Hospitalizations for hyponatremia and syndrome of inappropriate antidiuretic hormone hypersecretion at the Policlinic of Modena, Italy from January 2006 to September 2008

Introduction: Hyponatremia is the most common electrolyte imbalance among hospitalized patients. The cause is not always identified, but 30-40% of cases are the result of the syndrome of inappropriate antidiuretic hormone hypersecretion (SIADH). The aim of this study was to determine the frequency of hyponatremia in our hospital, its underlying causes (in particular the proportion of cases caused by SIADH), and approaches to treatment. Materials and methods: We retrospectively analyzed all nonpediatric hospitalizations occurring between 1 January 2006 and 30 September 2008 in which the discharge diagnosis was hyponatremia or SIADH. Results: A total of 64 cases were reviewed; 56 of the patients were aged > 60. The hyponatremia was classified as severe (< 125 mEq/L) in 36/64 cases (56%) (most involving patients aged >70); moderate (129-125 mEq/L) in 22 (34%); and mild (134-130 mEq/L) in 6 (9%). Neurologic symptoms were present in 11 patients with severe hyponatremia (confusion, hallucinations, coma) and 9 with moderate hyponatremia (weakness, impaired memory). Thirteen had hypovolemic hypotonic hyponatremia, 16 had hypervolemic hypotonic hyponatremia, and 35 had normovolemic hypotonic hyponatremia, including 23 who met the criteria for SIADH: 5 patients with neoplastic disease, 6 with pulmonary disorders, 3 with drug-related hyponatremia (a selective serotonin reuptake inhibitor in 1 case, a tricyclic antidepressant in 1, carbamazepine in 1), 2 whose SIADH developed after surgery, 4 with head trauma, 1 receiving noninvasive ventilatory support, and 2 elderly patients whose SIADH appeared idiopathic. Thirteen patients had hyponatremia related to the use of diuretics (thiazides in 7 cases), and one other had long-standing untreated hypothyroidism. In most cases, treatment consisted of infusion of hypertonic saline (sometimes with fluid restriction and/or furosemide administration). Discussion: Over one third (36%) of the adults hospitalized in the Policlinic of Modena for hyponatremia during the 3-year study had SIADH, a figure that is consistent with literature data. In most cases, the cause of hyponatremia can be identified on the basis of simple laboratory tests and complete patient histories.