Medical professionals are praising rugby's efforts to minimise the risk of players suffering regular head trauma.
Neurological Foundation medical adviser Jon Simcock said damage from regular concussion was common in American football, where players hid their symptoms from authorities to protect their earning potential.
However, rugby in New Zealand was "pretty careful" about compulsory stand-down periods after concussion.
"My impression is that the rugby union is well aware of problems with concussion and have mechanisms in place to take care of it," he said.
Repeated concussions can result in chronic encephalopathy, which has effects similar to those of Alzheimer's disease.
"They get, later in life, progressive impairment of memory and thinking ability."
Each concussion was different, with some people being OK after a few days. However, those still feeling the effects after a week, "they should not be playing at all".
Often too, knocks among young school-aged players turned out to be traumatic migraine misdiagnosed as concussion.
"There is lots of misleading information about it. The brain reacts differently to a knock on the head at different ages."
There were so many variables to head injury, "you can't easily write a protocol for everyone".
Clinical neuropsychologist Vincent Waide, who does concussion screening in his role at Wellington Hospital, said most sufferers had no problems.
However, there were some who suffered from cumulative and multiple head injuries, often presented towards the end of rugby seasons.
The more frequent the head injury, the more damaging and sensitive to them they became.
"Every knock you have, you're doing some damage."
New Zealand rugby standards were internationally recognised as using a good measure, meeting the middle ground between protecting the player and keeping the sport going.
"There is increasing recognition here that you've got to manage it."
- The Dominion Post
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