Lithium Toxicity Severity — MRCPsych Paper B MCQ

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ModerateMood DisordersLithium Toxicity SeverityMRCPsych Paper B

A patient receiving maintenance lithium develops nausea and a fine resting tremor after an intercurrent gastrointestinal illness. A correctly timed 12-hour trough serum lithium concentration is 1.7 mmol/L. They remain alert, with normal gait and coordination, and have no confusion, myoclonus, seizures or cardiac abnormalities. How should the severity of lithium toxicity be classified?

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Correct answer: E — Mild lithium toxicity

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

This is **mild lithium toxicity**. The lithium concentration is above the level at which toxicity is usually expected, and nausea with a fine resting tremor is characteristic of mild toxicity. Moderate toxicity would be suggested by increasing confusion, hyperreflexia, fasciculation, myoclonus or progression towards stupor. Severe toxicity involves features such as coma, convulsions, cerebellar signs, cardiovascular instability or dysrhythmias. The absence of these neurological and cardiac findings excludes options C and D. The correctly timed level and compatible symptoms make therapeutic exposure or an unclassifiable result untenable. Lithium should be withheld immediately, with urgent clinical assessment, serial lithium measurements and attention to renal function, fluid and electrolyte balance. Haemodialysis is not automatically required for mild or moderate toxicity; it is principally considered for severe poisoning or marked neurological features.

Reference: Electronic Medicines Compendium, Lithium Carbonate Essential Pharma 250 mg film-coated tablets, Summary of Product Characteristics, sections 4.4 and 4.9, updated 2025. https://www.medicines.org.uk/emc/product/10828/smpc