A 58-year-old woman with Parkinson disease of 8 years' duration, currently on pramipexole 1.5 mg TID and levodopa 250 mg QID, develops motor fluctuations ('on-off' phenomena) and peak-dose dyskinesias. Which agent should be added to reduce dyskinesias and smooth motor response?

Explanation

## Management of Levodopa-Induced Dyskinesias **Key Point:** Amantadine is the only non-dopaminergic agent with proven efficacy in reducing established levodopa-induced dyskinesias (LID). It works via NMDA receptor antagonism and is the drug of choice for symptomatic management of peak-dose or biphasic dyskinesias. **High-Yield:** Amantadine reduces dyskinesia severity by 20–50% and can improve motor fluctuations. It is particularly effective in patients with moderate to severe dyskinesias and should be added before considering surgery (DBS) in most cases. ## Mechanism and Efficacy of Amantadine in LID ```mermaid flowchart TD A[Chronic levodopa exposure]:::outcome --> B[Sensitization of striatal neurons]:::outcome B --> C[Abnormal glutamate signaling<br/>via NMDA receptors]:::outcome C --> D[Dyskinesia + motor fluctuations]:::urgent E[Amantadine]:::action --> F[NMDA receptor antagonism]:::action F --> G[Reduces abnormal motor output]:::action G --> H[Dyskinesia reduction<br/>Smoother motor response]:::outcome ``` ## Comparison of Agents for LID Management | Agent | Mechanism | Efficacy for LID | Role | Onset | |-------|-----------|-----------------|------|-------| | **Amantadine** | NMDA antagonist | ★★★★ (20–50% reduction) | First-line adjunct | 2–4 weeks | | Selegiline | MAO-B inhibition | ★ (minimal) | Adjunct for mild symptoms | Weeks | | Entacapone | COMT inhibition | ★★ (reduces off time, not dyskinesia) | Extends levodopa duration | Hours | | Ropinirole | D2/D3 agonist | ★★ (indirect via dose reduction) | Adjunct; increases dyskinesia risk if added | Days | **Clinical Pearl:** Amantadine also improves rigidity and bradykinesia independent of dyskinesia reduction, making it a dual-benefit agent. Typical dosing is 100–200 mg BID, with careful monitoring for anticholinergic side effects (confusion, hallucinations, urinary retention). **Mnemonic:** **AMANTADINE = Anti-dyskinesia, NMDA antagonist, Antiviral (historical), Dopamine-independent** — the only non-dopaminergic agent that reliably reduces LID. ## Why Not Other Options? - **Selegiline:** Provides only marginal symptomatic benefit; no proven effect on dyskinesias. Better suited for early PD. - **Ropinirole:** A dopamine agonist; adding more dopaminergic stimulation typically worsens dyskinesias, not improves them. - **Entacapone:** A COMT inhibitor that extends levodopa half-life and reduces off-time, but does not reduce dyskinesias and may worsen them by increasing peak levodopa levels. **Tip:** In this patient with 8 years of disease, motor fluctuations, and dyskinesias, the next step after amantadine would be to optimize levodopa dosing (lower dose, more frequent) or consider DBS if dyskinesias remain severe despite amantadine.