Learning Objective

Understand which drugs cause pupil dilation (mydriasis) versus pupil constriction (miosis) and the mechanisms behind these effects.

Pupil Dilation (Mydriasis, ↑ Pupil Size)

• Anticholinergics: Atropine, tricyclic antidepressants (TCAs), tropicamide, scopolamine, first-generation antihistamines
  Mechanism: Block muscarinic receptors on the pupillary sphincter.
• Indirect sympathomimetics: Amphetamines, cocaine, LSD, meperidine
  Mechanism: Increase norepinephrine release or inhibit reuptake → stimulation of α₁ receptors on the iris dilator.
• Direct sympathomimetics: Phenylephrine
  Mechanism: Direct α₁ agonism → pupillary dilator contraction.

Pupil Constriction (Miosis, ↓ Pupil Size)

• Parasympathomimetics: Pilocarpine, organophosphates
  Mechanism: Stimulate muscarinic receptors on the pupillary sphincter.
• Opioids (except meperidine): Morphine, heroin, fentanyl
  Mechanism: Central stimulation of the Edinger–Westphal nucleus → sphincter contraction.
• Sympatholytics: α₂-agonists (eg, clonidine, brimonidine)
  Mechanism: Decrease sympathetic outflow → reduced dilator activity.

Step 1 High-Yield Pearls

• “COPS” mnemonic for mydriasis: Cocaine, Opiates (meperidine exception), Phenylephrine, Scopolamine/anticholinergics
• Pilocarpine is used therapeutically for glaucoma (miosis → improved aqueous outflow).

Activity

Activity