Case: A 73-year-old lady presented with hyponatraemia six days post-transphenoidal surgery for a non functioning pituitary macro-adenoma. Peri-operatively she developed diabetes insipidus requiring short term desmopressin whilst on the neurosurgical ward. At post-operative presentation she complained of weakness, confusion and nausea; sodium 125 mmol/l, serum osmolality 268 mOsmol/kg, urine osmolality 474 mOsmol/kg. Over the next few days she became symptomatically worse and her hyponatraemia more severe; sodium 114 mmol/l, serum osmolality 252 mOsmol/kg, urine sodium 52 mmol/l. A diagnosis of post-operative SIADH was made and she was admitted for optimisation of fluid balance. Because she was clinically dehydrated, she was given slow intravenous normal saline. After 5 days her sodium gradually increased to 140 mmol/l and she was symptomatically much improved. Nine days later, her sodium increased to 145 mmol/l, serum osmolality 311 mOsmol/kg, urine osmolality 464 mOsmol/kg. Subsequently her sodium rose to a peak of 150 mmol/l; she developed polyuria and polydipsia and also reported numbness in the face and both feet and general loss of balance. A diagnosis of diabetes insipidus was made on the basis of the high urine osmolality and polyuria, and the patient was started on DDAVP. We have not yet explained her neurological symptoms and she awaits a post-operative MRI brain and pituitary fossa.
Discussion: This case is interesting as it displays the tri-phasic response pattern of sodium that can be seen post pituitary surgery. The development of SIADH is thought to be due to degeneration of paraventricular neurones, which may predispose to permanent diabetes insipidus. This case highlights the need to closely observe sodium levels in the short to mid-term post pituitary surgery. We discuss a possible short term role for the vaptans in temporary post operative SIADH which might prevent hospital admission in this situation.