The Autonomic Nervous System and Pupil Control
The size of the human pupil is controlled by two opposing muscles within the iris, regulated by the autonomic nervous system. The parasympathetic nervous system causes pupil constriction (miosis) in bright light via the release of acetylcholine, which acts on muscarinic receptors on the iris sphincter muscle. The sympathetic nervous system causes pupil dilation (mydriasis) in dim light by stimulating the iris dilator muscle.
How Anticholinergics Tip the Balance
Anticholinergic drugs interfere with the parasympathetic pathway. They block muscarinic acetylcholine receptors on the iris sphincter muscle, preventing contraction and causing it to relax. This allows the sympathetic system's effect on the iris dilator muscle to dominate, resulting in mydriasis. This blockade also paralyzes the ciliary muscle, causing cycloplegia and affecting the eye's ability to focus, leading to blurred vision and difficulty reading.
Systemic vs. Topical Administration
Anticholinergic effects on the eye can occur through topical application or systemic administration.
- Topical Application: Ophthalmic solutions like atropine, cyclopentolate, and tropicamide are used in eye exams to dilate pupils for better viewing of internal structures. The duration of dilation varies by drug, from a few hours with tropicamide to up to two weeks with atropine.
- Systemic Administration: Many non-ophthalmic medications have anticholinergic properties and can cause bilateral mydriasis as a side effect after being absorbed into the bloodstream. The cumulative effect of these medications is known as the "anticholinergic burden," which has been linked to adverse outcomes, particularly in older adults, such as cognitive decline and increased fall risk.
Examples of Drugs with Anticholinergic Effects
Medications with anticholinergic properties that can cause mydriasis include:
- OTC antihistamines (e.g., diphenhydramine)
- Tricyclic antidepressants (e.g., amitriptyline)
- Antipsychotics (e.g., quetiapine, chlorpromazine)
- Medications for overactive bladder (e.g., oxybutynin, trospium)
- Antispasmodics (e.g., dicyclomine)
- Muscle relaxants (e.g., methocarbamol)
- Antiparkinsonian drugs (e.g., benztropine)
- Motion sickness medications (e.g., scopolamine)
Comparison of Ocular Effects: Cholinergic vs. Anticholinergic
Comparing anticholinergic effects to cholinergic drugs, which mimic acetylcholine, clarifies their distinct actions.
| Feature | Anticholinergic Drugs | Cholinergic Drugs |
|---|---|---|
| Effect on Pupil | Mydriasis (Dilation) | Miosis (Constriction) |
| Mechanism | Blocks muscarinic receptors on iris sphincter | Stimulates muscarinic receptors on iris sphincter |
| Effect on Focus | Cycloplegia (paralysis of focus) | Ciliary muscle contraction (focus for near vision) |
| Visual Symptoms | Blurred vision, light sensitivity | Dim vision (from small pupil), blurred vision (from ciliary spasm) |
| Common Examples | Atropine, Scopolamine, Diphenhydramine | Pilocarpine, Neostigmine |
Conclusion
Anticholinergic drugs definitively cause mydriasis (pupil dilation). They block acetylcholine's action on the iris sphincter muscle, preventing constriction and leading to dilation, blurred vision, and light sensitivity. While used therapeutically in ophthalmology, this effect is also a common side effect of many systemic medications. Recognizing a medication's anticholinergic burden is important for managing potential ocular and systemic effects, particularly in older adults.
For more detailed information from a leading source, you can visit the EyeWiki page on Pharmacologic Dilation.
