Uses of Cholinergic Drugs

Classification

1. Direct-Acting Cholinergic Agonists

Acetylcholine (ACh)

• Has almost no systemic therapeutic use due to rapid hydrolysis by cholinesterase and diffuse, non-selective action.
• Topical ACh 1% eye drops: used during cataract extraction to produce rapid miosis (pupil constriction) intraoperatively. Systemic effects are negligible because it is hydrolyzed quickly.

Methacholine

• Primarily a muscarinic agonist; more resistant to hydrolysis than ACh.
• Bronchial provocation test (inhaled): diagnoses bronchial hyperreactivity in asthma by inducing bronchoconstriction.
• Supraventricular tachyarrhythmias (especially paroxysmal tachycardia): slows the heart when other measures fail.
• Also increases intestinal tone.
• Contraindicated in asthmatic patients and those with hypertension (may cause severe hypotension). Overdose treated with atropine.

Bethanechol

• Resistant to hydrolysis; selective for GI and urinary tracts; minimal cardiac effects.
• Urinary retention / neurogenic bladder atony: stimulates detrusor muscle contraction.
• Postoperative / postpartum GI atony: restores bowel motility.
• Gastroesophageal reflux disease (GERD): increases lower esophageal sphincter tone and gastric emptying.

Carbamylcholine (Carbachol)

• Has both muscarinic AND nicotinic activity.
• Glaucoma: topical for open-angle (wide-angle) glaucoma - reduces intraocular pressure by causing miosis and facilitating aqueous outflow.
• Intraocular miosis: used during ophthalmic surgery.

Pilocarpine (Alkaloid)

• A natural alkaloid; acts at muscarinic receptors.
• Glaucoma (open-angle and angle-closure): the most widely used miotic; lowers intraocular pressure.
• Xerostomia (dry mouth): used in patients with Sjogren's syndrome or after radiation therapy to head and neck (stimulates salivary secretion).
• Diagnosis of cystic fibrosis: pilocarpine iontophoresis sweat test - stimulates eccrine sweat glands to collect sweat for chloride measurement.

2. Indirect-Acting Agents (Anticholinesterases / Cholinesterase Inhibitors)

Physostigmine

• Tertiary amine - crosses the blood-brain barrier (BBB).
• Central anticholinergic syndrome: reversal of central effects of anticholinergic overdose (atropine, tricyclic antidepressants, antihistamines) - agitation, delirium, hallucinations.
• Glaucoma: historically used topically; largely replaced by newer agents.

Neostigmine

• Quaternary amine - does NOT cross BBB (peripheral effects predominate).
• Reversal of non-depolarizing neuromuscular blockade: most common clinical use in anesthesia (reverses vecuronium, rocuronium, atracurium). Given with glycopyrrolate to prevent muscarinic side effects.
• Myasthenia gravis: symptomatic treatment by increasing ACh at the neuromuscular junction.
• Postoperative urinary retention and paralytic ileus.

Pyridostigmine

• Quaternary amine; longer duration than neostigmine.
• Myasthenia gravis: the drug of choice for long-term symptomatic management.
• Reversal of neuromuscular blockade (less commonly than neostigmine).
• Prophylaxis against nerve agent (soman) exposure in military personnel.

Edrophonium

• Very short-acting (minutes).
• Tensilon (edrophonium) test: diagnosis of myasthenia gravis - transient improvement in muscle weakness confirms the diagnosis.
• Differentiation of myasthenic crisis vs. cholinergic crisis.
• Reversal of non-depolarizing neuromuscular blockade (short procedures).

Echothiophate (Irreversible)

• Organophosphate; available as topical ophthalmic drops.
• Glaucoma: prolonged duration of action (2-3 weeks after stopping); used when other agents fail.
• Note: reduces pseudocholinesterase activity, causing prolonged action of succinylcholine - important anesthetic concern.

Donepezil, Rivastigmine, Galantamine

• Reversible cholinesterase inhibitors; CNS-active.
• Alzheimer's disease: first-line symptomatic treatment; slow cognitive decline.
• Dementia with Lewy bodies, vascular dementia.
• Rivastigmine also approved for Parkinson's disease dementia.

3. Summary of Key Clinical Uses by Indication

Key Points

• All cholinergic drugs act through either muscarinic or nicotinic receptors (or both), mimicking parasympathetic stimulation.
• Adverse effects of excess cholinergic activity follow the "SLUDGE" mnemonic: Salivation, Lacrimation, Urination, Defecation, GI distress, Emesis. Bradycardia, bronchoconstriction, and miosis are also seen.
• The antidote for cholinergic toxicity/overdose is atropine (muscarinic blocker); pralidoxime is used specifically for organophosphate poisoning to reactivate cholinesterase.
• Cholinesterase inhibitors used in anesthesia (neostigmine) require co-administration of an antimuscarinic (atropine or glycopyrrolate) to block unwanted muscarinic side effects while allowing nicotinic reversal at the neuromuscular junction.