I've had a painful condition called dystonia in my wrist for many years and have gone to lighter rods and reels--specifically from a 9 ft. 5 wt to a 8.5 4 wt. and a 8 .5 3 wt. What I've found most helpful is a simple wrist brace (Mueller made the one I'm using and I got it from an online speciality house, but have been told a like product can be had at Wal-Mart).
The wrist brace is so important in lessening pain that I wll not fish without it. Three hours of casting is still my limit, though, and after 5 hours on the SF of the Rio Grande today, I quit, even though trout were still taking, because of the pain (and oh, yes, the 17 trout brought to hand contributed to the strain).
No man can have too many fly rods;
no woman too many shoes.