The more mood swings, the more deterioration. They only looked at people with 1 to 6 swings over four years. The news story doesn’t say whether they looked at frequency of mood episodes versus total number of days, so we don’t know whether someone with long, infrequent episodes would have the same damage as short, frequent ones (something of concern to people who rapid cycle). (I unfortunately don’t have access to this article.)
They did not find a decline in intelligence, although the article talks about declines that weren’t significant. That just means the scientists want to say they found a decline, so they’re putting it forth even though the rules of science designed to keep that kind of thing under control say it isn’t sufficiently certain for people to count it as a finding. (We have this kind of rule for a variety of good reasons, but mostly to keep people from overstating their findings.)
The lead author of the study says medication didn’t appear to be making the deterioration worse. He doesn’t say that helps, although both he and the writer of the news article hope that it does (as do I, and I hope someone follows up and tells us whether that’s the case).
What you can actually conclude from the news article is that people with infrequent episodes in bipolar disorder show gray matter loss in proportion to the number of episodes they have, and that we don’t have any evidence showing that this is harmful. My money is on yes, we will find it’s harmful, and maybe we’ll find that medication averts it (instead of, for example, finding that it’s related to the tendency to have more frequent episodes, rather than to the actual episodes). But we don’t know that right now. The news article (and possibly the lead researcher) are constructing a logical story around how things work, and telling that; and it makes sense. But real life is often less intuitive than our stories.
On a related topic, I hope to post about kindling tomorrow.