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Idiopathic nocturnal polyuria — MSRA MCQ

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HardLUTSIdiopathic nocturnal polyuriaMSRA

A 70-year-old man reports waking three times nightly to pass large volumes of urine. He has no urgency, urge incontinence, hesitancy, weak stream, intermittency or sensation of incomplete emptying. A 3-day frequency-volume chart shows a mean 24-hour urine output of 2.1 L, of which 1.2 L is passed overnight. Urine dipstick is negative for blood, leucocytes, nitrites and glucose, and post-void residual volume is 25 mL. HbA1c, adjusted calcium, liver function and thyroid function are normal. eGFR is 72 mL/min/1.73 m² and serum sodium is 139 mmol/L. He has no peripheral oedema, heart failure, liver disease, obstructive sleep apnoea symptoms, polydipsia or previous hyponatraemia. His blood pressure is 132/76 mmHg. His medicines are ramipril and atorvastatin; he takes no diuretic, SSRI, tricyclic antidepressant, carbamazepine or NSAID. Reducing evening fluids and caffeine has not helped. Following discussion, he elects treatment with desmopressin for idiopathic nocturnal polyuria. Which is the most appropriate prescribing and monitoring plan?

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Correct answer: C — Start desmopressin 50 micrograms sublingually 1 hour before bedtime, restrict fluid intake from 1 hour before dosing until 8 hours afterwards, and measure serum sodium before treatment, after 3 days, at 4–8 days and at 1 month.

This is idiopathic nocturnal polyuria: the frequency-volume chart shows disproportionately high nocturnal urine production with normal 24-hour output, large nocturnal voids, negligible residual volume and no clinically important voiding or overactive-bladder symptoms. Relevant systemic and medication-related causes have also been excluded. For a man, the licensed sex-specific dose of Noqdirna is desmopressin 50 micrograms sublingually 1 hour before bedtime. Fluid intake must be minimised from 1 hour before administration until 8 hours afterwards to reduce water retention and hyponatraemia. NICE advises checking sodium 3 days after initiation. Additionally, because he is aged 65 years or over, the SmPC requires normal sodium before treatment and monitoring in the first week (4–8 days) and again at 1 month. Option C incorporates both the NICE and product-specific safety requirements. A follows the SmPC elderly monitoring schedule but omits NICE's day-3 sodium check. B lacks the required fluid restriction and omits the further elderly monitoring points. C uses the female dose, which is inappropriate for a man. E provides inadequate fluid restriction and unsafe symptom-triggered rather than scheduled sodium monitoring.

Reference: NICE CG97: Lower urinary tract symptoms in men: management, recommendation 1.4.10 (Last reviewed December 2024) — https://www.nice.org.uk/guidance/CG97/chapter/recommendations Noqdirna 50 microgram oral lyophilisate Summary of Product Characteristics (Updated 15 June 2026) — https://www.medicines.org.uk/emc/product/4372/smpc