Which mechanism best explains the acute dystonia and akathisia caused by first-generation antipsychotics?

Explanation

## Extrapyramidal Side Effects (EPS) — Mechanism **Key Point:** First-generation antipsychotics block dopamine D2 receptors in the nigrostriatal pathway, disrupting the dopamine–acetylcholine balance and causing extrapyramidal symptoms. ### The Dopamine–Acetylcholine Imbalance **Normal State:** - Dopamine and acetylcholine maintain equilibrium in the basal ganglia - This balance is essential for smooth motor control **With FGA Antipsychotics:** 1. D2 dopamine blockade in nigrostriatal neurons → ↓ dopamine activity 2. Acetylcholine activity becomes relatively unopposed (relative cholinergic excess) 3. Imbalance manifests as rigidity, bradykinesia, tremor, dystonia, and akathisia ```mermaid flowchart TD A[FGA Antipsychotic]:::action --> B[D2 Dopamine Blockade<br/>Nigrostriatal Pathway]:::action B --> C[Dopamine Activity ↓]:::outcome C --> D[Relative Acetylcholine Excess]:::outcome D --> E{EPS Manifestation}:::decision E -->|Acute| F[Dystonia, Akathisia]:::urgent E -->|Subacute| G[Parkinsonism]:::urgent E -->|Chronic| H[Tardive Dyskinesia]:::urgent ``` ### Why Anticholinergics Work **High-Yield:** Anticholinergic drugs (benztropine, trihexyphenidyl) **reduce** acetylcholine activity, restoring the dopamine–acetylcholine balance and relieving acute EPS within minutes to hours [cite:Stahl's Psychopharmacology 6e]. **Clinical Pearl:** Acute dystonia (muscle spasms, oculogyric crisis, torticollis) responds dramatically to IV/IM benztropine — this rapid response confirms the cholinergic excess hypothesis. **Mnemonic:** **DOPA-CHOL** — Dopamine ↓ = Cholinergic excess = EPS. When dopamine is blocked, acetylcholine "wins" the balance. ### Contrast with Atypical Antipsychotics Atypical antipsychotics (e.g., risperidone, olanzapine) have **lower affinity** for D2 receptors in the nigrostriatal pathway and **faster dissociation** from the receptor, resulting in significantly lower EPS risk compared to FGAs.