U01.05.041 Drugs affecting pupil size

Understanding the pharmacological effects of various drugs on pupil size is crucial for USMLE Step 1 preparation. This topic integrates principles of autonomic pharmacology and clinical toxicology.

Classification of Drugs Based on Their Effect on Pupil Size

1. Drugs That Cause Pupil Dilation (Mydriasis)

These drugs stimulate sympathetic activity or inhibit parasympathetic activity.

Examples:

Category	Drug Examples	Mechanism of Action
Anticholinergics	Atropine, Tropicamide, TCAs, Scopolamine	Block muscarinic receptors, inhibiting parasympathetic input
Sympathomimetics	Amphetamines, Cocaine, LSD	Increase norepinephrine or serotonin activity
Drugs of Abuse	Amphetamines, Cocaine, LSD	Central and peripheral sympathomimetic effects

2. Drugs That Cause Pupil Constriction (Miosis)

These drugs stimulate parasympathetic activity or inhibit sympathetic activity.

Examples:

Category	Drug Examples	Mechanism of Action
Parasympathomimetics	Pilocarpine, Organophosphates	Stimulate muscarinic receptors, increasing parasympathetic input
Sympatholytics	α2-Agonists (e.g., Clonidine)	Reduce sympathetic outflow
Drugs of Abuse	Heroin, Opioids	Stimulate central parasympathetic pathways

Points to Remember

Mydriasis:
- Associated with anticholinergic and sympathomimetic effects.
- Clinical uses: eye exams (tropicamide), bradycardia (atropine).
- Toxicity signs: “Blind as a bat, dry as a bone, red as a beet, mad as a hatter.”
Miosis:
- Linked to parasympathetic stimulation or opioid use.
- Clinical uses: glaucoma (pilocarpine), nerve agent exposure (organophosphates).
Mnemonic for Mydriasis-Inducing Drugs:
“A Stimulated CAT”:
- Anticholinergics
- Sympathomimetics
- Cocaine
- Amphetamines
- Tropane alkaloids (e.g., atropine)
Mnemonic for Miosis-Inducing Drugs:
“POP a Pinpoint”:
- Parasympathomimetics
- Opioids
- Phosphates (organophosphates)

Quick Comparison Table

Effect on Pupil Size	Mechanism	Examples
Mydriasis (Dilation)	↓ Parasympathetic / ↑ Sympathetic	Atropine, Cocaine, Amphetamines
Miosis (Constriction)	↑ Parasympathetic / ↓ Sympathetic	Pilocarpine, Heroin, Organophosphates

USMLE Focus

Expect questions on drug overdose presentations, distinguishing between mydriasis and miosis.
Focus on clinical applications, such as eye exams (mydriasis) or opioid toxicity (miosis).

Bibliography

Katzung BG, Trevor AJ. Basic & Clinical Pharmacology.
USMLE-Rx and First Aid USMLE Step 1 Resources

USMLE Step 1 Review Series

Curriculum

U01.05.041 Drugs affecting pupil size

Classification of Drugs Based on Their Effect on Pupil Size

1. Drugs That Cause Pupil Dilation (Mydriasis)

Examples:

2. Drugs That Cause Pupil Constriction (Miosis)

Examples:

Points to Remember

Quick Comparison Table

USMLE Focus

Bibliography

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