Fluids and Electrolytes
F i g u r e 5 . Evaluation of hypotonic hyponatremia. SIADH = syndrome of inappropriate antidiuretic hormone secretion; UNa = urine sodium concentration; Uosm = urine osmolality.
CONT.
hyponatremia with a urine osmolality >100 mOsm/kg H2O is classified into three groups based on the clinical volume status: hypovolemic, hypervolemic, and euvolemic.
Hypovolemic Hyponatremia Hypovolemia causes stimulation of the sympathetic nervous system, activation of the renin-angiotensin-aldosterone (RAA) axis, and release of ADH. These adaptive responses allow volume maintenance at the expense of a low serum sodium with excessive water intake. Extrarenal causes of hypovolemia are common and include loss of sodium from the gastrointestinal tract (vomiting or diarrhea) and insensible loss of sodium chloride (sweating, burns, respiratory tract); in these conditions, the urine sodium concentration is typically <20 mEq/L (20 mmol/L). Renal causes of hypovolemic hyponatremia result in excessive loss of salt and water in the urine with volume depletion; the urine sodium concentration is typically >20 mEq/L (20 mmol/L). Diuretic therapy is the most common cause. Less common are
adrenal insufficiency and salt-wasting nephropathies with impaired renal tubular function; typical causes include reflux nephropathy, interstitial nephropathies, post-obstructive uropathy, cystic kidney diseases, and the recovery phase of acute tubular necrosis. The syndrome of cerebral salt wasting is a rare cause due to inappropriate natriuresis from intracranial disease such as subarachnoid hemorrhage, traumatic brain injury, craniotomy, encephalitis, and meningitis.
Hypervolemic Hyponatremia Patients with hypervolemic hyponatremia have increased total body sodium and water, with the latter dominating and leading to a reduced serum sodium concentration. The pathophysiology of hyponatremia in sodium-avid edematous disorders (heart failure, cirrhosis, and the nephrotic syndrome) is similar to that in hypovolemic hyponatremia, with the kidney sensing a decreased arterial blood volume despite excess total body sodium and water. The urine sodium concentration is typically <20 mEq/L (20 mmol/L) in the absence of diuretic therapy. 11