A 40-year-old man with Parkinson's disease on levodopa/carbidopa for 5 years develops sudden involuntary writhing movements of the face and limbs 45 minutes after each dose, lasting 30–60 minutes. Which is the most likely phenomenon and its mechanism?
- A Wearing-off; declining levodopa plasma levels
- B Diphasic dyskinesia; occurring at rising and falling levodopa plasma levels
- C Peak-dose dyskinesia; excessive dopaminergic stimulation at peak plasma levodopa levels ✓
- D Off-period dystonia; prolonged dopamine receptor block
Correct answer: C. Peak-dose dyskinesia; excessive dopaminergic stimulation at peak plasma levodopa levels
Explanation
Peak-dose dyskinesia (choreiform/ballistic movements) occurs 45–60 minutes after levodopa administration, coinciding with peak plasma and CNS drug levels. The mechanism involves pulsatile stimulation of sensitised striatal dopamine receptors due to short-acting levodopa. Wearing-off is reduced duration of benefit before the next dose. Diphasic dyskinesia occurs at both rising and falling phases of levodopa. Off-period dystonia is painful cramping during 'off' states.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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