Anticholinergic Drugs

Anticholinergics (more precisely, antimuscarinics) block acetylcholine at muscarinic receptors, producing a predictable "dry-hot-red-blind-mad" toxidrome and a wide therapeutic footprint from bradycardia to bronchospasm to organophosphate poisoning. This is a perennial NEET PG hot-spot — atropine titration in OP poisoning, the anticholinergic toxidrome, and drug-specific niches (tropicamide, ipratropium, glycopyrrolate) recur almost every year.

Definition & classification

Acetylcholine acts on two receptor families: nicotinic (Nm at neuromuscular junction, Nn at ganglia) and muscarinic (M1–M5, all G-protein coupled). The drugs in this topic are muscarinic antagonists — they competitively and reversibly block ACh at M receptors. They do not block nicotinic receptors at therapeutic doses (that is the domain of ganglion blockers and neuromuscular blockers).

Muscarinic receptor subtypes and where they matter:

Receptor	Location	G-protein	Main effect of blockade
M1	CNS, gastric parietal/ECL cells, ganglia	Gq (↑IP3/DAG)	↓ gastric acid, CNS effects
M2	Heart (SA/AV node)	Gi (↓cAMP)	Tachycardia, ↑AV conduction
M3	Smooth muscle, glands, eye, bladder	Gq	Bronchodilatation, ↓secretions, mydriasis, urinary retention
M4/M5	CNS	Gi / Gq	Modulatory CNS roles

Classification by source/structure:

1. Natural alkaloids (belladonna) → Atropine (dl-hyoscyamine), Scopolamine/Hyoscine (tertiary amines, cross BBB).
2. Semisynthetic/synthetic tertiary amines → Homatropine, Cyclopentolate, Tropicamide (ophthalmic); Dicyclomine, Oxybutynin, Tolterodine (smooth muscle); Benztropine, Trihexyphenidyl (CNS/parkinsonism); Pirenzepine (M1-selective, gastric).
3. Quaternary ammonium compounds → Ipratropium, Tiotropium (inhaled), Glycopyrrolate, Hyoscine butylbromide, Propantheline. Being charged, they poorly cross the BBB and GI mucosa → minimal CNS effects, poor oral absorption.

High-yield: Tertiary amines (atropine, scopolamine, benztropine, trihexyphenidyl) cross the blood–brain barrier → CNS effects (sedation, delirium, antiparkinsonian action). Quaternary compounds (glycopyrrolate, ipratropium, hyoscine butylbromide) do not → no central effects. This single fact answers many MCQs.

Pharmacological effects (system-wise)

Effects are dose-dependent, and atropine classically affects organs in a recognisable sequence as the dose rises:

Low dose → ↓ salivary/sweat/bronchial secretions → mydriasis + cycloplegia + tachycardia → ↓ GI/urinary tone → at toxic doses CNS excitation.

• CVS: Low doses may cause transient bradycardia (central vagal/presynaptic M1 effect — paradoxical); higher doses block M2 → tachycardia. Atropine is the antidote for vagally-mediated bradyarrhythmias.
• Eye: M3 blockade on sphincter pupillae → mydriasis; on ciliary muscle → cycloplegia (loss of accommodation). Can precipitate angle-closure glaucoma and causes photophobia + blurred near vision.
• Respiratory: Bronchodilatation + ↓ secretions (basis of ipratropium/tiotropium in COPD; pre-anaesthetic antisialagogue use).
• GIT: ↓ motility & secretions → antispasmodic, mild antiacid (pirenzepine).
• GU: Detrusor relaxation → urinary retention (oxybutynin/tolterodine for overactive bladder; but retention in BPH).
• Glands: ↓ sweat → hyperthermia ("atropine fever," esp. in children — dangerous); dry mouth, dry skin.
• CNS: Scopolamine → sedation, amnesia, antiemetic (motion sickness). Atropine in toxic doses → restlessness, hallucinations, delirium. Benztropine/trihexyphenidyl → reduce tremor/rigidity in parkinsonism.

High-yield: Scopolamine (hyoscine) is more potent than atropine on the eye, secretions and CNS (sedation/amnesia) but less potent on heart, bronchi and GIT. Scopolamine = sedation; atropine = excitation at toxic doses.

