13.2 Mydriatics, Cycloplegics, and Anesthetics

13.2 Mydriatics, Cycloplegics, and Anesthetics

Mydriatics (sympathomimetics)

Phenylephrine is the prototype. At 2.5% it dilates safely for routine exams; at 10% it produces strong dilation but carries real systemic risk: it can spike blood pressure and cause tachycardia or reflex bradycardia. The COA must know that phenylephrine 10% is contraindicated in infants, the elderly, and patients with hypertension or cardiac disease, and that the 10% concentration is avoided when fewer risks suffice.

Cycloplegics (anticholinergics)

These block the muscarinic receptors of the iris sphincter and ciliary muscle, producing both mydriasis and accommodative paralysis. Memorize the speed/duration ladder:

A standard adult dilation combo is tropicamide 1% plus phenylephrine 2.5%, because combining a sympathomimetic with an anticholinergic gives wider, faster dilation than either alone. Allow about 20-30 minutes before the exam.

Anticholinergic side effects (the "can't see, can't pee, can't spit, can't poop, hot as a hare" mnemonic)

Systemic absorption of cyclopentolate/atropine can cause dry mouth, flushing, fever, tachycardia, urinary retention, and CNS changes. Children and the elderly are most vulnerable. Punctal occlusion (pressing the inner canthus for 1-2 minutes after instillation) reduces nasolacrimal drainage and systemic absorption.

Topical anesthetics

Proparacaine 0.5% and tetracaine 0.5% anesthetize the corneal surface in roughly 10-20 seconds, lasting 10-20 minutes. They are used for Goldmann applanation tonometry, foreign-body removal, gonioscopy, and corneal scraping.

Critical safety rule the exam loves: never dispense a topical anesthetic for home use. Repeated self-instillation causes epithelial toxicity, delayed healing, and corneal melts. An anesthetized cornea also has no protective blink reflex, so the patient must avoid rubbing the eye until sensation returns.

Worked instillation scenario

Before tonometry you instill proparacaine, then a fluorescein strip. The drop stings briefly, the cornea numbs in seconds, and the fluorescein highlights the tear meniscus under the cobalt-blue light. If the patient says the eye still feels everything after 30 seconds, re-check that the drop actually entered the eye rather than running down the cheek. Document the drug, concentration, time, and eye (OD/OS/OU).

Why combination dilation works

The sphincter and dilator muscles of the iris are antagonists. Phenylephrine stimulates the dilator (sympathetic) while tropicamide paralyzes the sphincter (parasympathetic block). Attacking both pathways at once produces faster, wider, more reliable dilation than either drop alone, which is why the standard adult exam combination is tropicamide 1% plus phenylephrine 2.5%. This also explains why a patient on pilocarpine (a sphincter stimulant for glaucoma) may dilate poorly: the constricting force opposes your dilating drops.

Pediatric and elderly cautions

Children absorb anticholinergics readily and can show CNS toxicity from cyclopentolate, including drowsiness, disorientation, ataxia, and rarely hallucinations. For infants, lower concentrations (cyclopentolate 0.5%) limit risk, and you observe the child afterward. The elderly are sensitive to phenylephrine 10% cardiovascular effects and to anticholinergic urinary retention. In both groups, punctal occlusion is the simplest protective step the COA controls.

Anesthetic safety details

Topical anesthetics abolish the corneal blink reflex, so an anesthetized eye is vulnerable to injury until sensation returns (10-20 minutes). Counsel the patient not to rub the eye and not to drive if vision is blurred. Anesthetics can also produce transient stinging on instillation and, rarely, allergic reactions. Because repeated use is toxic to the corneal epithelium, an anesthetic is an in-office diagnostic tool only, never a take-home analgesic. A patient who returns repeatedly with a non-healing epithelial defect after "borrowing" anesthetic drops is a classic abuse pattern the exam may describe.

Reversing and managing residual dilation

There is no routine "antidote" you instill to reverse diagnostic dilation; the patient simply waits while tropicamide wears off over 4-6 hours. Counsel patients that they will be light-sensitive and unable to focus on near work during that window, advise sunglasses, and warn that driving may be impaired. For longer-acting agents like cyclopentolate (up to 24 hours) or homatropine (1-3 days), the blur lasts substantially longer, which is why those agents are chosen deliberately and not for a quick exam. Setting accurate expectations is part of the COA's counseling role and is a frequent "best patient instruction" answer.

Onset timing and exam flow

Knowing onset times lets you pace the clinic. Tropicamide-phenylephrine reaches adequate dilation in about 20-30 minutes, cyclopentolate cycloplegia in 30-60 minutes, and atropine maximal effect in 30-40 minutes (but with a multi-day tail). If a physician is ready early and the pupils are still small, the correct action is to wait and recheck, not to over-instill additional drops, because stacking doses raises systemic absorption without much added benefit. A dark-irised patient may dilate more slowly and need a few extra minutes, another timing nuance worth recognizing.