syrinx

One problem with second hand science reporting like this is you just get the headline and not the details.
For instance, how well did they match the groups? Since the study was done in the Netherlands, where various psychotropic drugs are legal, and which one might logically reason that someone with frequent pain would use to relieve such pain (marijuana, for instance), it could be argued that some of the brain damage might be due to chronic use of such substances to relieve the pain, IF those substances also caused brain damage. (I don't think THC has been shown to cause brain damage at normal consumption levels, although it certainly impairs function, but booze and a lot of the other stuff people put into their bodies do cause damage to the brain, liver, and more - from what I've read, of course.)
For all we know, the original research covered all this, but the little blurb doesn't address it. Actually, you about covered it all in your title...there wasn't much more at the link.