Cholinergic miotics are a group of drugs that stimulate the parasympathetic nervous system in the eye, leading to pupil constriction (miosis) and reduction of intraocular pressure (IOP). These drugs are especially important in the management of glaucoma and are frequently tested in pharmacology, ophthalmology, and physiology examinations.
The most commonly discussed cholinergic miotics include Pilocarpine, Carbachol, Demecarium, and Acetylcholine. Although they share a common final effect—miosis—their mechanisms, duration of action, and clinical uses differ, making them important to understand individually.
How Cholinergic Miotics Work in the Eye
Cholinergic miotics act primarily by stimulating muscarinic (M3) receptors in the eye.
Activation of these receptors causes:
- Contraction of the sphincter pupillae → miosis
- Contraction of the ciliary muscle → opening of the trabecular meshwork
- Increased aqueous humor outflow via the canal of Schlemm
- Reduction in intraocular pressure
This mechanism is especially useful in open-angle glaucoma and acute angle-closure glaucoma.
Pilocarpine
Pilocarpine is the most commonly used cholinergic miotic in clinical practice.
It is a direct-acting muscarinic receptor agonist, meaning it directly stimulates M3 receptors without affecting acetylcholinesterase.
Pilocarpine produces:
- Miosis
- Ciliary muscle contraction
- Increased aqueous humor drainage
Therapeutic Uses of Pilocarpine
Pilocarpine is widely used in:
- Chronic open-angle glaucoma
- Acute angle-closure glaucoma (initial management)
- Xerostomia (dry mouth) in Sjögren syndrome and post-radiation therapy
Adverse Effects of Pilocarpine
Because of its parasympathetic activity, pilocarpine can cause:
- Brow ache and eye pain
- Blurred vision (especially in low light)
- Headache
- Sweating, salivation, and nausea (systemic absorption)
Pilocarpine should be used cautiously in patients with asthma and cardiac disease.
Carbachol
Carbachol is a direct-acting cholinergic agonist with both muscarinic and nicotinic activity.
Unlike pilocarpine, carbachol is resistant to acetylcholinesterase, giving it a longer duration of action.
Clinical Uses of Carbachol
Carbachol is primarily used:
- In ophthalmic surgery to produce rapid and sustained miosis
- Occasionally in glaucoma (less commonly than pilocarpine)
It is often administered intraocularly during surgery rather than as long-term topical therapy.
Adverse Effects of Carbachol
Due to its potency, carbachol may cause:
- Excessive miosis
- Ocular irritation
- Systemic cholinergic effects if absorbed
Demecarium
Demecarium is an indirect-acting cholinergic drug.
It works by irreversibly inhibiting acetylcholinesterase, leading to increased acetylcholine levels at muscarinic receptors.
Because of this mechanism, demecarium has a very long duration of action.
Therapeutic Uses of Demecarium
Demecarium is used in:
- Chronic open-angle glaucoma
- Patients not responding to other antiglaucoma drugs
However, its use has declined due to safety concerns.
Adverse Effects and Limitations
Demecarium can cause:
- Prolonged miosis
- Cataract formation (with long-term use)
- Risk of systemic organophosphate-like toxicity
Because of these risks, demecarium is rarely used today and is mainly of exam importance.
Acetylcholine
Acetylcholine is the natural neurotransmitter of the parasympathetic nervous system.
In ophthalmology, it is used in a very specific setting.
Clinical Use of Acetylcholine
Acetylcholine is used:
- Intraoperatively during eye surgery
- To produce rapid, short-acting miosis
It is not used for chronic glaucoma because it is:
- Rapidly degraded by acetylcholinesterase
- Extremely short-acting
Key Point for Exams
Acetylcholine causes:
- Immediate miosis
- Very short duration of action
This makes it ideal for surgical procedures, not long-term therapy.
Comparison of Cholinergic Miotics (Conceptual Overview)
- Pilocarpine is a direct muscarinic agonist used commonly in glaucoma.
- Carbachol is a stronger, longer-acting direct agonist mainly used during surgery.
- Demecarium is an indirect cholinergic drug with prolonged action but higher toxicity.
- Acetylcholine is a natural transmitter used only intraoperatively due to its very short action.
Adverse Effects Common to Cholinergic Miotics
Cholinergic miotics may cause:
- Miosis leading to poor night vision
- Ciliary spasm and brow ache
- Headache
- Risk of retinal detachment (rare)
Systemic absorption can cause:
- Bradycardia
- Bronchoconstriction
- Excessive salivation and sweating
Contraindications
These drugs should be used cautiously or avoided in:
- Asthma and COPD
- Severe cardiac disease
- Uveitis or iritis
- Retinal detachment risk
Exam-Oriented High-Yield Points
- Pilocarpine is the drug of choice among cholinergic miotics for glaucoma.
- Carbachol and acetylcholine are commonly used during eye surgery.
- Demecarium is an irreversible acetylcholinesterase inhibitor.
- All cholinergic miotics reduce intraocular pressure by increasing aqueous humor outflow.
Simple Mnemonic for Memory
“PCDA” for Cholinergic Miotics
P – Pilocarpine (most common)
C – Carbachol (surgical miosis)
D – Demecarium (indirect, long-acting)
A – Acetylcholine (short-acting)
FAQs on Cholinergic Miotics
Which cholinergic miotic is most commonly used in glaucoma?
Pilocarpine is the most commonly used cholinergic miotic for glaucoma.
Why do cholinergic miotics reduce intraocular pressure?
They contract the ciliary muscle, opening the trabecular meshwork and increasing aqueous humor outflow.
Why is acetylcholine not used long-term?
It is rapidly broken down by acetylcholinesterase and has a very short duration of action.
Why is demecarium rarely used today?
Because of its long duration and risk of systemic toxicity and cataract formation.
What is a common side effect of pilocarpine?
Brow ache, headache, and blurred vision due to ciliary muscle spasm.
