Understanding Which of the Following Causes the Pupil to Constrict (Miotics)

October 9, 2025 •

5 min read

The constriction of the pupil, a process known as miosis, is a vital physiological response primarily controlled by the parasympathetic nervous system. Many medications, referred to as miotics, can induce this effect by mimicking or enhancing the action of the neurotransmitter acetylcholine. This article clarifies which of the following causes the pupil to constrict (miotics).

Quick Summary

This article examines how the parasympathetic nervous system induces miosis and details the classes of drugs that cause pupil constriction. It outlines the specific mechanisms of direct-acting and indirect-acting miotics, their therapeutic applications in conditions like glaucoma, and other non-miotic drugs causing miosis. The piece also contrasts miotics and mydriatics.

Key Points

Miotics Target the Parasympathetic System: Medications that cause miosis, or pupil constriction, primarily act by stimulating the parasympathetic nervous system, which controls the iris sphincter muscle.
Direct-Acting Miotics Mimic Acetylcholine: Drugs like pilocarpine and carbachol are direct-acting miotics that activate cholinergic receptors on the iris, causing the pupil to constrict.
Indirect-Acting Miotics Block Acetylcholinesterase: Cholinesterase inhibitors such as physostigmine increase the amount of available acetylcholine by blocking its breakdown, leading to sustained miosis.
Opioids are a Common Non-Miotic Cause: The use of opioids like morphine and fentanyl is a well-known non-miotic cause of miosis, producing the characteristic 'pinpoint pupils'.
Clinical Uses Include Glaucoma and Presbyopia: Miotics are used therapeutically to lower intraocular pressure in glaucoma and to improve near vision for patients with presbyopia.
Side Effects Include Blurry Vision and Brow Ache: Common side effects of miotics can include temporary blurred vision, ciliary spasm leading to brow ache, and poor vision in dim light.

The Physiology Behind Pupillary Constriction

The size of the pupil is regulated by two opposing muscles within the iris: the iris sphincter and the iris dilator. Miosis, or pupil constriction, is a function of the parasympathetic nervous system. When stimulated, parasympathetic nerve fibers release the neurotransmitter acetylcholine, which acts on muscarinic receptors (specifically the M3 subtype) located on the iris sphincter muscle. This causes the sphincter muscle to contract, reducing the pupil's size. The opposite process, mydriasis (pupil dilation), is controlled by the sympathetic nervous system acting on the iris dilator muscle.

Miotic Medications: The Main Culprits

Miotics are pharmacologic agents that specifically cause pupil constriction. They are typically categorized into two primary classes based on their mechanism of action.

Direct-Acting Miotics

These medications mimic the effect of acetylcholine by directly activating cholinergic receptors on the iris sphincter muscle.

Pilocarpine: One of the most well-known and widely used direct-acting miotics. Pilocarpine ophthalmic solution causes the iris sphincter muscle to contract, leading to miosis. It also causes contraction of the ciliary muscle, which helps to increase the outflow of aqueous humor and reduce intraocular pressure, making it useful in treating glaucoma. More recently, a low-dose formulation of pilocarpine has been approved for treating presbyopia, or age-related farsightedness, by creating a pinhole effect that enhances near vision.
Carbachol: Similar to pilocarpine, carbachol is a direct-acting cholinergic agonist that constricts the pupil and is used to lower intraocular pressure, particularly for patients who may be refractory or hypersensitive to pilocarpine.
Acetylcholine: This endogenous neurotransmitter is sometimes used intracamerally (injected into the anterior chamber of the eye) during ophthalmic surgery to produce rapid and complete miosis.

Indirect-Acting Miotics (Cholinesterase Inhibitors)

Rather than directly stimulating the receptor, these agents inhibit the enzyme acetylcholinesterase, which is responsible for breaking down acetylcholine in the synaptic cleft. By doing so, they prolong and enhance the effect of naturally released acetylcholine, leading to sustained miosis.

Physostigmine: This is a reversible cholinesterase inhibitor that causes pupillary constriction.
Echothiophate: A long-acting, irreversible cholinesterase inhibitor, though it is no longer commercially available in the U.S..
Demecarium: Another long-acting cholinesterase inhibitor that prolongs the effects of acetylcholine.

Other Drugs That Cause Miosis

Several other classes of drugs cause miosis, but usually as an unintended side effect rather than for therapeutic pupillary constriction.

Opioids: A classic example, opioids such as morphine, heroin, fentanyl, and hydrocodone cause miosis, resulting in the characteristic 'pinpoint pupils'. This occurs due to their effect on the central nervous system, which enhances parasympathetic output to the eye.
Antipsychotics and Antihypertensives: Certain antipsychotic medications like haloperidol and some antihypertensive drugs, such as clonidine, can also induce miosis.
Benzodiazepines: These drugs can cause pupillary constriction.
Organophosphates: Found in some insecticides and nerve agents, these chemicals act as powerful, irreversible cholinesterase inhibitors and cause severe, persistent miosis.

The Clinical Role of Miotic Medications

Glaucoma Management

Historically, and in some current cases, miotics are a cornerstone of glaucoma treatment. Their ability to contract the ciliary muscle pulls the scleral spur, which opens the trabecular meshwork and increases the outflow of aqueous humor, thereby lowering intraocular pressure. This is especially useful in certain types of glaucoma, such as narrow-angle glaucoma, where it helps to relieve the blockage of the fluid drainage system.

