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Syndrome of inappropriate secretion of ADH (SIADH) (Investigations (Low…
Syndrome of inappropriate secretion of ADH (SIADH)
Symptoms
Anorexia/nausea and malaise
Weakness and ache
Varied and generic
Reduction in GCS and confusion with drowsiness
Symptoms as a result of hyponatraemia
Fits and coma - occurs later
Pathophysiology
Excess release of ADH will result in increased insertion of aquaporin 2 channels in apical membrane of the collecting duct
This will result in excess water retention which in turn will dilute blood plasma thereby resulting in hyponatraemia as the Na concentration will decrease
Investigations
Low plasma osmolality
High urine osmolality
High urine Na
Absence of hypokalaemia, hypotension and hypovolaemia
Euvolaemia - normal blood volume (in contrast to hypovolaemia of Na and water depletion state)
Normal renal, adrenal and thyroid function
Low serum Na
Hyponatraemia is common in the ill, frail patients (difficult to distinguish between the two) - test with 0.9% saline
Sodium depletion will respond
SIADH will NOT respond
Dilutional hyponatraemia due to excessive water retention
Treatment Plan
Hypertonic (concentrated with salt) saline if really symptomatic (to prevent swelling of the brain)
Can give oral demeclocycline (induces nephrogenic DI by inhibiting action of ADH on kidney)
Restrict fluid intake to 500 - 1000ml daily to increase Na concentration and thus reduce symptoms
Treat underlying cause where possible
Give vasopressin antagonist e.g. oral tolvaptan
Salt and loop diuretic e.g. oral furosemide