How Is SIADH Diagnosed and Managed?

Water restriction. Restriction of fluid intake is the first-line treatment for SIADH in patients without hypovolemia. The severity of fluid restriction is guided by the concentration of the urinary solutes.¹⁵ Restriction of water intake to 500 ml/day to 1,000 ml/day is generally advised for many patients, as losses from the skin, lungs, and urine exceed this amount, leading to a gradual reduction in total body water.²¹ The main drawback of fluid restriction is poor compliance due to an intact thirst mechanism.

Saline infusion. The infusion of normal saline theoretically worsens hyponatremia due to SIADH because the water is retained while the salt is excreted. However, a trial of normal saline sometimes is attempted in patients in whom the differentiation between hypovolemia and euvolemia is difficult. From a study of a series of 17 patients with chronic SIADH, Musch and Decaux concluded that the infusion of intravenous normal (0.9%) saline raises P_Na when the urine osmolality is less than 530 mosm/L.²²

Oral solutes (urea and salt). The oral intake of salt augments water excretion²³, and salt tablets are used as a second-line agent in patients with persistent hyponatremia despite fluid restriction.²³ The oral administration of urea also results in increased free-water excretion via osmotic diuresis,²⁴ but its poor palatability, lack of availability in the U.S., and limited user experience has restricted its usage.²⁴

Demeclocycline. Demeclo-cycline is a tetracycline derivative that causes a partial nephrogenic diabetes insipidus.²⁵ Its limitations include a slow onset of action (two to five days) and an unpredictable treatment effect with the possibility of causing profound polyuria and hypernatremia. It is also associated with reversible azotemia and sometimes nephrotoxicity, especially in patients with cirrhosis.

Lithium. Lithium also causes nephrogenic diabetes insipidus by downregulating vasopressin-stimulated aquaporin-2 expression and thus improves hyponatremia in SIADH.²⁶ However, its use is significantly limited by its unpredictable response and the risks of interstitial nephritis and end-stage renal disease with chronic use. Therefore, it is no longer recommended for the treatment of SIADH.

Vasopressin receptor antagonists. Due to the role of excessive levels of vasopressin in the pathophysiology of most types of SIADH, antagonists of the vasopressin receptor were developed with the goal of preventing the excess water absorption that causes hyponatremia. Two vasopressin receptor antagonists, or vaptans, have been approved by the FDA for the treatment of nonemergent euvolemic and hypervolemic hyponatremia. Conivaptan is a nonselective vasopressin receptor antagonist that is for IV use only. Tolvaptan is a selective V2 receptor antagonist that is taken orally. Both conivaptan and tolvaptan successfully increase P_Na levels while the drugs are being taken.^27,28,29,30 Tolvaptan increases P_Na levels in hyponatremia due to SIADH and CHF, and modestly so in cirrhosis.³⁰

^{click for large version}

Figure 2. Algorithm for Treating Severe Hyponatremia

The most common side effects of the vaptans include dry mouth, increased thirst, and increased urination, although serious side effects (hypernatremia or too-rapid rate of increase in P_Na) are possible.²⁹ It is unclear if treating stable, asymptomatic hyponatremia with vaptans has any reduction in morbidity or mortality. One study found that tolvaptan increased the patients’ self-evaluations of mental functioning, but a study of tolvaptan used in combination with diuretics in the setting of CHF did not result in decreased mortality.^29,31 Due to their expense, necessity of being started in the hospital, and unclear long-term benefit, the vaptans are only recommended when traditional measures such as fluid restriction and salt tablets have been unsuccessful.

Back to the Case

Our patient has hypotonic hyponatremia based on her low serum osmolality. The duration of her hyponatremia is unclear, but the patient is not experiencing seizures or coma. Therefore, her hyponatremia should be corrected slowly, and hypertonic saline is not indicated.