Treating Presbyopia

For older adults experiencing presbyopia, pilocarpine eye drops are a therapeutic option. The pupil constriction creates an increased depth of focus, which allows for improved near vision without the need for reading glasses.

Other Ophthalmic Uses

During various ophthalmic surgeries, such as cataract extraction, acetylcholine or carbachol is used to rapidly constrict the pupil. Miotics can also be used to reverse the effects of mydriatics (pupil-dilating agents) after an eye exam.

Comparison of Miotics and Mydriatics

To better understand the effects of miotics, it's helpful to compare them to their counterparts, mydriatics.


Feature	Miotics	Mydriatics
Effect on Pupil	Causes pupil constriction (miosis)	Causes pupil dilation (mydriasis)
Mechanism	Stimulates cholinergic receptors on the iris sphincter muscle	Stimulates adrenergic receptors on the iris dilator muscle or blocks cholinergic receptors on the iris sphincter muscle
Nervous System	Promotes the parasympathetic pathway	Promotes the sympathetic pathway or inhibits the parasympathetic pathway
Example Drugs	Pilocarpine, Carbachol, Physostigmine	Phenylephrine, Atropine, Tropicamide
Clinical Uses	Glaucoma, presbyopia, ophthalmic surgery	Eye exams, treating uveitis

Side Effects of Miotics

While effective, miotics are not without side effects, which can be ocular or systemic.

Ocular Side Effects:

Blurred Vision and Myopia: Contraction of the ciliary muscle can cause a spasm of accommodation, leading to blurred vision, particularly in younger patients, and temporary near-sightedness.
Poor Vision in Dim Light: A constricted pupil lets in less light, making it difficult to see in low-light conditions.
Eye and Brow Ache: The ciliary muscle spasm can cause a headache or brow ache.
Conjunctival Congestion: Redness in the eye can occur due to increased blood flow.
Retinal Detachment: Though rare, long-term use of certain potent miotics can increase the risk of retinal detachment, especially in predisposed individuals.

Systemic Side Effects:

Gastrointestinal Disturbances: Nausea, vomiting, diarrhea, and abdominal cramps are possible due to increased parasympathetic activity.
Bradycardia and Hypotension: Some patients may experience a slowed heart rate and low blood pressure.
Increased Secretions: Systemic absorption can lead to increased salivation, sweating, and bronchial secretions.

Other Causes of Miosis (Non-Pharmacological)

It is important to remember that miosis can also be caused by factors other than medication.

Normal Light Response: The pupil naturally constricts in bright light to protect the retina from excessive illumination.
Neurological Conditions: Certain disorders, like Horner's syndrome, disrupt the sympathetic nerve pathway to the eye, resulting in an unopposed parasympathetic action that causes miosis.
Injury or Trauma: Eye injuries, head trauma, or certain brain injuries can lead to constricted pupils.
Inflammation: Inflammation of the iris, or uveitis, can trigger miosis.

Conclusion

Medications that cause the pupil to constrict, known as miotics, primarily act by stimulating the parasympathetic nervous system, leading to the contraction of the iris sphincter muscle. These include direct-acting agents like pilocarpine and carbachol, as well as indirect-acting cholinesterase inhibitors. While miotics have important therapeutic uses, particularly in ophthalmology for conditions such as glaucoma and presbyopia, their effects can also occur as side effects from other medications, most notably opioids. Understanding the specific causes and mechanisms of miosis is critical for both medical professionals and patients to manage these conditions and address related side effects effectively.

For more detailed information on miotic pharmacology, you can refer to the National Institutes of Health's extensive resources: Drug-induced Acute Angle-closure Glaucoma: A Review

Frequently Asked Questions

Miosis is primarily caused by the activation of the parasympathetic nervous system. Parasympathetic nerve fibers release acetylcholine, which binds to muscarinic receptors on the iris sphincter muscle, causing it to contract and constrict the pupil.

Direct-acting miotics, like pilocarpine, mimic acetylcholine and directly activate cholinergic receptors to cause miosis. Indirect-acting miotics, or cholinesterase inhibitors, block the enzyme that breaks down acetylcholine, increasing its availability and prolonging its constricting effect.

Pilocarpine is a direct-acting miotic that activates M3 muscarinic receptors on the iris sphincter muscle. This stimulates the muscle to contract, resulting in a constricted pupil.

Yes, several drugs cause miosis as a side effect. A prominent example is opioids, which can cause 'pinpoint pupils'. Other drugs like certain antipsychotics, some antihypertensives, and organophosphates can also cause miosis.

Miotics help treat glaucoma by constricting the pupil and causing the ciliary muscle to contract. This action pulls on the trabecular meshwork, opening the drainage angle and increasing the outflow of aqueous humor, which lowers intraocular pressure.

Common side effects include blurred vision, poor vision in low-light conditions, ciliary muscle spasm causing brow ache, and conjunctival redness. Systemic effects can include gastrointestinal issues like nausea and diarrhea.

Non-pharmacological causes of miosis include a normal physiological response to bright light, neurological conditions such as Horner's syndrome, eye or head trauma, and inflammation of the iris (uveitis).