Curriculum · Nephrology and Urology
Evaluation of hyponatraemia
What it is
Hyponatremia is a low serum sodium, but the value alone does not name the problem. The evaluation moves in steps: first whether the low sodium is true, then the urine dilution, then what the volume state is.
Causes and risk
The causes of SIADH are CNS disturbance such as stroke, hemorrhage or trauma, medications such as carbamazepine, SSRIs and NSAIDs, lung disease such as pneumonia, ectopic ADH secretion as in small cell lung cancer, and pain and/or nausea. Excess ADH is secreted from the posterior pituitary or an ectopic source, and the elevated levels lead to water retention and excretion of concentrated urine, which gives hyponatremia and volume expansion. Despite that volume expansion, edema is not seen in SIADH, because natriuresis occurs despite the hyponatremia: volume expansion raises atrial natriuretic peptide and reduces proximal tubular sodium absorption. The other groups are set by volume. Hypovolemic causes are renal salt loss from excessive diuretic use, low aldosterone or ATN, and extrarenal salt loss from diarrhea, vomiting, diaphoresis or third spacing. Euvolemic causes are SIADH, psychogenic polydipsia, postoperative state, hypothyroidism and glucocorticoid deficiency. Hypervolemic causes are CHF, nephrotic syndrome, liver disease, cirrhosis and renal failure.