OBJECTIVE: A major cause of readmission after transsphenoidal surgery (TSS) is delayed hyponatraemia. The purpose of this study was to identify predictors of hyponatraemia one week post surgery and predictors of 30-day readmissions for hyponatraemia.

DESIGN: A retrospective cohort study including patients who had TSS performed for pituitary lesions.

METHOD: The risk of readmission for hyponatraemia was assessed in consecutive patients between January 2008 and March 2016. The risk of hyponatraemia one week post surgery was assessed in patients admitted for TSS between July 2011 and March 2016.

RESULTS: Of all included patients, 56/522 (10.7%) were readmitted within 30 days. Hyponatraemia was found in 14/56 (25%) of 30-day readmissions. We did not identify any predictive variable for hyponatraemia on readmission. The number of patients with hyponatraemia on the seventh post-operative day was 26/314 (8.3%). The risk of hyponatraemia one week post surgery was increased by an odds ratio of 2.40 (95% CI: 1.06-5.40) in patients with a tumour abutting the optic chiasm and by an odds ratio of 1.16 (1.04-1.31) per mmol/L decrease in sodium levels on the first post-operative day.

CONCLUSIONS: Hyponatraemia occurred in 25% of readmissions; however, we did not identify any predictive variable for readmission with hyponatraemia. One week post surgery, 8.9% had hyponatraemia. Tumours pressing on the optic chiasm as well as a fall in sodium levels on the first post-operative day were associated with an increased risk of hyponatraemia one week post surgery. We suggest that a day 7 serum sodium <130 nmol/L should lead to concern and the provision of patient advice.