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Preservatives in Eye Drops

Definition and Purpose

• Effectively inhibit a broad spectrum of microorganisms
• Be non-toxic to ocular surface tissues
• Be stable and soluble at required concentrations
• Not interfere with the active drug

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Commonly Used Preservatives

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Mechanism of Toxicity of BAK

1. Disruption of the tear film lipid layer - reduces tear film stability, causes faster evaporation
2. Damage to corneal epithelial cells - increases cell permeability, alters tight junctions, triggers apoptosis
3. Goblet cell loss - reduces mucin secretion, worsening dry eye
4. Neurogenic inflammation - damages corneal nerve endings, causes neuropathy
5. Trabecular meshwork toxicity - relevant in long-term glaucoma patients
6. Pro-inflammatory activation - upregulates inflammatory cytokines (IL-1, TNF-α), activates macrophages and mast cells
7. Sub-conjunctival fibrosis - compromises bleb function after filtration surgery

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Side Effects / Adverse Effects

Ocular Surface

• Punctate epithelial keratopathy (PEK) - most common finding; stains with rose bengal or fluorescein
• Toxic ulcerative keratopathy - in severe cases
• Dry eye syndrome / Keratoconjunctivitis sicca (KCS) worsening
• Conjunctival hyperemia and chemosis
• Follicular conjunctivitis (especially with thimerosal)
• Squamous metaplasia of conjunctival epithelium
• Goblet cell loss - reduced conjunctival mucin
• Cicatricial changes - subconjunctival fibrosis with chronic exposure
• Corneal sub-basal nerve fiber loss (detected on confocal microscopy)

Allergic/Immune-Mediated

• Contact allergy / allergic blepharoconjunctivitis - especially thimerosal
• Giant papillary conjunctivitis with chronic use

Clinical Consequences

• Compromised filtration surgery outcomes - BAK-induced conjunctival fibrosis reduces bleb success in trabeculectomy
• Poor compliance - symptoms cause patients to discontinue glaucoma drops
• Iatrogenic OSD in glaucoma patients - most clinically significant group (using multiple preserved drops chronically)

Systemic Effects

• All ophthalmic medications can be systemically absorbed via nasolacrimal drainage
• BAK: mild systemic toxicity at ophthalmic doses
• Chlorobutanol: CNS depression in large amounts (rarely relevant)
• Thimerosal: mercury toxicity with prolonged use (main reason it was discontinued)

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Risk Factors for Preservative Toxicity

• High frequency of instillation (>3-4 times/day)
• Multiple preserved preparations used simultaneously (additive BAK exposure)
• Pre-existing ocular surface disease (dry eye, blepharitis)
• Punctal occlusion (reduces drainage, increases contact time)
• Contact lens wear
• Long-term use (months to years, as in glaucoma)

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Treatment / Management

Principle: Reduce or eliminate preservative exposure

• First-line approach; use unit-dose (single-use) vials wherever possible
• PF artificial tears for dry eye
• PF versions of glaucoma drops (e.g., PF timolol, PF latanoprost, PF dorzolamide) are available
• Multi-dose PF systems (e.g., ABAK filter technology) - eliminate preservative at the nozzle

• Substitute BAK-containing drops with Polyquad-, Purite-, or SofZia-containing equivalents when PF unavailable
• Polyquad and Purite exhibit significantly lower ocular surface toxicity than BAK
• Caveat: benefits vs BAK largely still being established (2024 review - PMID 39098762)

• Fixed-dose combination drops (e.g., dorzolamide/timolol) - one bottle = less total BAK exposure
• Laser procedures (SLT - selective laser trabeculoplasty) to reduce need for topical drops
• Incisional surgery (trabeculectomy, MIGS) to allow drop reduction

• Preservative-free artificial tears / lubricants (hyaluronic acid, CMC)
• Topical cyclosporin A 0.05% (Restasis) - for inflammatory OSD
• Lifitegrast drops - LFA-1 antagonist for dry eye
• Punctal plugs (once preservative burden reduced) - increase tear retention
• Warm compresses and lid hygiene - for meibomian gland dysfunction component
• Bandage contact lenses - for severe corneal epithelial breakdown
• Autologous serum drops - for severe, refractory OSD

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Key Exam Points (High-Yield)

1. BAK is the most common and most studied toxic preservative in ophthalmic formulations
2. Mechanism: detergent disruption of lipid bilayer + inflammatory activation
3. Most common sign: punctate epithelial keratopathy (PEK)
4. Most at-risk group: glaucoma patients on multiple chronic drops
5. Newer preservatives (Polyquad, Purite, SofZia) are better tolerated but not entirely innocuous
6. Treatment cornerstone: switch to preservative-free formulations; use fixed combinations; consider laser/surgical alternatives
7. Thimerosal is largely obsolete due to allergy and mercury toxicity
8. Post-trabeculectomy: chronic BAK exposure causes conjunctival fibrosis that reduces surgical success
9. Preservative-free rule of thumb: use PF when drops needed >3-4 times/day, in dry eye, or after ocular surface surgery

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• Kanski's Clinical Ophthalmology: A Systematic Approach, 10th Ed. - Tear substitutes section
• Goodman & Gilman's Pharmacological Basis of Therapeutics - Ophthalmic toxicity
• Katzung's Basic and Clinical Pharmacology, 16th Ed. - Preservatives
• Kahook et al. "Preservatives and ocular surface disease: A review." Ocular Surface, 2024 (PMID 39098762) - a recent comprehensive review confirming current clinical recommendations

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Intraoperative Floppy Iris Syndrome (IFIS)

Definition

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Classic Triad (Must Know)

Not all three features need to be present - many patients show only one or two components.

