I've had carpal tunnel surgery on both hands last year, and had to sell my 1D MK IV because of the pain holding it. This year, I suddenly have tendonitis. It likely comes from zillions of hours spent using a mouse to edit photos. I've ordered a ergonomic mouse, I have 2500 photos that I can't edit because of the pain.Sorry to hear that. Hope you feel better now ... what about using a Wacom tablet, does that help? as opposed to a ergonomic mouse? ... I ask, coz I haven't used a Wacom pen/tablet yet, but plan on getting sometime soon.
Years ago I switched to using a tablet for this reason It's a great help. Now with my new 5DMkIII I got a Wacom Intuos Pro tablet as part of a Canon promotion, and that tablet also supports touch, so I hab even use handgestures now.
It's also good to learn to use the mouse with your other hand. When I was a CAD engineer I used both hands: A Wacom tablet on the right and a mouse on the left.