Antipsychotic-induced parkinsonism — MRCPsych Paper A MCQ

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EasyPsychopharmacologyAntipsychotic-induced parkinsonismMRCPsych Paper A

A 24-year-old man with first-episode psychosis starts haloperidol. Within one week, he develops tremor, rigidity and a shuffling gait. Which pharmacological action most directly causes this adverse effect?

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Correct answer: D — Blockade of dopamine D2 receptors in the nigrostriatal pathway

The correct answer is D. Tremor, rigidity and a shuffling gait arising shortly after haloperidol initiation indicate antipsychotic-induced parkinsonism. Haloperidol is a potent dopamine D2 receptor antagonist, and D2 blockade in the nigrostriatal pathway disrupts basal ganglia motor control, producing extrapyramidal symptoms. Alpha-1 blockade is principally associated with hypotension, while H1 blockade causes sedation and contributes to weight gain. Muscarinic M1 blockade causes anticholinergic effects and may impair cognition; antimuscarinic drugs can actually reduce drug-induced parkinsonism. Cortical 5-HT2A antagonism is characteristic of many second-generation antipsychotics and is not the principal mechanism of haloperidol-induced parkinsonism.

Reference: HALDOL 2 mg/ml oral solution, Summary of Product Characteristics, sections 4.4 and 5.1, revised 22 September 2023. https://www.medicines.org.uk/emc/product/15252/smpc