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Etiology and Pathophysiology

Primary Cause - Alpha-1 Adrenergic Antagonists

1. Pharmacological blockade of dilator muscle - prevents effective mydriasis
2. Structural changes / atrophy of iris dilator smooth muscle with chronic use
3. Loss of iris tone and rigidity - the iris becomes flaccid ("floppy")

Drug-Specific Risk (Incidence of IFIS)

Why Stopping the Drug Does NOT Help

• IFIS has been reported years after drug discontinuation (Nguyen et al., 2007)
• Chronic pharmacological inhibition leads to permanent structural atrophy of iris dilator muscle
• Stopping tamsulosin before surgery does NOT significantly reduce IFIS risk
• Therefore: always ask about past use of alpha-blockers, not just current use

Other Reported Causes (Less Common)

• Other medications: finasteride, minocycline, antipsychotics (with α-blocking properties)
• Pseudoexfoliation syndrome (associated with poor pupillary dilation)
• Small pupil from any cause

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Clinical Significance / Why It Matters

• Posterior capsule rupture - most feared; can lead to vitreous loss and poor visual outcomes
• Iris trauma / iris damage - from phaco tip or instruments
• Vitreous loss - secondary to capsule rupture
• Dropped nucleus - lens material into vitreous
• Endophthalmitis risk increases with vitreous loss
• Worse visual outcomes overall

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Preoperative Assessment

History (Critical)

• Always ask about alpha-blocker use (current AND past) in any male patient undergoing cataract surgery
• Age group: elderly men with BPH are the typical demographic
• Drug history must specifically prompt for tamsulosin, silodosin, alfuzosin, doxazosin, terazosin, prazosin

Kanski's Tip:

"Systemic alpha-blockers (e.g. tamsulosin) are the main cause of intraocular floppy iris syndrome when undertaking phacoemulsification."

• Kanski's Clinical Ophthalmology, 10th Ed.

Preoperative Workup Additions

• Biometry as usual
• Examine pupil size and dilation response preoperatively
• Inform the surgical team so appropriate adjuncts are prepared

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Management

Principle: Anticipate, Prepare, and Adapt

A. Preoperative Strategies

1. Do NOT stop alpha-blocker (stopping does not reduce risk and may worsen BPH)
2. Warn the patient about increased surgical risk and possible complications
3. Maximize preoperative mydriasis:
   • Tropicamide 1% + phenylephrine 2.5-10% (standard mydriatics)
   • Consider adding NSAIDs (e.g., ketorolac, diclofenac drops) preoperatively - reduce intraoperative miosis by blocking prostaglandin release
   • Atropine 1% preoperatively (longer-acting mydriatic)
4. Consider early referral to experienced vitreoretinal surgeon if posterior capsule rupture risk is deemed high

B. Intraoperative Strategies

Pharmacological

• Intracameral phenylephrine (e.g., 1:10,000 or 1:100,000 dilution) - injected into anterior chamber to directly dilate iris; bypasses systemic alpha-blockade
• "Shugarcaine" (intracameral cocktail): preservative-free lidocaine 1% + bisulfite-free epinephrine in BSS - provides analgesia + pupil dilation + iris stabilization; commonly used in IFIS
• Intracameral atropine - used in some centers
• Viscoelastics (OVDs - ophthalmic viscosurgical devices):
  • Dispersive OVD (e.g., Viscoat - sodium hyaluronate + chondroitin sulfate): used to coat and "cushion" the iris, prevent prolapse
  • Technique: "soft-shell" technique - dispersive OVD placed under a cohesive OVD to push iris back

Mechanical / Surgical Adjuncts

Surgical Technique Modifications

• Use smaller phaco tip to reduce fluid turbulence
• Reduce irrigation flow rates and aspiration - lower vacuum settings to minimize iris flutter
• Use cohesive OVD to maintain anterior chamber depth
• Avoid wound distortion - well-constructed incisions prevent iris prolapse
• Consider smaller capsulorhexis initially
• Bimanual microincision phacoemulsification (MICS) - smaller incisions reduce iris prolapse tendency
• Have a vitreoretinal surgeon on standby or alert for complex cases

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Key Exam Points - High Yield Summary

