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Parkinson disease psychosis — MRCPsych Paper A MCQ

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HardPsychopharmacologyParkinson disease psychosisMRCPsych Paper A

A 69-year-old man with Parkinson's disease and no cognitive impairment has his pramipexole dose increased for wearing-off symptoms. One week later, his bradykinesia has improved, but he develops formed visual hallucinations in a clear sensorium. Infection, metabolic disturbance and other medication changes have been excluded. Which receptor-level action of the increased drug most directly links the motor improvement with the new adverse effect?

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Correct answer: A — Agonism at dopamine D2-family receptors, especially D3

Pramipexole is a dopamine agonist acting at the D2 receptor subfamily, with preferential affinity for D3 receptors. Striatal dopamine-receptor stimulation improves parkinsonian motor symptoms, while increased central dopaminergic stimulation can precipitate hallucinations; the close temporal relationship to dose escalation and clear sensorium support a medication-related effect. Parkinson disease psychosis is nevertheless multifactorial and should not be reduced solely to mesolimbic dopamine excess. NMDA antagonism is associated with amantadine, not pramipexole. Serotonin 5-HT2A inverse agonism is an antipsychotic mechanism rather than the action causing this presentation. Alpha-2 agonism and GABA-A modulation do not explain pramipexole's combined motor and psychotic effects. NICE advises excluding medical triggers and reviewing potentially causative Parkinson's medicines.

Reference: electronic Medicines Compendium. MIRAPEXIN prolonged-release tablets, Summary of Product Characteristics, sections 4.4, 4.8 and 5.1. Updated 18 July 2024. https://www.medicines.org.uk/emc/product/4455/smpc