Chronic Idiopathic Hyponatremia in Older People Due to Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) Possibly Related to Aging

OBJECTIVE: To determine the prevalence of syndrome of inappropriate antidiuretic hormone secretion (SIADH) among older hyponatremic patients in a subacute geriatric facility, to identify patients with no apparent cause for the SIADH (idiopathic SIADH), and to determine their clinical characteristics. DESIGN: Prospective analysis of a cohort of older patients over a period of 3 months. SETTING: Two wards in a geriatric rehabilitation hospital. PARTICIPANTS: Patients aged 65 and older. MEASUREMENTS: All patients with hyponatremia (serum sodium <135 mmols/l) were clinically examined and relevant investigations were performed to determine the etiology of hyponatremia. Patients were observed for symptoms of hyponatremia. Hyponatremia was classified into possible SIADH and non-SIADH types. Patients with SIADH type hyponatremia were screened for possible causes. Past medical histories were obtained from the general practitioners. RESULTS: Of the 172 patients studied, 43 (25%) had hyponatremia. It was symptomatic in only four patients. Twenty-two (51%) had SIADH etiology. In nine (mean age 84 +/- 4), no cause for the SIADH was evident (presumed idiopathic SIADH) and in seven, hyponatremia (128-135 mmols/l) was chronic (12 to 72 months). Further reduction in serum sodium, which was symptomatic, was noted in two of these patients with the onset of pneumonia. CONCLUSION: Most older hyponatremic patients in a rehabilitation setting seem to have SIADH etiology. This study confirms the presence of a group of older individuals with chronic idiopathic hyponatremia in whom the underlying mechanism may be SIADH related to aging. Hyponatremia is modest in these patients and has little clinical significance. However, they may be at increased risk of developing symptomatic hyponatremia with intercurrent illnesses.