Contact lenses Flashcards
Why use contact lenses?
To correct visual defects: myopia, hyperopia, astigmatism Personal appearance Convenience Activity To increase peripheral vision Therapeutic necessity
Emmetropia, hyperopia and myopia
Emmetropia: normal eye
Hyperopia: longsighted
Myopia: shortsighted
Contraindications
History of allergy History of ocular infection/inflammation Insufficient tear production Diabetes, hypothyroid, pregnancy Exposure to dust/chemicals Inability to manipulate lenses Other medication- systemic/topical
The ideal lens
Allows good visual acuity Good oxygen permeability Good wettability Does not allow deposits to form Not prone to scratching/breaking Safe- not detrimental to eye health Comfortable and easy to use
Hard corneal lenses
Since late 1940s Polymerised products of acrylic acid and methacrylic acid (PMMA) Have small diameter Not very permeable to oxygen V rarely used now- superseded by RGP
Rigid Gas Permeable lenses
Small- smaller than diameter of cornea Good optical properties Safe Decreases myopia progression in young people Durable Uncomfortable Poorer wetting than soft lenses
Materials for rigid gas permeable lenses
Cellulose acetate butyrate (CAB)- moderate permeability, good wetting, prone to scratching/warping
Silicone/acrylate (S/A): hydrophobic but flexible
Fluorine-silicone/acrylate (F-S/A): lens of choice, less deposits, less of problem with dryness, good permeability
Soft Hydrogel lenses
Flexible plastic lens
Conforms to shape of cornea
Absorbs and binds water (hydroxyl and lactam groups)
Better wetting than RGP
More adherence of environmental contaminants
HEMA
2-hydroxyethyl methacrylate Most commonly used Forms hydrophilic gel network Holds 38% water alone With other monomers can hold 55-70% Very stable Good wettability
Ionic charges and water contents
Ionic charge may increase deposit formation
Increased water content increases gas permeation, fragility and deposit formation
Advantages of soft lenses
Comfort Easier to insert Reduced spectacle blur vs. RGP Less likely to trap particles Less likely to dislodge (larger size) Disposable options available
Disadvantages of soft lenses
Less efficient at attaining visual acuity
Absorption problems
More fragile than RGP (esp. if dried out)
Problems of non-compliance
Problems with some eye drops
Complications of use- lens deposits
E.g. cosmetics, cations, dust, lysozyme film, tear proteins, oils
Causes: allergy, decreased visual acuity, decreased antimicrobial activity of solutions, irritation, discolouration
Can lead to conjunctivitis, keratitis, corneal ulceration
Complications of use- infection (bacterial)
Source can be lens, lens case, solution
Bacterial e.g. S. aureus, P. aeruginosa, attaches to surface of lens, can lead to corneal ulceration
Complications of use- infection(fungal)
e.g. candida, fusarium solani
Can penetrate soft lenses to infect the cornea
Patient complains of irritation and blurred vision
Complications of use- infection (other)
Acanthamoeba- rare but serious infection by protozoa, found in water, causes irreversible keratitis
HIV and vCJD- unlikely but potential infection route through trial sets
Complications of use- discolouration
Lenses can get discoloured by: Local/systemic drugs e.g. rifampicin, sulphasalazine Tobacco Cosmetics Buffering agents e.g. sorbic acid
Contact lens care
Lenses accumulate secretions from eye, substances from fingers, cosmetics etc.
Should be cleaned daily, immediately following removal, and disinfected nightly
Cases should also be cleaned regularly
Don’t top up solution in cases- replace each time
NB daily disposable soft lenses remove need for cleaning (hygiene still important)
Contact lens care- cleaning
Clean after every use to remove deposits e.g. grease/cosmetics
Cleaning also improves disinfection
Wash hands first
Surfactant and mechanical friction in palm of hand then rinse with saline
Enzymatic cleaner also recommended to remove protein deposits (weekly)
Contact lens care- saline
Used for rinsing/cleaning lens case/solvent for enzyme tablets
Aerosols and squeezable bottles
Some bottles contain preservatives- these are not sufficient for disinfection of lenses
Contact lens care- multi-purpose solutions
Contain surfactant + disinfectant
One bottle = more convenient
Still need to clean before disinfecting
No-rub formulations now available but studies suggest poorer disinfection
Not generally as effective as separate surfactant
Contact lens care- disinfection
Heat- can bake one deposits, not for RGP and high water soft lenses
Chemical- easier, cheaper, portable, can combine with cleaners
Oxidative- hydrogen peroxide- faster than chemical, preservative free
Disinfectant lens solutions
Needed to decrease microbial contamination
Should have wide range of antimicrobial activity
Must be sterile, chemically stable, non-irritant, isotonic
Must comply with Medical Device Directive (CE mark)
Disinfection- chemical solutions
Old products- low concentration preservatives
Chlorhexidine gluconate- highly effective bactericidal, not used with soft lenses
Thiomersal- effective vs bacteria and fungi
Benzalkonium chloride- wide activity vs bacteria and fungi, not used with soft lenses
Problems with sensitivity reactions
Disinfection- MPS
Usually polyquats from same family as chlorhexidine
E.g. polyhexanide, polyquad
High molecular weight- don’t penetrate lens
Less risk of toxicity
Disinfection- oxidative
Hydrogen peroxide
Efficacious at 3%, good vs acanthamoeba
Must neutralise before reinsertion
Two-step: soak in peroxide (overnight) then neutralise (10-20 minutes)
One step: peroxide and neutraliser (time release catalase tablet or platinum coated disc)
Other components of contact lens solutions
Chelating agents- EDTA/disodium edetate, increases preservative action of other components
Buffering agents- e.g. borate, phosphate, reduces discomfort
Contact lens care- wetting
For RGP, increases comfort and makes reinsertion easier
Often polyvinyl alcohol-based comfort drops
