Lecture 6: Contact Lenses: Are they really safe?

Created by melvingarcia 

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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

Bugs
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 =

infection

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

Tears
Epithelium
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

Direct
- Changes to tear defense
Indirect
- 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

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