staff accept that and even in rugby league, there's been a dramatic shift there.
"I think the management going forward of head injury is much better, especially in the professional game. It's a little hard in the amateur game, the club game, the third-grade game where there's no one on the side line keeping an eye on the players so there's an educational process that needs to go on there, but in professional contact sport – certainly in New Zealand – we're doing a much better job at managing head injuries.
"But as people point out, it may be that one head injury is enough to end someone's career and we've seen examples of that. Whereas other people can have 20 head injuries and make a full recovery. There's a lot of unknowns and I think we have to be careful that just because we have a famous ex-player who has a medical problem that it's related to their rugby playing. It may be totally unrelated to that."
World Rugby guidelines
World Rugby recently adopted independent concussion consultants to support the graduated return to play process in the elite game, and last month issuing guidelines around reducing contact in training.
These includes limiting full contact to 15 minutes per week; 40 minutes of controlled contact utilising tackle shields and pads, and 30 minutes of live set-piece training with lineouts, scrums and mauls at a high intensity.
The recommendations could soon be mandated, with test teams potentially obliged to follow guidelines in order to compete at the 2023 World Cup.
Plumtree explained training measures the All Blacks adopt to minimise contact such as using tackle shields, rather than players, to practise cleanout drills, and closing the distance between contacts to reduce impact.
"We're minimising the risk. The players can train cleanout work on pads that are designed like a jackler, so we're not using a body for that type of training. It's all designed to look after the players and train the perfect technique. As professional coaches we've got that responsibility.
"The head is a protected place and those type of conversations are talked about all the time in the All Blacks."
Plumtree, meanwhile, confirmed Brodie Retallick will not feature against Italy this weekend due to a shoulder injury, paving the way for rookie lock Josh Lord's second test.
Sam Cane and Dane Coles will continue their respective returns this weekend while Highlanders loose forward Shannon Frizell is set for his first appearance for the All Blacks since July.
