Mydriatics

I. Introduction

Mydriatics are a class of ophthalmic medications used to induce pupil dilation (mydriasis) by either stimulating the dilator muscles or inhibiting the constrictor (sphincter) muscles of the iris. They are primarily used for diagnostic procedures such as fundoscopic examination, preoperative preparation, treatment of certain ocular disorders, and occasionally for therapeutic cycloplegia.

Mydriatics can be divided into two pharmacological subclasses:

1. Sympathomimetic mydriatics – directly stimulate the iris dilator muscle (adrenergic stimulation).

2. Anticholinergic mydriatics (cycloplegics) – block the parasympathetic innervation of the iris sphincter and ciliary body.

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II. Classification of Mydriatics

A. Sympathomimetic Agents (Alpha-Adrenergic Agonists)

These act on the α1-adrenergic receptors of the iris dilator muscle to cause mydriasis without cycloplegia (no paralysis of accommodation).

• Phenylephrine hydrochloride (2.5% or 10% ophthalmic solution)

• Hydroxyamphetamine (available in combination with tropicamide)

• Epinephrine (rarely used due to systemic effects)

B. Anticholinergic Agents (Cycloplegic Mydriatics)

These block muscarinic (M3) receptors, preventing iris sphincter and ciliary muscle contraction. Result: mydriasis with cycloplegia.

• Tropicamide (0.5% and 1%)

• Cyclopentolate hydrochloride (0.5%, 1%, 2%)

• Atropine sulfate (1%)

• Homatropine hydrobromide

• Scopolamine hydrobromide

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III. Mechanism of Action

Subclass	Mechanism
Sympathomimetics	Stimulate α1 receptors → contraction of radial iris muscle → dilation
Anticholinergics	Block M3 receptors → inhibit sphincter pupillae & ciliary muscle → dilation & cycloplegia

IV. Clinical Indications

1. Ophthalmic Diagnosis

   • Fundus examination

   • Refraction in children (requires cycloplegia)

2. Pre-operative Mydriasis

   • Cataract surgery

   • Retinal detachment repair

3. Uveitis and Iritis

   • Prevents posterior synechiae

   • Reduces pain from ciliary spasm (cycloplegia)

4. Amblyopia Treatment (Penalization Therapy)

   • Long-acting cycloplegics used to blur vision in the good eye

5. Cycloplegic Refraction

   • Particularly in children or those with latent hyperopia or accommodative esotropia

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V. Common Agents and Their Profiles

1. Phenylephrine Hydrochloride

• Strengths: 2.5% and 10%

• Onset: 15–30 minutes

• Duration: 3–6 hours

• Mydriasis without cycloplegia

• Indications: Diagnostic dilation, pre-surgical dilation

2. Tropicamide

• Strengths: 0.5% and 1%

• Onset: 20–40 minutes

• Duration: 4–8 hours

• Cycloplegia: Mild to moderate

• Preferred for routine fundoscopic exams

3. Cyclopentolate

• Strengths: 0.5%, 1%, 2%

• Onset: 30–60 minutes

• Duration: 6–24 hours

• Used in pediatric cycloplegic refraction

4. Atropine

• Strength: 1%

• Onset: 1–3 hours

• Duration: 7–14 days

• Strong cycloplegic and mydriatic action

• Used in penalization therapy, severe uveitis

5. Homatropine

• Duration: 1–3 days

• Less potent than atropine but longer acting than tropicamide

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VI. Dosage and Administration

• Single drop in each eye, may repeat depending on indication

• For maximal dilation: often combination of phenylephrine + tropicamide

• Apply light pressure to the lacrimal sac (punctal occlusion) for 1–2 minutes to reduce systemic absorption

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VII. Adverse Effects

A. Ocular

• Photophobia

• Blurred vision (due to cycloplegia)

• Increased intraocular pressure (especially in narrow-angle glaucoma)

• Ocular irritation, redness

B. Systemic (more common in children and elderly)

• Dry mouth

• Tachycardia

• Flushing

• Hallucinations (especially with atropine or cyclopentolate)

• Seizures (rare, associated with cyclopentolate in infants)

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VIII. Contraindications and Cautions

• Angle-closure glaucoma: Risk of acute attack

• Known hypersensitivity

• Use with caution in:

  • Children (especially infants): risk of systemic toxicity

  • Elderly with cardiovascular conditions (phenylephrine)

  • Patients with Down syndrome (enhanced response to anticholinergics)

  • Patients with prostate hypertrophy (due to anticholinergic burden)

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IX. Drug Interactions

Drug/Class	Effect / Risk
MAO inhibitors (with phenylephrine)	Hypertensive crisis
Tricyclic antidepressants	Potentiation of sympathomimetics
Antihistamines / antipsychotics	Additive anticholinergic burden
Beta-blockers (topical or systemic)	May reduce IOP elevation risk
Carbonic anhydrase inhibitors	Use in patients at risk of elevated intraocular pressure

X. Mydriatics vs Cycloplegics

Feature	Mydriatics (Sympathomimetics)	Cycloplegics (Anticholinergics)
Pupil dilation	Yes	Yes
Cycloplegia	No	Yes
Duration	Short (3–6 hrs)	Longer (6 hrs to 2 weeks)
Common use	Diagnostic dilation	Pediatric refraction, uveitis
Examples	Phenylephrine	Atropine, Tropicamide, Cyclopentolate

XI. Clinical Practice Tips

• Always measure intraocular pressure before and after using mydriatics in glaucoma-susceptible patients

• Inform patients not to drive until effects wear off

• Provide sunglasses to reduce photophobia post-exam

• For children: use lowest effective concentration, apply punctal occlusion to minimize systemic absorption

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XII. Special Populations

Pediatrics

• Cyclopentolate or atropine preferred for cycloplegic refraction

• Higher risk of systemic toxicity; avoid 10% phenylephrine

• Monitor for behavioral or CNS effects

Elderly

• Use lower concentrations

• Screen for glaucoma risk

• Monitor for cardiovascular side effects (especially with phenylephrine)

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XIII. Formulations and Commercial Brands

Active Ingredient	Brand Names
Tropicamide	Mydriacyl, Tropicacyl
Phenylephrine	Neo-Synephrine, Altafrin
Cyclopentolate	Cyclogyl
Atropine	Isopto Atropine
Homatropine	Isopto Homatropine
Combo (Tropicamide + Phenylephrine)	Mydrin-P, Cyclomydril

XIV. Storage and Handling

• Store at controlled room temperature (15–30°C)

• Protect from light

• Discard opened bottles within 30 days unless otherwise specified

• Use aseptic technique when applying

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XV. Alternative and Adjunctive Uses

• Atropine 0.01% in myopia control (off-label, emerging clinical use)

• Phenylephrine to break posterior synechiae in uveitis

• Cyclopentolate for pain control in corneal abrasions (reduces ciliary spasm)