Drug-specific high-yield profiles

Drug	Type	Signature use / MCQ point
Atropine	Tertiary alkaloid	OP/carbamate & mushroom (muscarinic) poisoning; symptomatic bradycardia; antisialagogue; mydriatic (long, ~7–10 days)
Scopolamine (Hyoscine)	Tertiary alkaloid	Motion sickness (transdermal patch behind ear), sedation, amnesia; "twilight sleep" historically
Glycopyrrolate	Quaternary	Preferred pre-anaesthetic antisialagogue (no CNS effect, less tachycardia); given with neostigmine to block muscarinic effects during NM block reversal
Ipratropium / Tiotropium	Quaternary inhaled	COPD (DOC bronchodilator class); ipratropium short-acting, tiotropium long-acting (once daily)
Tropicamide	Tertiary	Short-acting mydriatic/cycloplegic for fundus exam (onset fast, recovery ~6 h) — preferred for routine dilation
Cyclopentolate	Tertiary	Cycloplegic refraction in children (faster/shorter than atropine)
Atropine (ophthalmic)	Tertiary	Cycloplegic refraction in young children, anterior uveitis (prevents synechiae); long action
Pirenzepine	M1-selective	Peptic ulcer (↓ acid with fewer systemic effects)
Oxybutynin / Tolterodine / Solifenacin	—	Overactive bladder / urge incontinence
Trihexyphenidyl / Benztropine	Tertiary	Drug-induced parkinsonism & extrapyramidal symptoms; tremor-predominant Parkinson's
Dicyclomine / Hyoscine butylbromide	—	GI/biliary/renal colic antispasmodic

High-yield: Tropicamide is the mydriatic of choice for a quick fundus examination (short duration). Atropine is preferred for cycloplegic refraction in young children and for uveitis (prolonged dilatation prevents posterior synechiae).

High-yield: Glycopyrrolate is preferred over atropine as a pre-anaesthetic drying agent and during neostigmine-mediated reversal of neuromuscular blockade — it is quaternary (no CNS delirium) and causes less tachycardia.

Anticholinergic toxidrome (atropine poisoning)

This toxidrome is one of the most frequently tested clinical pictures. Classic teaching mnemonic:

"Blind as a bat, Dry as a bone, Red as a beet, Hot as a hare, Mad as a hatter, Full as a flask, Fast as a fiddle."

• Blind as a bat → cycloplegia + mydriasis → blurred vision, photophobia
• Dry as a bone → dry mouth, dry skin (↓ secretions)
• Red as a beet → cutaneous vasodilatation (flushing)
• Hot as a hare → hyperthermia (↓ sweating) — life-threatening, esp. children
• Mad as a hatter → delirium, agitation, hallucinations, seizures
• Full as a flask → urinary retention
• Fast as a fiddle → sinus tachycardia

High-yield: Compared with the sympathomimetic toxidrome, the anticholinergic patient has dry skin (sympathomimetics cause diaphoresis/sweating). Dry axillae + mydriasis + delirium + retention = anticholinergic.

Management of anticholinergic poisoning:

1. Supportive — ABC, cooling for hyperthermia, IV fluids, benzodiazepines for agitation/seizures, urinary catheterisation, cardiac monitoring.
2. Physostigmine — the specific antidote. A tertiary carbamate cholinesterase inhibitor that crosses the BBB, reversing both central and peripheral signs. Used for severe central anticholinergic syndrome.
   • Contraindicated with TCA overdose (risk of asystole/seizures) and in wide-QRS/conduction defects.
   • Neostigmine is quaternary → does not cross BBB → useless for central toxicity.

High-yield: Physostigmine (not neostigmine) is the antidote for central anticholinergic syndrome because it is a tertiary amine that crosses the BBB.

Organophosphate (OP) poisoning — the marquee topic

OP compounds (insecticides — parathion, malathion; nerve agents — sarin) irreversibly inhibit acetylcholinesterase → ACh accumulates at muscarinic, nicotinic and central synapses → a cholinergic crisis (opposite of anticholinergic toxidrome).

