A patient presented with a history of multiple episodes of vomiting. He was given an intravenous drug, and later, he developed abnormal movements. Which of the following can be used to treat him?

Correct Answer: B. Benzhexol

This question describes metoclopramide-induced acute dystonia, a common iatrogenic movement disorder in Indian clinical practice. Metoclopramide, a dopamine antagonist widely used as an antiemetic in India, blocks D2 receptors in the basal ganglia, disrupting the dopamine-acetylcholine balance. This excess cholinergic activity causes acute dystonic reactions—involuntary muscle contractions, oculogyric crisis, torticollis, and opisthotonus. Benzhexol (trihexyphenidyl) is an anticholinergic agent that restores the dopamine-acetylcholine equilibrium by blocking muscarinic receptors. It rapidly reverses acute dystonia within minutes to hours, making it the first-line treatment in Indian emergency departments. The mechanism is straightforward: anticholinergic activity counteracts the relative cholinergic excess caused by dopamine blockade. Benzhexol is preferred over other anticholinergics because of its rapid onset, good CNS penetration, and safety profile in acute settings. This is a high-yield concept in Indian NEET PG curricula, as metoclopramide-induced dystonia is frequently encountered in hospital practice.

Why the other options are wrong

A. Levodopa — Levodopa is used for Parkinson's disease (dopamine deficiency), not for drug-induced dystonia. While it increases dopamine, it has slow onset (hours to days) and is not suitable for acute dystonic reactions. NBE may trap students who think 'dopamine antagonist blockade → give dopamine replacement,' but acute dystonia requires immediate anticholinergic reversal, not dopamine augmentation. C. Hyoscine — Hyoscine (scopolamine) is an anticholinergic, but it is primarily used for motion sickness and has poor CNS penetration due to high lipophilicity and blood-brain barrier sequestration. It does not effectively reverse central dystonia. Benzhexol is superior because it crosses the BBB efficiently and has rapid, reliable CNS action in acute dystonia. D. Cyproheptadine — Cyproheptadine is a serotonin antagonist used for serotonin syndrome and allergic reactions, not for dopamine antagonist-induced dystonia. It has no anticholinergic properties and does not address the dopamine-acetylcholine imbalance. This is a distractor that confuses antihistamine/antiserotonergic action with anticholinergic action.

High-Yield Facts

Mnemonics

DOPA-ACH Balance Rule Dopamine ↓ (blocked) → Acetylcholine ↑ (relative excess) → Give ANTI-cholinergic (benzhexol). When dopamine is down, chop acetylcholine. Metoclopramide Dystonia = Anticholinergic Fix Metro (metoclopramide) blocks dopamine → dystonia → Benzhexol (anticholinergic) fixes it. Remember: dopamine blocker → anticholinergic rescue.

NBE Trap

NBE pairs dopamine antagonist-induced dystonia with dopamine replacement (levodopa) to trap students who think 'dopamine blockade → give dopamine.' The correct approach is anticholinergic reversal, not dopamine augmentation, because the problem is relative cholinergic excess, not absolute dopamine deficiency.

Clinical Pearl

In Indian emergency departments, metoclopramide-induced acute dystonia is a common presentation in young patients given the drug for gastroenteritis or postoperative nausea. A single dose of benzhexol (1–2 mg IV/IM) or diphenhydramine (25–50 mg) produces dramatic relief within 5–10 minutes, making anticholinergics the bedside gold standard for this iatrogenic emergency.

_Reference: KD Tripathi Pharmacology Ch. 8 (ANS Pharmacology); Harrison Ch. 476 (Movement Disorders)_