March 14, 2016

Dementia and rugby

Dylan Cleaver has a feature story in the Herald on the Taranaki rugby team who won the Ranfurly Shield in 1964. Five of the 22 have been diagnosed with dementia. Early on in the process he asked me to comment on how surprising that was.

The key fact here is 1964: the five developed dementia fairly young, in their 60s and early 70s. That happens even in people who have no family history and no occupational risks, as I know personally, but it’s unusual.

I couldn’t find NZ data, but I did find a Dutch study (PDF, Table 3) estimating that a man who is alive and healthy at 55 has a 1.5% risk of diagnosed dementia by 70 and 3.2% by 75. There’s broadly similar data from the Framingham study in the US.   The chance of getting 5 or more out of 22 depends on exact ages and on how many died earlier of other causes, but if these were just 22 men chosen at random the chance would be less than 1 in 10,000 — probably much less.  People who know about rugby tell me the fact they were all in the back line is also relevant, and that makes the chance much smaller.

There are still at least two explanations. The first, obviously, is that rugby — at least as played in those days — caused similar cumulative brain damage to that seen in American football players. The second, though, is that we’re hearing about the 1964 Taranaki team partly because of the dementia cases — there wouldn’t have been this story if there had only been two cases, and there might have been a story about some other team instead. That is, it could be a combination of a tragic fluke and the natural human tendency to see patterns.  Statistics is bad at disentangling these; the issue crops up over and over again in cancer surveillance.

In the light of what has been seen in the US, I’d say it’s plausible that concussions contributed to the Taranaki cases.  There have already been changes to the game to reduce repeated concussions, which should reduce the risk in the future. There is also a case for more systematic evaluation of former players, to get a more reliable estimate of the risk, though the fact there’s nothing that can currently be done about it means that players and family members need to be involved in that decision.

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Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »

Comments

  • avatar

    There are 520 Rugby clubs in New Zealand. (https://en.wikipedia.org/wiki/Rugby_union_in_New_Zealand) so should we be asking the probability of one of these 520 clubs have this number of dementia cases? Which comes to about 1 in 20. Interesting but not astounding.

    1 year ago

  • avatar

    There are 520 Rugby clubs in New Zealand. (https://en.wikipedia.org/wiki/Rugby_union_in_New_Zealand) so should’nt we be asking the probability of one of these 520 clubs have this number of dementia cases? Which comes to about 1 in 20. Interesting but not astounding.

    1 year ago

    • avatar
      Thomas Lumley

      If we knew this was the most extreme rugby club in New Zealand, that would be the p-value. But we don’t know that. There hasn’t been a systematic survey. If someone did a survey of even twenty clubs and there wasn’t an excess of dementia in any of the others, it would suggest that the Taranaki cases are a random tragedy. Also, if the excess was in, say, colon cancer or Legionnaire’s Disease, the lack of a good explanation would suggest that it was random.

      But here we have an excess that would be a bit surprising even as the worst of 520 clubs, and in a setting where there’s known to be a problem in other sports. I think there’s plenty of evidence to support looking further.

      1 year ago