A 58-year-old woman with Parkinson disease of 8 years' duration now experiences motor fluctuations (on-off phenomena) and peak-dose dyskinesias despite optimized levodopa dosing. What is the drug of choice to add to her current regimen to reduce dyskinesias and improve motor stability?

Explanation

## Management of Motor Complications in Advanced Parkinson Disease ### Clinical Context After 8 years of levodopa therapy, this patient has developed motor complications—on-off fluctuations and peak-dose dyskinesias—hallmarks of long-duration levodopa exposure. The goal is to reduce dyskinesias while maintaining motor control. **Key Point:** Amantadine is the only symptomatic agent with proven efficacy in reducing dyskinesias in advanced PD. It acts as an NMDA receptor antagonist and is particularly effective for peak-dose dyskinesias. ### Mechanism of Amantadine in Dyskinesias ```mermaid flowchart TD A[Chronic levodopa exposure]:::outcome --> B[Sensitization of striatal neurons]:::outcome B --> C[Excessive dopaminergic signaling]:::outcome C --> D[Peak-dose dyskinesias]:::urgent E[Amantadine: NMDA antagonist]:::action --> F[Blocks excessive glutamatergic input]:::action F --> G[Reduces dyskinesia severity]:::outcome G --> H[Improves motor stability]:::outcome ``` ### Comparison of Adjunctive Agents for Motor Complications | Drug | Mechanism | Effect on Dyskinesias | Effect on Motor Fluctuations | Use | |------|-----------|----------------------|------------------------------|-----| | **Amantadine** | NMDA antagonist | ↓↓ Reduces significantly | Minimal | **Dyskinesias (first-line)** | | **Entacapone** | COMT inhibitor | No direct effect | ↑ Prolongs levodopa effect | Motor fluctuations | | **Rasagiline** | MAO-B inhibitor | No effect | Minimal | Adjunctive, neuroprotection | | **Bromocriptine** | D2 agonist | May worsen | ↑ Improves | Advanced PD (not dyskinesias) | ### Why Amantadine Is Correct - **Proven efficacy:** Reduces dyskinesia severity by 30–50% in clinical trials - **Mechanism:** NMDA receptor blockade reduces excitotoxicity in sensitized striatal neurons - **Timing:** Most effective when added early in the course of motor complications - **Dosing:** 100 mg BD to TDS; requires renal dose adjustment **High-Yield:** Amantadine is the **only** agent with Level A evidence for reducing dyskinesias in PD. It should be offered to all patients with troublesome dyskinesias. **Clinical Pearl:** Amantadine may also improve bradykinesia and rigidity as a bonus, making it a multipurpose agent in advanced PD. ### Rationale for Rejecting Other Options - **Bromocriptine:** A dopamine agonist; may worsen dyskinesias by increasing dopaminergic tone - **Entacapone:** A COMT inhibitor that prolongs levodopa duration; useful for motor fluctuations, not dyskinesias - **Rasagiline:** A MAO-B inhibitor; provides mild symptomatic benefit but has no proven effect on dyskinesias