Normovolaemic (euvolaemic) hyponatraemia

• many different hypo-osmolar disorders can potentially present clinically with a normal extracellular fluid (ECF) volume, or euvolaemia, in part because it is difficult to detect modest changes in volume status using standard methods of clinical assessment
• most patients with hyponatraemia have clinical euvolaemia, in part because of the large number of diseases associated with SIADH
  • causes of normovolaemic hyponatraemia include:
    • SIADH
    • prolonged strenuous exercise (marathon, triathalon, ultramarathon, hot-weather hiking)
    • idiopathic
    • glucocorticoid deficiency
    • hypothyroidism
• normovolaemia
  • generally diagnosed clinically from the history, physical examination, and laboratory results
  • patients who do not have clinical signs of volume depletion (orthostatic decreases in blood pressure and increases in pulse rate, dry mucus membranes, decreased skin turgor) or volume expansion (subcutaneous oedema, ascites) should be considered to have normovolaemic unless there is alternative evidence suggesting an abnormal ECF volume status
  • laboratory results include a normal or low urea, and reduced serum uric acid level
  • spot urine [Na+] should be >=30 mmol/L in most patients with normovolaemic hyponatraemia unless they have become secondarily sodium depleted

Reference:

• (1) hyponatraemia Treatment Guidelines 2007: Expert Panel Recommendations The American Journal of Medicine 2007; 120 (11);S1:S1-S21.