Hyponatremia medical therapy

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Medical Therapy

• Hyponatremia is due to an excess of free water in the body, not due to a deficiency of sodium.
• The treatment of hyponatremia is therefore free water restriction.
• In patients who are receiving intravenous therapy, you should make sure that D₅W is not infusing, and that drug infusions are mixed in normal saline, and not D₅W.
• The patients access to water and juice should be restricted.
• In some refractory cases, the water to the room must be turned off.

The treatment of hyponatremia will depend on the underlying cause and whether the patient's volume status is hypervolemic, euvolemic, or hypovolemic. In the setting of hypovolemia, intravenous administration of normal saline may be effective, but caution must be exercised not to raise the serum sodium level too quickly (see below). Euvolemic hyponatremia is usually managed by fluid restriction and treatment to abolish any stimuli for ADH secretion such as nausea. Likewise, drugs causing SIADH should be discontinued if possible. Patients with euvolemic hyponatremia that persists despite those measures may be candidates for a so-called vaptan drug as discussed below. Hypervolemic hyponatremia should be treated by treating the underlying cause (e.g. heart failure, cirrhosis). In practice, it may not be possible to do so, in which case the treatment of the hyponatremia becomes the same as that for euvolemic hyponatremia (i.e. fluid restriction and/or use of a vaptan drug).

Hyponatremia must be corrected slowly in order to lessen the chance of the development of central pontine myelinolysis (CPM), a severe neurological disease. In fact, overly rapid correction of hyponatremia is the most common cause of that potentially devastating disorder. During treatment of hyponatremia, the serum sodium should not be allowed to rise by more than 8 mmol/l over 24 hours (i.e. 0.33 mmol/l/h rate of rise). In practice, too rapid correction of hyponatremia and thence CPM is most likely to occur during the treatment of hypovolemic hyponatremia. In particular, once the hypovolemic state has been corrected, the signal for ADH release disappears. At that point, there will be an abrupt water diuresis (since there is no longer any ADH acting to retain the water). A rapid and profound rise in serum sodium can then occur. Should the rate of rise of serum sodium exceed 0.33 mmol/l/h over several hours, vasopressin may be administered to prevent ongoing rapid water diuresis.

Pharmaceutically, vasopressin receptor antagonists can be used in the treatment of hyponatremia, especially in patients with SIADH, congestive heart failure or liver cirrhosis. A vasopressin receptor antagonist is an agent that interferes with the action at the vasopressin receptors. A new class of medication, the "vaptan" drugs has been specifically developed to inhibit the action of vasopressin on its receptors (V1A, V1B, and V2). These receptors have a variety of functions, with the V1A and V2 receptors are expressed peripherally and involved in the modulation of blood pressure and kidney function respectively, while the V1A and V1B receptors are expressed in the central nervous system. V1A is expressed in many regions of the brain, and has been linked to a variety of social behaviors in humans and animals.

Vaptan Drugs

The “vaptan” class of drugs contains a number of compounds with varying selectivity, several of which are either already in clinical use or in clinical trials as of 2010.

Unselective (mixed V1A, V2)

• Conivaptan

V1A selective

• Relcovaptan

V1B selective

• Nelivaptan

V2 selective

• Lixivaptan

• Mozavaptan

• Satavaptan

• Tolvaptan

The V2-receptor antagonists tolvaptan and conivaptan allow excretion of electrolyte free water and are effective in increasing serum sodium in euvolemic and hypervolemic hyponatremia.

Rate of Na Correction

The rate of correction of hyponatremia should be 0.5-1.0meq/L/hr, with not more than a 12 meq/l correction in 24 hrs. If the patient has ongoing seizures (or [Na⁺]<115 meq/li), correction can be attempted at up to 2 meq/L/hr, but only while seizure activity lasts and the [Na⁺] exceeds 125-130 meq/Li.