The patient described had hyponatremia for longer than a month. This puts him in the category of chronic hyponatremia, which may influence management. Nevertheless, on his second presentation, the patient had profound hyponatremia, with symptoms due to brain edema (Figure 1), which constitutes a medical emergency and an indication for emergent treatment.
A potentially serious pitfall during treatment of symptomatic hyponatremia is correction of hyponatremia that is too rapid; this can result in osmotic demyelination syndrome, which can be fatal (Figure 2).
Hyponatremia must be corrected by a combination of water intake restriction to levels usually less than 500 mL/d. In an individual with a massive excess of water that is causing brain edema, restriction of water intake to zero may be a reasonable intervention.
Administration of hypertonic salt solutions is also indicated. The rate of correction can be calculated and should be carefully gauged. Extensive data suggest that the serum sodium should be raised by no more than 10 mEq/L over 24 hours.[1,7] Correction by 6 mEq/L in 24 hours has been dubbed the "rule of sixes."[8,9] The rule of sixes is as follows: "Six-a-day makes sense for safety. Six in 6 hours for severe symptoms and stop."
Correction that is too rapid and consequent osmotic demyelination can occur in instances of hyponatremia caused by cerebral salt wasting or with beer potomania, although the pathophysiology of hyponatremia in these disorders is different from that of SIADH. Individuals with hypovolemia who replace their volume losses mostly by hypotonic fluids or electrolyte-free water and become hyponatremic are especially at risk for overcorrection. When such individuals are given crystalloid solutions to correct their hyponatremia and their volume losses are restored, the high antidiuretic hormone levels triggered by hypovolemia return to baseline, and the patients may start excreting copious amounts of maximally dilute urine, resulting in correction that is too rapid. In such situations, the overcorrection should be reversed to avoid osmotic demyelination and potential death. This can be accomplished by administration of electrolyte-free water to partially lower the serum sodium concentration and by simultaneously administering desmopressin to avoid too rapid dilution of urine.
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