Hyponatremia is a relatively rare condition. In reference to the New England Journal of Medicine, the incidence of hyponatremia was about 13%, but only 0.6% (a level of <120 mmol) had critical levels. In the hospital, we see many people with sodium levels in the 130's and even the high 120's. Unless the person is symptomatic, we are not overly concerned. In fact, we often correct people with hyponatremia to around 130, and then we discharge them home as long as the offending cause is removed. The reason for this is that the levels will gradually return to normal by itself.
So what exactly is the cause of hyponatremia in people performing high endurance activities. Well, the exact cause is not known, but it is most likely due to excessive free water intake in combination with sweating. Now, sweating is actually hypotonic, or it contains less sodium than regular fluid in our body. The loss of salt in sweat is minimal and we don't normally care about it. Usually, normal rehydration wouldn't affect salt levels and even salt replacement is not necessary. But it can be problematic in endurance athletes. In general, these athletes tend to drink a lot and not eat. Hikers tend to both eat and drink. Most stop for lunch and I'm sure at least most backpackers stop for dinner. And most hikers do not hike 26 miles within 4-5 hours (unless you're Cavedog). What does this mean for "hikers?" Usually, a meal with a moderate exercise throughout the day is more than enough salt replacement. Hikers generally do not have all the risk factors as marathon runners and triathletes.
For hyponatremia, someone already posted a website with a list of symptoms. Most people are not symptomatic, and usually our body can compensate pretty well, especially the kidneys which can concentrate salt to up to 600meq/L to prevent excessive salt loss. By the time a person eats a meal, there should be enough salt to keep salt in balance.
Also, when physicians treat for a person with hyponatremia, we have to correct for it very slowly as there is significant problems if we correct it too quickly. That being said, we usually start off a person with normal saline as that is usually adequate. Usually, an athlete will probably suffer from dehydration, and even if they have hyponatremia, they usually have hyponatremia AND dehydration. So we start off with normal saline, check the sodium levels and the sodium levels in the urine, and then adjust the fluids appropriately.
Generally speaking, an endurance athlete should drink a maximum of 400-800 ml of free water per hour. This is true for hikers as well. In fact, I believe that hikers can easily drink more than that. If you are really concerned about hyponatremia, then just snack on GORP, energy bars, or have a small snack when you enjoy the view from the summit you're on. As for someone with epilepsy, any change in electrolytes can potentially cause seizures, including both hyponatremia and hypernatremia. As long as you are drinking enough and have a dinner or snack, you should be fine. I don't think there is anything to worry about. Hope this helps. If i missed a question, just let me know and hopefully I can answer it.
