[Hyponatremia secondary to inappropriate antidiuretic hormone secretion]

The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a disorder of sodium and water balance characterized by hypotonic hyponatremia and impaired water excretion in the absence of renal insufficiency , adrenal insufficiency or any recognized stimulus for the antidiuretic hormone (ADH). An inappropriate increase in ADH release of any cause produces hyponatremia by interfering with urinary dilution, thereby preventing the excretion of ingested water. Despite being the most common cause of hyponatremia in hospitalized patients, SIADH remains a diagnosis of exclusion. SIADH should be suspected in any patient with hyponatremia, hyposmolarity, urine osmolality above 100 mosmol/hgH2O, urine sodium concentration usually above 40 mEq/L, and clinical euvolemia. a number of modalities can be used to correct hyponatremia in SIADH, with water restriction and salt administration being the most important. The rate of correction is dependent upon the degree of hyponatremia and the presence or absence of symptoms. Patients with severe neurological symptoms require prompt correction; however, excessively rapid correction should be avoided because it can lead to the late onset of neurological complications from osmotic demyelination.