23.2 Mydriatics, Cycloplegics, Anaesthetics & Diagnostic Agents
Key Takeaways
- Phenylephrine dilates the pupil by stimulating the iris dilator and has no cycloplegic effect.
- Tropicamide, cyclopentolate, homatropine, scopolamine and atropine are antimuscarinics that both dilate and cycloplege, in increasing order of duration.
- Phenylephrine 10% carries a risk of hypertensive crisis and is avoided in cardiovascular disease; 2.5% is the routine strength.
- Proparacaine and tetracaine are topical anaesthetics for diagnostic use only; they must never be dispensed for pain relief.
- Repeated topical anaesthetic use causes epithelial toxicity, delayed healing and can lead to corneal melting.
Two ways to dilate a pupil
The iris has two muscles with opposing innervation, so there are two pharmacologic routes to mydriasis.
Sympathomimetics stimulate the dilator muscle:
| Drug | Strength | Onset / duration | Notes |
|---|---|---|---|
| Phenylephrine | 2.5% routine, 10% rarely | 20–30 min / 3–5 h | No cycloplegia; also blanches conjunctival vessels |
| Hydroxyamphetamine | 1% | — | Diagnostic use in Horner syndrome localisation |
| Apraclonidine | 0.5–1% | — | Reverses the miosis of Horner syndrome (diagnostic); also lowers pressure |
Antimuscarinics (parasympatholytics) block the sphincter and the ciliary muscle, so they produce both mydriasis and cycloplegia:
| Drug | Strength | Onset | Duration | Typical use |
|---|---|---|---|---|
| Tropicamide | 0.5–1% | 15–20 min | 4–6 h | Routine dilation; weak cycloplegic |
| Cyclopentolate | 0.5–2% | 20–30 min | 6–24 h | Paediatric cycloplegic refraction |
| Homatropine | 2–5% | 30–60 min | 1–3 days | Uveitis cycloplegia |
| Scopolamine (hyoscine) | 0.25% | 20–30 min | 3–7 days | Uveitis |
| Atropine | 0.5–1% | 30–40 min | 7–14 days | Deep cycloplegia; uveitis; amblyopia penalisation |
The routine dilation combination is tropicamide 1% plus phenylephrine 2.5%, which uses both mechanisms and gives a wider, faster dilation than either alone.
Phenylephrine safety
Phenylephrine 10% is a potent alpha-1 agonist and has been associated with hypertensive crisis, tachycardia, arrhythmia, myocardial infarction and subarachnoid haemorrhage after topical ocular use, particularly with repeated instillation, in infants, in the elderly and in patients on monoamine oxidase inhibitors or tricyclic antidepressants.
Practical rules: use 2.5% for routine dilation; avoid 10% in cardiovascular disease, uncontrolled hypertension, aneurysm, thyrotoxicosis and in infants; never instil onto an abraded cornea (absorption rises sharply); apply punctal occlusion; and record the strength used.
Phenylephrine in children: use the lowest effective concentration. 2.5% is standard, and 10% is generally avoided in infancy.
Contraindications and cautions for dilation
- Narrow anterior chamber angle — assess anterior chamber depth before dilating. A patient with a shallow chamber, a history of haloes and brow ache, or previously documented narrow angles must be assessed by the clinician first. Acute angle closure after dilation is uncommon but real.
- Iris-supported intraocular lens or anterior chamber lens — the clinician should direct.
- Known allergy to the agent or its preservative.
- Recent trauma with a suspected open globe — instil nothing.
- Patients who must drive — warn about photophobia and near blur, advise sunglasses, and confirm arrangements.
- Neurological monitoring — pupil size is a neurological observation; check before dilating a patient under head injury observation.
Counselling every dilated patient: blurred near vision and light sensitivity for several hours (longer with cyclopentolate, days with atropine), sunglasses, no driving until vision is comfortable, and to return immediately for severe pain, redness or haloes.
Topical anaesthetics
| Drug | Onset | Duration | Notes |
|---|---|---|---|
| Proparacaine (proxymetacaine) 0.5% | 10–20 s | 10–20 min | Least stinging; the routine choice; refrigerate |
| Tetracaine (amethocaine) 0.5% | 10–20 s | 10–20 min | More stinging, slightly longer acting |
| Oxybuprocaine (benoxinate) 0.4% | 10–20 s | 10–15 min | Often combined with fluorescein |
| Lidocaine gel 2% | 1–2 min | 20–30 min | Used before injections and minor procedures |
Uses: applanation tonometry, pachymetry, gonioscopy and contact lens examination, foreign body removal, punctal dilation, corneal scraping, and comfort during Schirmer basal secretion testing.
The hard rule: topical anaesthetics are for diagnostic and procedural use in the clinic only and are never dispensed to a patient for pain relief. Repeated use causes:
- Epithelial toxicity and punctate keratopathy
- Delayed epithelial healing — the drug is directly toxic to migrating epithelial cells
- Loss of corneal sensation, removing the protective blink and the warning of further injury
- Neurotrophic ulceration, stromal melting and perforation in chronic abuse
- Ring infiltrates that mimic Acanthamoeba keratitis
Anaesthetic abuse is a recognised and serious clinical entity, usually arising when a patient obtains a bottle after a painful abrasion. Always warn the patient that the anaesthetic will wear off and that the eye must not be rubbed while it is numb.
Diagnostic agents
| Agent | Use | Key points |
|---|---|---|
| Sodium fluorescein (strips or drops) | Epithelial defects, applanation, Seidel test, tear break-up time, contact lens fitting | Stains soft contact lenses — remove them first; use the cobalt blue filter |
| Intravenous fluorescein | Fluorescein angiography | Causes transient yellowing of skin and bright yellow urine for 24 to 36 hours — warn the patient; nausea is common; anaphylaxis is rare but possible |
| Lissamine green | Devitalised cells, mucin-deficient epithelium | Well tolerated; view 1 to 4 minutes after instillation |
| Rose bengal | Same targets as lissamine green | Stings considerably; largely superseded |
| Indocyanine green | Choroidal angiography | Contains iodine — contraindicated in iodine or shellfish allergy and in significant liver disease |
| Trypan blue | Capsule staining in white cataract surgery | Intraoperative |
Reversal and related agents
There is no reliable routine reversal agent for pharmacologic mydriasis in common clinical use, so the correct answer to "can you undo this dilation?" is to explain the expected duration rather than to instil a miotic. Pilocarpine will constrict a pharmacologically dilated pupil only partially and introduces its own effects, and it is not used routinely for this purpose.
Apraclonidine is used diagnostically: in Horner syndrome the denervated dilator is supersensitive, so apraclonidine reverses the anisocoria, dilating the smaller pupil — a useful confirmatory test. Cocaine 4% blocks noradrenaline reuptake and fails to dilate a Horner pupil; hydroxyamphetamine 1% dilates a pre-ganglionic but not a post-ganglionic Horner pupil, localising the lesion.
Which agent dilates the pupil without producing any cycloplegia?
Why must topical anaesthetic never be dispensed to a patient for ongoing pain relief?
Which patient requires particular caution before phenylephrine 10% is instilled?
Which diagnostic dye is contraindicated in a patient with a documented iodine allergy?
A patient asks how long their pupils will stay dilated after tropicamide 1%. What is the appropriate answer?