skip to main content

Cyclophosphamide Hyponatraemia — ESENeph MCQ

Instant feedback + full explanation. One question, done properly.

ModerateElectrolyte DisordersCyclophosphamide HyponatraemiaESENeph

A 55-year-old man develops serum sodium of 112 mmol/L over 48 hours after commencing high-dose Cyclophosphamide for ANCA vasculitis. He has received 3 litres of IV fluids as part of the hydration protocol. He is confused. What is the mechanism of hyponatraemia?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: C — Cyclophosphamide potentiates ADH action at the collecting duct, causing water retention (drug-induced SIADH); compounded by aggressive IV hydration

High-dose Cyclophosphamide causes hyponatraemia through enhanced ADH effect at the collecting duct (essentially drug-induced SIADH). When combined with the obligatory IV hydration given to prevent haemorrhagic cystitis, water retention is compounded. This is a well-recognised complication. Prevention includes using isotonic (not hypotonic) fluids for hydration, monitoring sodium closely during and after infusion, and restricting free water intake. Treatment includes stopping excess fluid, fluid restriction, and hypertonic saline if symptomatic.

Reference: KDIGO 2024 – AAV Guideline; De Fronzo et al 1973 – Cyclophosphamide SIADH