What is the primary action of phenylephrine in ocular meds and a caution for its use?

• A. It's a beta-2 agonist causing accommodation; caution in myopia
• B. It's a cholinergic agent causing miosis; caution in dry eye
• C. It's a serotonin receptor antagonist; caution in glaucoma
• D. It is an alpha-1 agonist causing mydriasis; caution in hypertension, tachyarrhythmias, and narrow-angle glaucoma

Answer: (D)

Phenylephrine works by activating alpha-1 adrenergic receptors in the eye, which stimulates the iris dilator (radial) muscle to contract. This causes pupil dilation (mydriasis) without significantly changing accommodation. It’s commonly used to produce dilation for eye exams or preoperative preparation.

Because it can be absorbed systemically, it can raise blood pressure by vasoconstriction and may provoke tachyarrhythmias in sensitive individuals. It can also precipitate or worsen angle-closure glaucoma in people with narrow angles due to further widening of the iris–lens contact. So the main caution is to monitor or avoid use in patients with hypertension, tachyarrhythmias, and narrow-angle glaucoma.