Euvolemic
hyponatremia is frequently encountered in hospitalized patients and the syndrome of inappropriate
antidiuretic hormone secretion (
SIADH) is the most common cause in most patients.
SIADH diagnosis is confirmed by decreased serum osmolality, inappropriately elevated urine osmolality (>100 mosmol/L), and elevated urine
sodium (Na) levels. Patients should be screened for
thiazide use and adrenal or thyroid dysfunction should be ruled out before making a diagnosis of
SIADH. Clinical mimics of
SIADH like cerebral
salt wasting and reset osmostat should be considered in some patients. The distinction between acute (<48 hours) versus chronic (>48 hours or without baseline labs)
hyponatremia and clinical symptomatology are important to initiate proper
therapy. Acute
hyponatremia is a medical emergency and osmotic
demyelination syndrome (ODS) occurs commonly when rapidly correcting any chronic
hyponatremia. Hypertonic (3%) saline should be used in patients with significant
neurologic symptoms and maximal correction of serum Na level should be limited to <8 mEq over 24 hours to prevent the ODS. Simultaneous administration of parenteral
desmopressin is one of the best ways to prevent overly rapid Na correction in high-risk patients. Free water restriction combined with increased solute intake (e.g.,
urea) is the most effective
therapy to treat patients with
SIADH.
0.9% saline acts as a
hypertonic solution in patients with
hyponatremia and should be avoided in the treatment of
SIADH due to rapid fluctuations in serum Na levels. Dual effects of
0.9% saline resulting in rapid correction of serum Na during infusion (inducing ODS) and post-infusion worsening of serum Na levels are described in the article with clinical examples.