Psychogenic Polydipsia — MRCPsych Paper B MCQ

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ModerateSchizophrenia & PsychosisPsychogenic PolydipsiaMRCPsych Paper B

A 35-year-old man with schizophrenia is brought to the emergency department with confusion. Ward staff report that he has been drinking 8–10 litres of water daily. He is clinically euvolaemic. His serum sodium is 122 mmol/L, serum osmolality is 258 mOsm/kg, blood glucose is normal, and urine osmolality is 70 mOsm/kg. Which condition most likely explains these findings?

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Correct answer: C — Psychogenic polydipsia

The correct answer is C, psychogenic polydipsia. Documented excessive water intake, hypotonic hyponatraemia and maximally dilute urine indicate appropriate suppression of vasopressin in response to free-water excess. SIADH causes inappropriately concentrated urine, usually with urine osmolality above 100 mOsm/kg. Diabetes insipidus also produces dilute urine, but water loss typically results in normal or raised serum sodium and osmolality unless intake fully compensates. Diabetes mellitus causes hyperglycaemia and an osmotic diuresis with increased urinary solute. Addison's disease more often causes volume depletion, hyperkalaemia and non-suppressed vasopressin. Confusion with a sodium of 122 mmol/L requires urgent medical assessment and carefully monitored correction; subsequent prevention includes supervised fluid restriction and treatment of behavioural or psychotic drivers of drinking.

Reference: Spasovski G, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia, diagnostic algorithm section. European Journal of Endocrinology. 2014;170:G1–G47. https://www.rcpsych.ac.uk/docs/default-source/improving-care/better-mh-policy/college-reports/college-report-cr222.pdf