Have you checked all advice about desk layout, seating position, type of chair and so on? A mouse arm develops quickly if your seating posture is not good.
Yes, but too late, I'm afraid.

My posture have not been that bad generally, but I have had my arm angled out AND forward from my body. That's not ideal. So I've straightened that out now.
But to be honest, I'm not sure if the right posture would have prevented this as the sheer volume of what I've been doing is simply too much anyway, I think? I'm just not around in an office for 60 hours per week. Most of that time I spend holding a mouse and clicking a LOT.
What may have prevented this is to have been more aware of taking frequent breaks, massages, stretches and so on. At least that's what I'm thinking...
Howard
had the usual laparoscopic via vein in the groin but they had to go in through my side as well so larger wound) recommended this instead of traditional anti-inflammatory meds. I was very surprised by two things, 1. I was getting 6-7 hours per tablet instead of Ibuprofin etc. usual 2-3 and 2. Healing really seemed unusually fast.