Former Maori All Blacks scrum-half Billy Guyton, who died last year, has become the first professional rugby player to be diagnosed with chronic traumatic encephalopathy (CTE).
Guyton's brain, which was donated by his family to the Human Brain Bank at the Auckland Neurological Foundation for examination, showed signs of Stage 2 CTE. He died last May aged 33.
Guyton played for the Blues, Crusaders and Hurricanes in Super Rugby during his career.
The report said Guyton had "background changes consistent with global hypoxic ischemic encephalopathy", a term for a brain injury.
CTE was noted by a New Zealand-based pathologist and the brain was sent to an Australian pathologist for a second opinion and "it was he who gave it the Stage 2 designation," Professor Maurice Curtis of the Brain Bank said.
CTE is a degenerative brain disease known to cause violent moods, depression, dementia and other cognitive difficulties. It has been linked to the repeated hits to the head endured by football, rugby and hockey players, boxers and members of the military.
Researchers have found evidence that the severity of the symptoms is increased for those who sustained concussive or sub-concussive blows at a younger age.
CTE can only be diagnosed posthumously. Stage 1 is classified as mild, with Stage 4 severe.
"There are a limited number of modifiable risk factors in this case and concussions and head knocks are certainly one of them," Curtis said.
Stage 2 symptoms include irrational outbursts and severe depressive episodes.
Guyton's father John Guyton told Radio New Zealand those symptoms clearly described his son's behaviour.
"The poor guy would spend hours in a small, dark cupboard because he couldn't handle being in the light," John Guyton said.
"Some mornings he'd just sit in the bottom of his shower tray crying, trying to muster up the energy to get moving."
In a statement, New Zealand Rugby said it will continue to take steps to reduce the danger of head impacts at all levels of the sport.
"This includes the introduction of smart mouthguards, lower tackle height, contact guidelines, law changes, compulsory education for community coaches and many other initiatives," it said.
"NZR is also supporting world-leading research to better understand the long-term impacts of participation in rugby including a focus on understanding any link between concussion and long-term brain health."
Last December, former England captain Phil Vickery and ex-Wales fly-half Gavin Henson were among more than 200 retired rugby players bringing a legal claim against three of the sport's governing bodies alleging they suffered brain injuries during their careers.
Vickery, 47, a member of England's World Cup-winning team in 2003, and Henson, 41, who won Six Nations Grand Slams in 2005 and 2008, waved their anonymity in their claims against World Rugby, the Rugby Football Union and the Welsh Rugby Union.
Mark Regan, 51, another member of England's triumphant World Cup squad, was also named.
Lawyers for the group of players have previously alleged the governing bodies failed to take reasonable steps to protect players from injury caused by repetitive blows and that many now have permanent neurological injuries including early onset dementia, Parkinson's disease and CTE.
Steve Thompson, who played with Vickery and Regan, had already been revealed to be among the claimees after he was the first to go public three years ago.
A joint statement on behalf of World Rugby, WRU and RFU read: "Player welfare is rugby's top priority, and will continue to be our top priority. Rugby is committed to leading the welfare agenda in sport, driven by evolving science and research to protect and support players at all levels.
"This includes world first initiatives, such as the use of smart mouthguard technology by all elite players to facilitate in-game monitoring and treatment of concussive and sub concussive impacts. A lower tackle height is also being trialled in the community game to ensure that the benefits of our great sport can be enjoyed by all."