The signs and symptoms of dehydration and the signs and symptoms of hyponatremia are very similar. However, they can both be fatal if not treated and the treatment is very different for both. In fact, the treatment for one can cause death if the problem is the other.
This whole topic is how to keep water and salt balanced in the body. Putting one or both out of balance causes various problems.
- Dehydration is lower than normal amounts of water in the body.
- Hyponatremia is lower than normal amounts of sodium in the body (and is normally associated with hypervolemia (too much water in the body) because the athlete has been over-drinking).
Ultra-runners know of these dangers and carefully monitor the fluid they take and supplement with salt tablets. For example, @Sparafusile said, "Cramps would be a dead giveway". I would disagree. If I see cramps then I would be wondering if the salt balance was high enough (because the body uses sodium and potassium as part of the signalling pathway for muscle contractions - along with calcium and magnesium in lower concentrations).
The best way to learn how to manage these issues is the nine-way hydration/salt-balance table from the scientist Karl King. This table shows the interaction between hydration being low, ok or high and electrolytes being low, ok or high (hence nine-way).
In determining dehydration vs hyponatremia I would be looking at weight changes and puffiness in fingers. If in doubt, I would transport the athlete to hospital and make sure the serum sodium levels were taken before the medical staff assume dehydration (often they treat for dehydration instead of hyponatremia and the patient dies).
The scientist Lulu Weschler posts on the Ultra mailing list quite a lot and was a co-author on the 2008 consensus statement on Exercise Associated Hyponatremia. I will list some of her papers here from her email to that list on Dec 14, 2008. That email is available in the password-protected but free Ultra mailing list archive. In that email she gives a nice summary but I won't breach her copyright by repeating it here without her permission.
- Hew-Butler T, Jordaan E, Stuempfle KJ, Speedy DB, Siegel AJ, Noakes TD,
Soldin SJ, and Verbalis JG. Osmotic and Non-Osmotic Regulation of Arginine
Vasopressin during Prolonged Endurance Exercise. J Clin Endocrinol Metab 2008.
- Hew-Butler T, Noakes TD, and Siegel AJ. Practical management of
exercise-associated hyponatremic encephalopathy: the sodium paradox of
non-osmotic vasopressin secretion. Clin J Sport Med 18: 350-354, 2008.
- Rae DE, Knobel GJ, Mann T, Swart J, Tucker R, and Noakes TD. Heatstroke
during Endurance Exercise: Is There Evidence for Excessive Endothermy? Med
Sci Sports Exerc 40: 1193-1204, 2008.
- Hew-Butler T, Ayus JC, Kipps C, Maughan RJ, Mettler S, Meeuwisse WH, Page
AJ, Reid SA, Rehrer NJ, Roberts WO, Rogers IR, Rosner MH, Siegel AJ, Speedy
DB, Stuempfle KJ, Verbalis JG, Weschler LB, and Wharam P. Statement of the
Second International Exercise-Associated Hyponatremia Consensus Development
Conference, New Zealand, 2007. Clin J Sport Med 18: 111-121, 2008.
- Weschler LB. Sweat electrolyte concentrations obtained from within occlusive
coverings are falsely high because sweat itself leaches skin electrolytes. J
Appl Physiol 105: 1376-1377, 2008.
- Hew-Butler T, Noakes TD, Soldin SJ, and Verbalis JG. Acute changes in
endocrine and fluid balance markers during high-intensity, steady-state, and
prolonged endurance running: unexpected increases in oxytocin and brain
natriuretic peptide during exercise. European journal of endocrinology /
European Federation of Endocrine Societies 159: 729-737, 2008.