From Handshoe Mouse a Good Ergonomic Option at

There are are six different variations [of the Horseshoe Mouse] offered, with hand sizes (left and right) corresponding to hands measured from the tip of the ring finger to the wrist crease in large (210 mm or 8.25'), medium (190 mm or 7.5'), and small sizes (170 mm or 6.75'). Left-hand sizes are manufactured on demand and cost up to four times the price of right-hand versions. (emph mine)

Sheesh. Guess it's a good thing I can switchhit with the mouse. Had a coworker once who started getting carpel in her mouse hand, so she switched hands and was starting to get it in the left as well. I've managed to keep the pain in a single hand, and wonder if I wouldn't benefit from something fancy like the horseshoe mouse. Ergonomic keyboards help the tendons in my forearms noticeably.

Can't imagine how badly I'll feel at 70.