The electrolyte disturbance hyponatremia exists when the sodium level in the plasma falls below 135 mmol/l.
Most patients are asymptomatic of the hyponatremia, but usually have symptoms related to the underlying cause.
Severe hyponatremia may cause osmotic shift of water from the plasma into the brain cells. Typical symptoms include nausea, vomiting, headache and malaise. As the hyponatremia worsens, confusion, stupor or coma may occur.
An abnormally low plasma sodium level is best considered in conjunction with the person's plasma osmolality and extracellular fluid volume status.
Most cases of hyponatremia are associated with reduced plasma osmolality.
A normal or high plasma osmolality with hyponatremia is called pseudohyponatremia. Pseudohyponatremia may be caused if high lipid levels in the plasma interfere with the sodium assay, or if a different solute (such as glucose) is abnormally abundant.
When the plasma osmolality is low, the extracellular fluid volume status may be in one of three states:
Low volume. Loss of water is accompanied by loss of sodium.
SIADH (syndrome of inappropriate antidiuretic hormone)
- Some cases of psychogenic polydipsia
High volume. There is retention of water.
Severe hyponatremia may result from a few hours of heavy exercise in high temperature conditions, such as hiking in desert areas, or from endurance athletic events when electrolytes are not supplied. (Such an incident notably happened to long-distance athlete Craig Barrett in 1998).
Last updated: 05-07-2005 03:17:52
Last updated: 05-13-2005 07:56:04