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PH3114 > Contact lenses > Flashcards

Flashcards in Contact lenses Deck (28):
1

Why use contact lenses?

To correct visual defects: myopia, hyperopia, astigmatism
Personal appearance
Convenience
Activity
To increase peripheral vision
Therapeutic necessity

2

Emmetropia, hyperopia and myopia

Emmetropia: normal eye
Hyperopia: longsighted
Myopia: shortsighted

3

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

4

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

5

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V rarely used now- superseded by RGP

6

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

7

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

8

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

9

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

10

Ionic charges and water contents

Ionic charge may increase deposit formation
Increased water content increases gas permeation, fragility and deposit formation

11

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

12

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

13

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

14

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

15

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

16

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

17

Complications of use- discolouration

Lenses can get discoloured by:
Local/systemic drugs e.g. rifampicin, sulphasalazine
Tobacco
Cosmetics
Buffering agents e.g. sorbic acid

18

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)

19

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)

20

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

21

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

22

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

23

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)

24

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

25

Disinfection- MPS

Usually polyquats from same family as chlorhexidine
E.g. polyhexanide, polyquad
High molecular weight- don't penetrate lens
Less risk of toxicity

26

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)

27

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

28

Contact lens care- wetting

For RGP, increases comfort and makes reinsertion easier
Often polyvinyl alcohol-based comfort drops