Occupational Health Medicine. Sports Injury treatment. If you think of your wrist & fingers as miniature athletes, and treat 'em accordingly...
Anyhow, I've had both tendonitis and carpal tunnel syndrome, your basic repetitive motion injury and I can't say I enjoyed the experience. Because I was at the time of injury, and still am a full-time professional performer, I had to produce, just like any football, baseball, Olympic professional athelete, even when injured. Here's the various treatments my medical folks suggested. It all helped, some to greater degree than others, but I suspect one's individual physiology makes the difference.
1. Mega-doses of anti-inflamatories. You'll need a prescription - me they tried first something like a 600mg horsepill of ibuprophen 3 times a day, but for me, that caused water retention, which further exacerbated the swelling at the injured area. It's very hard on the liver and if you forget to take any ant-inflamatory with bland food (crackers or bread) or milk, you can rip up your stomach, too. For some people, ibuprophen can make one feel mentally unfocused as well. It varies - some of my musician friends preferred ibuprophen because the other main anti-inflamatory, naproxen, made them feel weird. For me, a twice a day 800mg of naproxen worked when I had a flare-up, with no water retention. SEE A DOCTOR!!!!! What if you've got some other exciting problem like a pinched nerve? See a doctor, get second opinions if need be until your problem is effectively addressed.
2. Before you perform, ice down the injured part for 5 minutes.
3. Wear a carpal-tunnel ELASTICIZED wrist brace while performing or practicing, and at night, wear the NON-elasticized carpal-tunnel wrist brace to support the wirst, and to keep it in a neutral position as you sleep. SOmetimes sleeping people bend the wrist in toward the arm, which can make the injury worse. If you experience numbness at night after having been asleep, or the sensation of your hand having "gone-to-sleep" with the attendant "pins & needles" or tingling feeling, you might be unconsciously bending your wrist in. You want the non-elastic brace at night so you don't cut off circulation, and the elastic one for performance flexibility. Don't laugh, but the medical term here for the brace is a "wrist cock-up brace!" Here, both styles can be found in drugstores, pharmacies, and medical supply stores.
3. For specifically keyboard work, either typing or mouse or (rarely) piano, my occupational health experts made me a molded to my arm & wrist plaster (or is it a resin now-a-days?) half-cast, to be velcro-ed on as needed. A foam wrist rest is essential for both keyboard and mouse, but the half-cast gives the best support, and has (I believe) prevented re-injury.
4. Physical Therapy. See your doctor. And, if you can find a banjo coach who has studied the Alexander Technique, or Feldenkreisse method, get some re-training. What worked for me and saved my career was a superb harp teacher, who had herself had a superb teacher - Eddie Druzinksy, principal harpist of the Chicago Symphony Orchestra, who both believe that it is important to diagnose, to analyse the movement, the motion that is comfortable FOR THE INDIVIDUAL!!!! What might be perfectly wonderful harp (or banjo) technique for one person's physiology might be perfectly awful for another. Basically, if it hurts, don't do it. Pain is your friend in that it is telling you that injury is occurring, so STOP doing whatever it is that is hurting you. If it is uncomfortable, it is wrong for your hand and wrist. What is correct technique for you, is that which is comfortable and efficient for you.
Good Luck -HEAL SOON - Write again & let us all know how things are going. Regards, A Celtic Harper