Mydriatics vs. Cycloplegics
Two categories of drops are used to dilate the pupil for examination and imaging:
- Mydriatics: Dilate the pupil (cause mydriasis) without significantly affecting accommodation. They work by stimulating the dilator muscle of the iris (sympathomimetics) or relaxing the sphincter (anticholinergics).
- Cycloplegics: Paralyze the ciliary muscle (causing cycloplegia), which relaxes accommodation. Cycloplegics also dilate the pupil because the iris sphincter and ciliary muscle share the same parasympathetic innervation. Every cycloplegic is also a mydriatic, but not every mydriatic is cycloplegic.
Common Mydriatic and Cycloplegic Agents
| Drug | Class | Onset | Duration | Primary Use |
|---|---|---|---|---|
| Tropicamide 0.5% or 1% | Anticholinergic | 15-20 min | 4-6 hours | Routine dilation; mild cycloplegia |
| Phenylephrine 2.5% or 10% | Sympathomimetic | 15-20 min | 3-5 hours | Pupil dilation without cycloplegia |
| Cyclopentolate 1% or 2% | Anticholinergic | 25-45 min | 12-24 hours | Cycloplegic refraction, uveitis |
| Atropine 1% | Anticholinergic | 30-60 min | 7-14 days | Maximum cycloplegia, severe uveitis, amblyopia penalization |
| Homatropine 2% | Anticholinergic | 30-45 min | 24-72 hours | Cycloplegic refraction, uveitis (intermediate duration) |
💡 Clinical Tip: For routine funduscopic examination and photography, the most common combination is tropicamide 1% (dilation) plus phenylephrine 2.5% (additional dilation via a different mechanism). Using two drops from different drug classes produces better dilation than either drop alone, especially in patients with dark irides who are harder to dilate.
CPOA Role in Instilling Mydriatics/Cycloplegics
- Verify the order: Confirm which drops, which eye(s), and which concentration are ordered. Never assume -- verify before instilling.
- Screen for contraindications: Ask the patient before instilling any dilating drops:
- "Do you have glaucoma?" (narrow-angle or angle-closure glaucoma is a relative contraindication for cycloplegics and sympathomimetics).
- "Do you have any heart conditions or high blood pressure?" (phenylephrine 10% can raise blood pressure -- use 2.5% for elderly and cardiovascular patients).
- Note any known drug allergies.
- Instill correctly: 1 drop per eye in the inferior fornix. Ask the patient to look up, gently retract the lower lid, instill the drop, then ask the patient to close their eyes and look down for 1-2 minutes. Applying gentle nasolacrimal pressure for 2 minutes reduces systemic absorption.
- Document the drops, time, and concentration in the patient chart immediately.
- Counsel the patient: "Your vision will be blurry up close and your eyes will be sensitive to light for several hours. Do not drive until your vision has fully recovered."
⚠️ Common Mistake: Failing to document the time and concentration of dilating drops. The doctor needs to know when drops were instilled (to estimate how much dilation has occurred) and what was given (to assess cycloplegia adequacy for a cycloplegic refraction or to counsel the patient about recovery time). Always document immediately after instillation.
Side Effects and Adverse Reactions
- Photophobia: Dilation removes the pupil's ability to constrict to bright light -- all patients will be light-sensitive. Sunglasses are recommended.
- Blurred near vision: Cycloplegics impair accommodation -- reading will be impossible for the duration of effect. Patients should be warned not to plan activities requiring close work.
- Acute angle-closure: In susceptible individuals (narrow angles), dilation can push the dilated iris into the angle, blocking aqueous drainage and causing a sudden IOP spike. A risk that requires gonioscopic assessment before routine dilation in at-risk patients.
- Systemic effects of cyclopentolate (especially in children): Flushing, tachycardia, dry mouth, hallucinations (psychomotor symptoms). Rare but possible with cyclopentolate -- use nasolacrimal occlusion and the lowest effective concentration in children.
- Phenylephrine 10%: Can cause significant systemic hypertension, arrhythmia, and pallor. Use only 2.5% in elderly, hypertensive, and cardiovascular patients. The 10% concentration is rarely needed for routine dilation.
Key Takeaways
- Mydriatics dilate the pupil; cycloplegics paralyze accommodation and also dilate the pupil.
- Routine dilation: tropicamide 1% + phenylephrine 2.5%. Cycloplegic refraction: cyclopentolate or atropine.
- Screen for glaucoma and cardiovascular disease before instilling dilation drops.
- Document drug, concentration, eye(s), and time immediately after instillation.
- Counsel all patients about photophobia, near blur, and the driving restriction.
- Use nasolacrimal occlusion to minimize systemic absorption, especially in children and for cycloplegics.