Severe Hyponatraemia Following Underwater Colonoscopy and Polypectomy in an Elderly Woman: A Multifactorial Case Involving Bowel Prep, Procedural Stress, and Occult Malignancy.

We report a case of a 77-year-old woman with a previously normal serum sodium of 141 mmol/L one week earlier, who developed acute severe symptomatic hyponatraemia (serum sodium 106 mmol/L) two days after an uneventful underwater colonoscopy and polypectomy for a caecal lateral spreading tumour (LST-G). The procedure was preceded by a positive faecal immunochemical test (FIT) in April 2025, followed by a CT colonogram showing a caecal lesion. She completed a low-solute Plenvu® (Norgine, Amsterdam, Netherlands) bowel preparation prior to endoscopy. Collateral history revealed three falls and two episodes of vomiting in the 24 hours before hospital presentation. On admission, she was found confused, with laboratory features consistent with syndrome of inappropriate antidiuretic hormone secretion (SIADH), including a urine sodium of 83 mmol/L. She received 3% hypertonic saline in intensive care, with full neurological recovery. Histology confirmed a T2N0M0 adenocarcinoma. This case highlights the importance of recognising risk factors for hyponatraemia - including procedural stress, low-solute preparation, age, and occult malignancy - particularly in the setting of water-assisted colonoscopy, and supports early biochemical monitoring in high-risk patients.