Actually I know several mtb guides who have had to deal with clients with this problem (like rushing them to the ER from the outback). Naturally, when you are dealing with clients who may not be used to low humidity and high altitude (and of questionable fitness), you encourage them to drink lots. These guides are receiving better training on recognizing the problem. I still think dehydration is the more prevalent problem, but hyponatremia is cropping up here and there.