Endocrine Abstracts

Hyponatraemia is the most common fluid and electrolyte disorder encountered in clinical medicine, with incidences as high as 15% to 30% in both acutely and chronically hospitalized patients. Differential diagnosis is complicated by a long list of potential etiologies. Traditional diagnostic strategies entail an initial characterization of the patient's extracellular fluid volume status to differentiate euvolemic hyponatraemia from hypovolemic hyponatraemia (generally indicating dehydration) and hypervolemic hyponatraemia (usually reflecting congestive heart failure or cirrhosis). Patients with euvolemic hyponatraemia are candidates for the syndrome of inappropriate antidiuretic hormone secretion (SIADH), which entails differentiating among a further long list of potential etiologies. Although the cause of the hyponatraemia may not be diagnosed initially in some cases, the numbers of patients in whom an etiology cannot be established after careful follow up are relatively few. One exception appears to be elderly patients, who often develop hyponatraemia in the absence of any apparent etiology. Coupled with an increased incidence of hyponatraemia in older patients (7% to 53% in institutionalized geriatric patients), this suggests that the normal aging process may be accompanied by abnormalities of vasopressin secretion that predispose to SIADH. A recent study of elderly hyponatraemic patients concluded that approximately 60% remained 'idiopathic' despite rigorous evaluation, leading to a suggestion that extensive diagnostic procedures may not be warranted in elderly patients if routine evaluation fails to indicate a diagnosis. However, because unexplained hyponatraemia can precede the diagnosis of underlying disease processes, this strategy could potentially miss occult malignancies in some cases. Therefore, an individualized approach to differential diagnosis of hyponatraemia is necessary, in which multiple factors should dictate the extent of diagnostic testing undertaken in each case. Adoption of a more standardized method of evaluation and diagnosis is especially important in view of the imminent availability of vasopressin receptor antagonists for the treatment of hyponatraemic patients.