We present two cases of severe
hyponatremia secondary to syndrome of inappropriate secretion of
antidiuretic hormone (
SIADH) with very high urine
sodium concentrations (>130 mmol/L). The first patient had
hyponatremia from
traumatic brain injury (TBI) while the second case had a history of recurrent
SIADH triggered by various causes including
gastritis. In both cases, fluid administration and/or consumption worsened the
hyponatremia. Although a low urine
sodium of <30 mmol/L is highly suggestive of
hypovolemic hyponatremia and good response to saline infusion, there is lack of clarity of the threshold of which high urine
sodium concentration can differentiate various causes of natriuresis such as
SIADH, renal or cerebral
salt wasting. Apart from high urine osmolality (>500 mOsm/kg), persistence of high urine
sodium concentrations may be useful to predict poor response to fluid restriction in
SIADH. More studies are needed to delineate treatment pathways of patients with very high urine osmolality and urine
sodium concentrations in
SIADH.