Hello everyone. I've fought a kind of autoimmune arthritis for years (psoriatic symmetric polyarthritis). My hands work fine for typing but using a computer mouse can get intensely uncomfortable. The reason is that almost all mice are too narrow--if my palm has to arch and can't stay flat, then the mouse will become painful to use. The problem is when I pull my thumb in to touch the side of almost all mice, it makes the palm arch and ache.
I've looked for years. I can tell you some that are almost good -- some of which sadly are no longer made. The Logitech MX1100 wasn't a great gaming mouse but it was a pretty comfortable mouse for me. The Mad Catz Rat 7/9 and MMO-7 weren't very comfortable out of the box, but they could be adjusted to be--in the case of the MMO-7, adjusted in a way that wasn't intended, as the thumb tray had to be made to swing out and they tried to stop you from doing that, probably because RAT 7/9 models frequently broke there.
The problem with the Mad Catz mice was that they all lose tracking and die after a couple of years. Every last one of them I have had. There is some problem in their electronics.
The Func M3 was pretty comfortable except that the shaped rest for the little finger is too straight for mine and while my palm doesn't hurt, after a while my little finger does. The Roccat Leadr / Tyon are almost wide enough to be comfortable--also true of the Steelseries Rival 500 and the Elecom MMO Dux. If the Cougar 700M were a bit wider, or the M910 didn't slant down to the right...
Here are some models that are far too small / narrow for my hands and quickly become painful when I try to use them: Corsair M95, Logitech G502 (and I *really* wanted this one to work!--nice design), Logitech G700, Corsair M65, or the G.Skill Ripjaws MX780 (far too small!).
I've gone through a lot of computer mice and spent more than I like to think about trying to solve this problem.