Features mnemonics:

• Muscarinic — "DUMBBELSS": Diarrhoea, Urination, Miosis, Bradycardia/Bronchorrhoea/Bronchospasm, Emesis, Lacrimation, Salivation, Sweating.
• Nicotinic — "MTWtHF" (days of week): Mydriasis (can occur), Tachycardia, Weakness, twitching/fasciculations, Hypertension, Fasciculations — i.e. muscle fasciculations, weakness, tachycardia, hypertension.

High-yield: Bronchorrhoea and bronchospasm, not arrhythmia, are the usual cause of death in OP poisoning. The therapeutic endpoint of atropine is drying of secretions / clear chest, NOT pupil size or heart rate.

Stepwise management:

1. Decontamination & ABC → remove clothes, wash skin; secure airway; high-flow oxygen before atropine (atropine in a hypoxic patient → ventricular fibrillation).
2. Atropine → competitively reverses muscarinic effects. Give 2–5 mg IV (0.05 mg/kg in children), double the dose every 5 minutes until atropinisation (clear chest on auscultation, HR > 80, SBP > 80, dry axillae, pupils no longer pinpoint). Then maintenance infusion.
   • Atropine does NOT reverse nicotinic effects (muscle weakness, fasciculations, respiratory muscle paralysis).
3. Pralidoxime (2-PAM) / Oxime → reactivates acetylcholinesterase by removing the phosphate group — works on both muscarinic and nicotinic signs (especially the nicotinic weakness atropine cannot touch). Must be given early, before "ageing".
4. Benzodiazepines for seizures.

High-yield: "Ageing" is the irreversible covalent loss of an alkyl group from the OP–enzyme complex, after which pralidoxime can no longer reactivate the enzyme. Oximes must be given before ageing occurs — hence give 2-PAM early.

Atropine vs Pralidoxime — the must-know contrast:

Feature	Atropine	Pralidoxime (2-PAM)
Mechanism	Blocks muscarinic receptors	Reactivates cholinesterase
Muscarinic signs	Reverses	Reverses
Nicotinic signs (weakness, fasciculations)	No effect	Reverses
CNS	Some central effect	Poor CNS penetration
Endpoint	Atropinisation (dry chest)	Restored muscle power
Timing critical?	Titrate to effect	Yes — before ageing

High-yield: Atropine treats the muscarinic crisis; only the oxime restores nicotinic/respiratory muscle function. Both are given together — they are complementary, not alternatives. Carbamate poisoning needs atropine but oximes are usually not required (spontaneous reactivation; classically avoided/controversial in pure carbamate toxicity such as carbaryl).

Diagnosis & investigation of choice

• OP poisoning → clinical (cholinergic toxidrome + garlicky odour, miosis). Confirm with red cell (true) acetylcholinesterase — best correlates with synaptic enzyme and severity; plasma (pseudo)cholinesterase / butyrylcholinesterase falls earlier and is a sensitive screening marker but less specific.
• Anticholinergic poisoning → clinical diagnosis (toxidrome). ECG to exclude TCA (wide QRS) before considering physostigmine.

High-yield: RBC acetylcholinesterase = true cholinesterase = best severity marker in OP poisoning; plasma pseudocholinesterase = most sensitive early screening test (also low in liver disease, malnutrition, and succinylcholine apnoea).

Therapeutic indications — quick map

• Bradycardia/AV block: Atropine 0.5–1 mg IV (DOC for symptomatic sinus bradycardia).
• Pre-anaesthetic antisialagogue: Glycopyrrolate > atropine > scopolamine.
• Reversal of NM blockade: Glycopyrrolate/atropine + neostigmine.
• COPD/asthma: Ipratropium (acute, with salbutamol), tiotropium (maintenance COPD).
• Mydriasis/cycloplegia: Tropicamide (exam), atropine (children/uveitis).
• Motion sickness/PONV: Transdermal scopolamine.
• Overactive bladder: Oxybutynin, tolterodine, solifenacin.
• Parkinsonism/EPS: Trihexyphenidyl, benztropine.
• GI colic / IBS: Dicyclomine, hyoscine butylbromide.
• Mushroom (muscarinic, Inocybe/Clitocybe) poisoning: Atropine.

