A mydriatic drop is used for pupil dilation. The act of widening the pupil is called mydriasis. A mydriatic drug usually works by stimulating the iris dilator muscle. That muscle pulls the pupil open. A wider pupil gives the doctor a much larger window into the back of the eye. That is exactly what she needs to study the retina and the optic nerve. Without dilation, the iris blocks most of the view of the back of the eye.
Mydriatics are used mainly to make the inside of the eye easier to examine. Phenylephrine is a commonly used one. Do not mix mydriatics up with cycloplegics. A cycloplegic paralyzes the ciliary muscle, so it blocks accommodation, the eye's focusing effort. It also relaxes the iris sphincter, so the pupil dilates as well. A mydriatic only dilates the pupil; a cycloplegic dilates and blocks focusing. Tropicamide is a common cycloplegic. Both classes share a red cap under the standard color code.
Before you dilate, ask about narrow angles. In a patient with a narrow drainage angle, a dilating drop may set off an angle-closure attack. Stinging and light sensitivity are common and expected after these drops. Warn the patient that bright light will bother her for a few hours. Offer sunglasses before she leaves. Knowing what each class of drop does before you instill it is a core part of the COA pharmacology content area.
Incorrect Answers
- A. No routine eye drop is given in order to raise pressure. Dilating a narrow-angle eye may raise pressure as a complication, not as the goal.
- B. Pupil constriction is the job of a miotic, such as pilocarpine. Miotics carry a dark green cap.
- D. Lowering pressure is the goal of glaucoma drops, such as timolol or dorzolamide. Those drops do not dilate the pupil.
Testing Pearls
- Mydriasis means the pupil gets bigger; miosis means it gets smaller.
- Mydriatics dilate only; cycloplegics dilate and block accommodation.
- Phenylephrine is a mydriatic; tropicamide is a cycloplegic.
- Mydriatic and cycloplegic caps are red in the standard color code.
- Check the angle before dilating, since dilation can trigger angle-closure glaucoma.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 6, Basics of Ophthalmic Pharmacology.
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 8, Comprehensive Medical Eye Examination.
