Howdy HikeUp
You asked:
"Is hyponatremia a more serious condition that dehydration? I'm sure it's not a clear cut answer, but if you had a choice of drinking too much (leading to hyponatremia) vs. not drinking enough (leading to dehydration) which is the least risky choice on a strenuous hike?"
Yes -- hyponatremia is more serious than dehydration. As a sports med MD friend of mine Randy Eichner said, "Dehydration kills performance, hyponatremia kills." Bottom line, I would err on the side of not drinking enough and being slightly dehydrated.
The idea is to closely match your sweat rate to prevent losing more than 2% of your weight. Losing 1 to 2 lb is fine, as this represents less than 2% of your body weight. It sounds to me like you're matching your sweat losses pretty well and doing a good job "listening" to your body.
Hyponatremia is scary because the early symptoms can resemble those of dehydration. Early signs and symptoms of hyponatremia tend to develop when the plasma sodium concentration falls below 130 mmol/L and include bloating, “puffiness,” nausea, vomiting, and headache.
Symptoms of impending heat illness due to dehydration include weakness, chills, goose bumps on the chest and upper arms, nausea, headache, faintness, disorientation, muscle cramping, and cessation of sweating.
As the severity of hyponatremia increases — with serum sodium concentration below 125 mmol/L — more serious signs and symptoms occur due to increased cerebral edema (brain swelling). These include altered mental status (confusion, disorientation, and agitation), seizures, respiratory distress (due to pulmonary edema), unresponsiveness, coma and death.
Treatment in the hospital ER may include hypertonic solutions of sodium chloride or hypertonic mannitol to immediately decrease brain edema. Time is of the essence in treatment.
This is why dehydration is preferable to hypnonatremia in the wilderness.
Miles of smiles,
Ellen