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Lecture 6: Contact Lenses: Are they really safe?

What does the cornea need to be transparent?
No blood vessels
Not many cells
How does the cornea get what it needs?
Most nutrients come the inside of the eye (aqueous)
Gas exchange from the atmosphere - oxygen is taken up and CO2 is released
Tear exchange
Sweeping action of lids
Correct environmental conditions
Eye and brain resist inflammation
• Inflammation is bad for cornea
o Causes tissue destruction and can lead to scarring to cornea
o Front of eye constantly bombarded by things that can inflammation but strongly resists it
Two major lens types - Hard (rigid) CLs
o Not flexible
o Tear exchange under lens (~10%)
o Don't cover entire cornea
Two major lens types - Soft CLs
o Flexible, take up shape of cornea
o Very little tear exchange under lens (~1%)
o Cover entire and extend over conjunctiva
Vessel Ingrowth
When your eye does not have enough oxygen and yoru eye will send signals when it needs oxygen, if not applied, then blood vessels will converge to cornea to supply as much oxygen
Surface scratches
Can be caused from debris that had got caught in eye and when you blink, it scratches surface of cornea
Cornea infection
• Bacteria scattering
o Why cornea is transparent?
• Immune cells make up opaque part of eye
Contact Lens infections
• Mostly soft contact lenses
• Extended wear is a risk factor
o More nights worn, increase of contact lens infection
• Most worrisome cause
o Pseudomonas aeruginosa
• Really good at destroying cornea
How do bacteria get into the eye?
• Pseudomonas is everywhere
• Solutions are often contaminated
• Bacteria stick to contact lenses
o Transfers bacteria into eye

• Normally eyes resists infection by these bacteria
o Lens wear MUST compromise this resistance
• Or else contact lens infections wouldn't happen
• Normally we're healthy but lens wears changes situation
o Whats normally resistant is not anymore
How do we figure out why lenses cause infection so we can prevent it?
1. Study how the eye normally resists infections when lens is not worn
2. Then determine how contact lens wear reduces that resistance
How is disease caused in the eye?
bacteria must get past the multi-cell layered epithelium to get into the underlying stroma
--> once it get in stroma, bacteria gets direct disruption with calligen
--> Alarms white cells, immune response and bacteria destroying stroma
How do bacteria get to the stroma?
• Directly if stroma is exposed
o Eg after a scratch
• By first traveling through epithelium that has lost the ability to resist
o Likely happens with contact lens wear
Two types of P. aeruginosa bacteria
While some P. aeruginosa strains invade cells (invasive strains)
Cytotoxic P. aeruginosa strains kill cells (don't invade cells)
Epithelial cells of the cornea play critical roles in defense
Passive and active roles
• Passive defense 1: Tight junctions between cells provide physical barrier
• Passive defense 2: the glycocalyx
-> The glycocalyx (white squiggly line) repels bacteria
• Passive Defense 3: cell sloughing removes infected cells
Epithelial cells also have active defenses
1. Toll like (TLR) receptor recognizes bug components
2. Signal sent to nucleus
3. Defense factor (s) secreted
Filter paper damage + bacteria =
no infection
Scratch to strome + bacteria =
What doesn't epithelial injury always predispose to infection?
Because there are other defenses
Tear fluid - fluid that bathes the ocular surface
Basal Lamina - the basement membrane underlying the epithelium
Three critical defense layers
Basal Lamina
--> Redundancy
All three need to be compromised to get infection
The Basal Lamina of the Cornea
Separates the epithelium from the cornea
- Pores are smaller than bacteria
- Acts as a filter
--> basal lamina may be the key, explains filter paper injury, resistant to infection
Why do CLs cause infection?
• Is it lack of oxygen (hypoxia)?
o Some thought so..
o But new lenses allow lots of oxygen through and haven't stopped infections
• 2 cents worth:
o when changing CL
• infection follows changes to the biochemistry of the ocular surface under the lens
changes to molecules on cell surfaces or
changes to molecules in tear fluid where it makes contact with the surface cells
Ocular Surface Biochemistry - What would alter it?
1. Compartmentalization of tear fluid
2. Changes to tear production
3. Trapping of bugs/debris
4. Modification fo tear components after they are produced
Ocular Surface Biochemistry - Consequences
- Changes to tear defense
- Effects on corneal cells and basal lamina defenses
- Tears normally maintain epithelium cells
- Epithelium cells in tuen make the basal lamina
Contact lens-related infections
My opinion (prof): all 3 defense layers must be compromised to get infection
Lens wear might do this by changing ocular surface biochemistry