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• Kanski's Clinical Ophthalmology: A Systematic Approach, 10th Ed. - Systemic preoperative assessment
• Campbell-Walsh-Wein Urology, 3-Volume Set - Alpha-blocker complications
• Barash's Clinical Anesthesia, 9th Ed. - Ophthalmic anesthesia; IFIS management
• Lippincott Illustrated Reviews: Pharmacology - Alpha-blocker adverse effects

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Botulinum Toxin (Botox) in Ophthalmology

Background

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Mechanism of Action

1. BoNT is produced by Clostridium botulinum (gram-positive, anaerobic)
2. Injected toxin binds to presynaptic cholinergic neurons
3. Enters the cell by endocytosis
4. Cleaves SNARE proteins (specifically SNAP-25 for type A) - prevents docking and fusion of acetylcholine-containing vesicles
5. Results in flaccid paralysis of injected muscle (skeletal) and reduced activity at cholinergic autonomic synapses
6. Effect onset: 3-7 days; maximal effect: 1-2 weeks; duration: 3-4 months
7. Recovery: requires nerve sprouting (not receptor regeneration) - hence reversible but slow

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Ophthalmic Indications

1. Strabismus (Chemodenervation)

• Takes several days to develop
• Maximal at 1-2 weeks post-injection
• Usually worn off by 3 months
• ~16% adults and 25% children develop temporary ptosis as side effect

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2. Benign Essential Blepharospasm (BEB)

• Meige syndrome = blepharospasm + oromandibular dystonia
• Brueghel syndrome = similar combination

• Botulinum toxin injection: 2.5-5 units injected subcutaneously at 3-4 periocular sites (orbicularis oculi)
• Affords relief in ~95% of patients
• Repeat injections required every 3 months
• Surgery (myectomy) reserved for non-responders/intolerant patients

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3. Hemifacial Spasm

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4. Spastic / Involutional Entropion

• BoNT injected into the lower lid pre-tarsal orbicularis muscle
• Temporarily paralyzes the overriding orbicularis
• Useful as temporary measure or in patients unfit for surgery
• Effect lasts 3-4 months; may need repeat

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5. Endocrine (Thyroid) Orbitopathy (Graves' Ophthalmopathy)

• Upper eyelid retraction - injection into Müller's muscle (superior tarsal muscle) or levator aponeurosis via conjunctival approach; reduces lid retraction
• Restricted extraocular muscles - injection into fibrotic/overacting muscle (e.g., inferior rectus causing hypotropia) as alternative or bridge to surgery
• Useful in active phase when surgery is contraindicated

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6. Facial Palsy (CN VII Palsy)

• Protective ptosis: BoNT into levator palpebrae superioris → intentional ptosis to protect cornea from exposure keratopathy (lagophthalmos)
• Used while awaiting recovery of facial nerve function
• Alternative to tarsorrhaphy (reversible)

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7. Convergence Spasm (Spasm of Near Reflex)

• Rare; presents with intermittent esotropia, miosis, and accommodative spasm
• BoNT to medial recti can provide relief

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8. Cosmetic Uses (Oculoplastic)

• Glabellar lines (frown lines between brows) - procerus and corrugator supercilii muscles
• Crow's feet (lateral orbital rhytids) - lateral orbicularis oculi
• Brow lift - injection of frontalis inferior fibers to achieve brow elevation
• Hyperfunctional forehead lines

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9. Other / Emerging Uses

• Gustatory hyperlacrimation (Crocodile tears) - after aberrant regeneration of CN VII; BoNT into lacrimal gland
• Chronic migraine with ocular features - periorbital injection sites
• Nystagmus - retrobulbar injection (experimental, rarely used)
• Dacryocystorhinostomy (DCR) adjunct - prevent pump failure
• Brow ptosis - corrugator/procerus injection for brow lift

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Contraindications

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Side Effects / Complications

Local (at injection site)

• Ptosis (most common - from spread to levator) - especially in strabismus and brow/glabellar injections
• Lagophthalmos (inadequate lid closure) - from orbicularis paralysis
• Diplopia - from spread to adjacent extraocular muscles
• Dry eye - from orbicularis paralysis reducing blink/pumping
• Ecchymosis / bruising at injection site
• Epiphora (watering eye) - reduced blink
• Ectropion - lower lid

Systemic (rare - due to distant spread)

• Generalized weakness
• Dysphagia, dysphonia
• Respiratory paralysis (rare but potentially fatal) - FDA black box warning
• Flu-like symptoms

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Dosing Reference (Approximate)

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High-Yield Exam Summary

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• Kanski's Clinical Ophthalmology, 10th Ed. - Botulinum Toxin (strabismus), Blepharospasm section
• Goodman & Gilman's Pharmacological Basis of Therapeutics - Botulinum Toxin mechanism and ophthalmic use
• Leszczynska et al. "Five decades of the use of botulinum toxin in ophthalmology." Indian J Ophthalmol, 2024 (PMID 38317306) - comprehensive recent review