The syndrome of inappropriate antidiuretic hormone secretion is the most common cause of hyponatremia in clinical practice, but current management of hyponatremia and outcomes in patients with syndrome of inappropriate antidiuretic hormone secretion are not well understood. The objective of the Hyponatremia Registry was to assess the current state of management of hyponatremia due to syndrome of inappropriate antidiuretic hormone secretion in diverse hospital settings, specifically which diagnostic and treatment modalities are currently used and how rapidly and reliably they result in an increase in serum sodium concentration ([Na(+)]). A secondary objective was to determine whether treatment choices and outcomes differ across the United States and the European Union.

The Hyponatremia Registry recorded selected diagnostic measures and use, efficacy, and outcomes of therapy for euvolemic hyponatremia diagnosed clinically as syndrome of inappropriate antidiuretic hormone secretion in 1524 adult patients with [Na(+)] ≤130 mEq/L (1034 from 146 US sites and 490 from 79 EU sites). A subgroup of patients with more rigorously defined syndrome of inappropriate antidiuretic hormone secretion via measurement of relevant laboratory parameters was also analyzed.

The most common monotherapy treatments for hyponatremia in syndrome of inappropriate antidiuretic hormone secretion were fluid restriction (48%), isotonic (27%) or hypertonic (6%) saline, and tolvaptan (13%); 11% received no active agent. The mean rate of [Na(+)] change (mEq/L/d) was greater for all active therapies than no active treatment. Hypertonic saline and tolvaptan produced the greatest mean rate of [Na(+)] change (interquartile range, both 3.0 [6.0] mEq/L/d) compared with lower interquartile range rates of [Na(+)] change for isotonic saline (1.5 [3.0] mEq/L/d) and fluid restriction (1.0 [2.3] mEq/L/d). The general pattern of responses was similar in both the US and EU cohorts. At discharge, [Na(+)] was <135 mEq/L in 75% of patients and ≤130 mEq/L in 43% of patients. Overly rapid correction occurred in 10.2% of patients.

Current treatment of hyponatremia in syndrome of inappropriate antidiuretic hormone secretion often uses therapies with limited efficacy; the most commonly chosen monotherapy treatments, fluid restriction and isotonic saline, failed to increase the serum [Na(+)] by ≥5 mEq/L in 55% and 64% of monotherapy treatment episodes, respectively. Appropriate laboratory tests to diagnose syndrome of inappropriate antidiuretic hormone secretion were obtained in <50% of patients; success rates in correcting hyponatremia were significantly higher when such tests were obtained. Few outcome differences were found between the United States and the European Union. A notable exception was hospital length of stay; use of tolvaptan was associated with significantly shorter length of stay in the European Union but not in the United States. Despite the availability of effective therapies, most patients with syndrome of inappropriate antidiuretic hormone secretion were discharged from the hospital still hyponatremic.