From what I've read so far, brain metabolism of glucose is altered, blood flow is reduced the first four weeks or so, and damage need not show up on imaging, except DTI can pick up axonal/dendritic damage that's too small to show up on other imaging. I've got a lot of links put away but it might take a while to find stuff.

I'm baffled by the kinds of setbacks, including my own, often reported, where things seem to work fine for a while then out of nowhere symptoms return.