Adverse effects, contraindications & key differentials

Adverse effects: dry mouth, blurred vision, constipation, urinary hesitancy/retention, tachycardia, hyperthermia, confusion (esp. elderly — anticholinergic burden contributes to delirium and dementia risk).

Contraindications:

• Narrow/closed-angle glaucoma (mydriasis raises IOP — classic absolute contraindication).
• Benign prostatic hyperplasia / bladder outlet obstruction (precipitates retention).
• Tachyarrhythmias, thyrotoxicosis, paralytic ileus, reflux oesophagitis.
• Caution in elderly and febrile children.

High-yield: A patient on an antimuscarinic who develops a painful red eye with a fixed mid-dilated pupil = acute angle-closure glaucoma until proven otherwise.

Key differentials of the dilated pupil / altered mental status:

• Sympathomimetic toxidrome — sweaty skin (vs dry), hypertension; cocaine/amphetamine.
• Serotonin syndrome — clonus, hyperreflexia, recent serotonergic drug.
• Neuroleptic malignant syndrome — lead-pipe rigidity, normal/sluggish reflexes, slow onset.
• OP/cholinergic crisis — miosis, wet (SLUDGE), bradycardia — the mirror image.
• TCA overdose — anticholinergic features plus wide QRS, seizures, hypotension — here avoid physostigmine; give sodium bicarbonate.

Recently asked / exam angle

• "Endpoint of atropine therapy in OP poisoning?" → Clearing of bronchial secretions / dry chest (NOT pupillary dilatation, NOT heart rate alone).
• "Drug acting on nicotinic component of OP poisoning?" → Pralidoxime.
• "Most sensitive early lab marker of OP poisoning?" → Plasma (pseudo)cholinesterase; severity → RBC AChE.
• "Antidote for atropine/datura (belladonna) poisoning?" → Physostigmine.
• "Why glycopyrrolate over atropine as antisialagogue?" → Quaternary, no CNS effect, less tachycardia.
• "Shortest-acting mydriatic for fundoscopy?" → Tropicamide.
• "Cycloplegic of choice in children for refraction / in uveitis?" → Atropine.
• "Antimuscarinic contraindicated in glaucoma — mechanism?" → mydriasis → ↓ aqueous outflow → ↑ IOP.
• "Cause of death in OP poisoning?" → Respiratory failure from bronchorrhoea/bronchospasm + paralysis.
• "Why give oxygen before atropine in OP poisoning?" → atropine in hypoxia → VF.
• Image-based: garlic-smelling, miotic, frothing patient (OP) vs flushed, dry, dilated, delirious patient (anticholinergic).

Rapid revision

1. Antimuscarinics block M1–M5; no nicotinic block at therapeutic doses.
2. Tertiary amines cross BBB (atropine, scopolamine, benztropine); quaternary do not (glycopyrrolate, ipratropium, hyoscine butylbromide).
3. Scopolamine > atropine on eye/secretions/CNS (sedation); atropine > scopolamine on heart/bronchi/gut.
4. Anticholinergic toxidrome: dry, hot, red, blind, mad, fast, full — skin is dry (distinguishes from sympathomimetics).
5. Antidote for central anticholinergic syndrome = physostigmine (tertiary, crosses BBB); contraindicated in TCA overdose.
6. OP poisoning = irreversible AChE inhibition → DUMBBELSS muscarinic + nicotinic weakness; death from bronchorrhoea/respiratory failure.
7. Atropine endpoint = clear chest/dry axillae, titrate by doubling dose; give O₂ first.
8. Pralidoxime reactivates AChE and is the only agent reversing nicotinic weakness; give before ageing.
9. RBC AChE = best severity marker; plasma pseudocholinesterase = earliest/most sensitive screen.
10. Tropicamide = short mydriatic for fundoscopy; atropine = cycloplegic for children/uveitis; cyclopentolate = paediatric refraction.
11. Glycopyrrolate = pre-op antisialagogue & neostigmine partner; ipratropium/tiotropium = COPD.
12. Absolute contraindications: angle-closure glaucoma and BPH/bladder outlet obstruction.