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

Abstract

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.

Keywords: bowel cleansing agents; colonoscopy complications; colorectal cancer; elderly people; gastroenterology and endoscopy; hypertonic sodium chloride; hyponatraemia; paraneoplastic syndromes; syndrome of inappropriate secretion of antidiuretic hormone (siadh); underwater polypectomy.

Figure 1. Serum sodium trend during admission and follow-up.

Serum sodium fell from a baseline of 141 mmol/L (12/05/2025) to 106 mmol/L on admission (23/05/2025), four days after water-assisted colonoscopy and polypectomy. Following initial isotonic saline, hypertonic saline was administered in the intensive therapy unit (ITU), resulting in a rise from 106 to 120 mmol/L within 22 hours. An interim blood gas sample at 02:00 on 24/05/2025 showed sodium of 121 mmol/L, prompting 5% dextrose to prevent further correction. Subsequent serum sodium stabilised, with recovery supported by spontaneous aquaresis. The patient was discharged on 28/05/2025 with sodium at 130 mmol/L and maintained normal sodium on follow-up. Reference range: 135–145 mmol/L (green shaded area).

Figure 2. Suggested mechanisms contributing to acute hyponatraemia in this patient

Procedural stress and vomiting likely stimulated hypothalamic ADH release, providing a potent non-osmotic trigger. The low-solute PEG bowel preparation, supplemented with ascorbic acid, reduced renal osmotic load and promoted intraluminal fluid retention, limiting free water excretion. Water immersion during colonoscopy may have permitted transmucosal absorption of hypotonic irrigation fluid, analogous to TURP syndrome. Finally, advanced age and the presence of a caecal adenocarcinoma predisposed to impaired water clearance and possible paraneoplastic SIADH, compounding the risk. Source: "Suggested mechanisms of hyponatraemia in our patient" by Ramakrishnan S, 11 August 2025. Created in BioRender. https://BioRender.com/4vbbdf0 Abbreviations: ADH, anti-diuretic hormone; PEG, polyethylene glycol; SIADH, syndrome of inappropriate antidiuretic hormone secretion; TURP, transurethral resection of prostate