Correct Answer: B. Trihexyphenidyl
Antipsychotic-induced parkinsonism is an extrapyramidal side effect (EPS) caused by dopamine blockade in the basal ganglia, leading to relative cholinergic predominance. The gold standard treatment is an anticholinergic agent, which restores the balance between dopamine and acetylcholine in the basal ganglia. Trihexyphenidyl is a centrally acting anticholinergic drug that crosses the blood-brain barrier and effectively reverses antipsychotic-induced parkinsonism by blocking muscarinic receptors in the striatum. It is the most commonly prescribed anticholinergic in Indian clinical practice for this indication and is listed in the NLEM as a first-line agent for drug-induced parkinsonism. The typical dose is 1–2 mg thrice daily, titrated based on response. Anticholinergics are preferred over dopamine agonists in acute antipsychotic-induced EPS because they work faster and have a more predictable effect in this specific context. Other anticholinergics like benztropine are also effective but trihexyphenidyl is the most widely used in India due to cost and availability.
Why the other options are wrong
A. Pramipexole — Pramipexole is a dopamine agonist used primarily for idiopathic Parkinson's disease and restless leg syndrome. While dopamine agonists can theoretically help parkinsonism, they are NOT the first-line choice for antipsychotic-induced EPS because: (1) they may worsen the underlying psychotic condition by increasing dopamine activity, and (2) anticholinergics work faster and more reliably in this acute drug-induced setting. NBE trap: confusing dopamine agonists (for primary PD) with anticholinergics (for drug-induced parkinsonism). C. Rasagiline — Rasagiline is a selective MAO-B inhibitor used as monotherapy or adjunct in idiopathic Parkinson's disease to slow disease progression. It has NO role in acute antipsychotic-induced parkinsonism because: (1) it does not provide immediate symptomatic relief, (2) it is not indicated for drug-induced EPS, and (3) it carries risk of serotonin syndrome if combined with certain antipsychotics. NBE trap: grouping all Parkinson's drugs together without distinguishing between primary PD treatment and acute EPS management. D. Entacapone — Entacapone is a COMT inhibitor used as an adjunct to levodopa in idiopathic Parkinson's disease to extend dopamine availability. It is NOT indicated for antipsychotic-induced parkinsonism because: (1) it requires concurrent levodopa therapy (not used in drug-induced EPS), (2) it has no anticholinergic properties, and (3) it is used for chronic PD management, not acute EPS. NBE trap: listing multiple Parkinson's medications to confuse students about which class treats drug-induced versus primary parkinsonism.
High-Yield Facts
- Anticholinergics (trihexyphenidyl, benztropine) are first-line for antipsychotic-induced parkinsonism by restoring dopamine-acetylcholine balance in the basal ganglia.
- Dopamine agonists (pramipexole, bromocriptine) are avoided in antipsychotic-induced EPS because they may exacerbate psychosis.
- Trihexyphenidyl typical dose: 1–2 mg three times daily; onset of action within hours (faster than dopamine agonists).
- MAO-B inhibitors (rasagiline) and COMT inhibitors (entacapone) are used only in idiopathic Parkinson's disease, not drug-induced EPS.
- Anticholinergic side effects (dry mouth, urinary retention, constipation, tachycardia) must be monitored; contraindicated in glaucoma and urinary obstruction.
Mnemonics
ACNE for Antipsychotic-induced EPS management Anticholinergics (first-line) → Central dopamine agonists (second-line, risky) → Not MAO-B or COMT inhibitors → Extrapyramidal symptoms resolve. Use this to remember that anticholinergics come first, dopamine agonists are risky, and other PD drugs don't apply. Memory hook: 'ANTI-psychotic → ANTI-cholinergic' Antipsychotics block dopamine (cause parkinsonism) → Anticholinergics restore balance. This 'opposite' pairing helps recall that the antidote to dopamine blockade is cholinergic blockade, not dopamine replacement.
NBE Trap
NBE pairs all Parkinson's-related drugs (dopamine agonists, MAO-B inhibitors, COMT inhibitors) in the options to trap students who know 'Parkinson's drugs' but don't distinguish between idiopathic PD treatment (where dopamine agonists, rasagiline, entacapone apply) and acute antipsychotic-induced EPS (where only anticholinergics are first-line). The key discriminator is the clinical context: drug-induced EPS requires anticholinergics, not dopaminergic agents.</trap> <parameter name="textbookRef">KD Tripathi Pharmacology Ch. 9 (Antipsychotics and EPS management); Harrison Ch. 476 (Drug-induced movement disorders)</textbookRef> <parameter name="clinicalPearl">In Indian psychiatric practice, when a patient on chlorpromazine, haloperidol, or risperidone develops acute parkinsonism (rigidity, bradykinesia, tremor), trihexyphenidyl 1 mg BD–TDS is prescribed immediately and often continued for the duration of antipsychotic therapy. This is far more practical and cost-effective than switching antipsychotics or adding dopamine agonists, which risk psychotic relapse.</clinicalPearl> </invoke>