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Nocturnal polyuria in a man with benign prostatic enlargement — MSRA MCQ

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ModerateBPHNocturnal polyuria in a man with benign prostatic enlargementMSRA

A 71-year-old man with benign prostatic enlargement takes tamsulosin MR 400 micrograms daily. His voiding symptoms are controlled, but he remains troubled by nocturia due to idiopathic nocturnal polyuria confirmed on a frequency-volume chart. Heart failure, diabetes mellitus, obstructive sleep apnoea, dependent oedema, renal impairment and diuretic use have been excluded. Evening fluid modification and caffeine reduction have not helped. He started desmopressin oral lyophilisate 50 micrograms sublingually at bedtime 6 days ago. His baseline serum sodium was 139 mmol/L and eGFR was 71 mL/min/1.73 m². He has followed fluid-restriction advice and reports improved nocturia. Routine serum sodium today is 133 mmol/L. He is well, with no headache, nausea, confusion, weight gain or seizures. What is the most appropriate management now?

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

Reveal the answer and explanation

Correct answer: B — Stop desmopressin and maintain fluid restriction while serum sodium is reassessed

Explanation lettering: D = shown as A · E = shown as B · A = shown as C · C = shown as D · B = shown as E

Desmopressin should be stopped because his serum sodium has fallen below the normal range (133 mmol/L), even though he is asymptomatic and nocturia has improved. NICE recommends checking sodium shortly after starting desmopressin for nocturnal polyuria and stopping treatment if it falls below normal. The UK SmPC for desmopressin oral lyophilisate similarly specifies discontinuation below 135 mmol/L. At age 71, he is at increased risk of clinically significant hyponatraemia. A is inappropriate because the sodium result already requires action; waiting until the one-month check risks further water retention and hyponatraemia. B is plausible because lower doses are used in some contexts, but a subnormal sodium concentration requires cessation rather than dose reduction; 25 micrograms is also the licensed sex-specific dose for women, not a routine reduced dose for men. C is unsafe because restarting after transient omission does not address the demonstrated susceptibility to hyponatraemia without reassessment. D is insufficient: fluid restriction is important for prevention but does not override the requirement to stop treatment once sodium is below normal. E appropriately removes the causative drug and supports correction of the dilutional hyponatraemia.

Reference: NICE CG97: Lower urinary tract symptoms in men: management — Recommendations 1.4.10 (2010; accessed 15 August 2026) — https://www.nice.org.uk/guidance/cg97/chapter/recommendations Noqdirna 50 micrograms oral lyophilisate — Summary of Product Characteristics (Updated 2026) — https://www.medicines.org.uk/emc/product/4372/smpc