Desmopressin intravenously with 5% dextrose is the most appropriate treatment. The patient presented with a 5-day history of gastroenteritis symptoms and severe hyponatremia with a normal neurologic examination. She has chronic asymptomatic hyponatremia that was likely from volume depletion, causing an appropriate elevation in antidiuretic hormone (ADH) secretion, which caused the hyponatremia by retaining the free water she was drinking to maintain hydration. Volume repletion in the hospital led to suppression of ADH production with a subsequent water diuresis, leading to rapid correction. Rapid correction in chronic asymptomatic hyponatremia increases the risk for osmotic demyelination syndrome (ODS), especially in women, which may be delayed for a few days after overcorrection. Overcorrection is considered to be an increase of >8.0 mEq/L (8.0 mmol/L) of the serum sodium within the first 24 hours or >16 mEq/L (16 mmol/L) within the first 48 hours. Desmopressin will terminate the water diuresis, and if given alone will maintain the serum sodium around the current level of 128 mEq/L (128 mmol/L), which still represents overcorrection. Desmopressin intravenously with 5% dextrose will stop the water diuresis and further lower the serum sodium. The 5% dextrose should be given in sufficient volume to reverse the serum sodium level to 120 mEq/L (120 mmol/L) over the first 24 hours and then allowing slow correction.

Fluid restriction alone is inappropriate because the serum sodium will continue correction toward normal and increase the risk of ODS, as will fluid restriction with salt tablets, which will increase the rate of correction.

Providing no further intervention would not reverse the overcorrection, and the patient's serum sodium would likely continue to return to normal, further increasing her risk for ODS.