CORNEAL RESPONSE TO ANOXIA STRESS FROM CONTACT LENS WEAR

Asst.Prof.Lt.Col.Theeratep Tantayakom, MDCornea and Refractive Surgery specialistPhramongkutklao hospital

Corneal physiology and metabolism

Epithelium

Tear film

Bowman’s layer

Stroma

Descemet’smembrane

Endothelium

Tear film

Cover the corneal surface

Volume 6.5uL, thickness 7um

Consist of 3 layers;

A superficial lipid layer; 0.1um

An aqueous layer; 7um

A mucinous layer; 0.05um

Component:

More than 98% is water

Also contains many biologically ions and molecules

Function

Protect the cornea from dehydration

Maintain the smooth epithelial surface

Source of nutrients for the corneal epithelium

Corneal epithelium

Nonkeratinized, stratified, squamous cells

Thickness 50um (10% of total thickness)

Consists of 5-6 layers of epithelial cells

Only the basal cells of the epithelium proliferate

The cells differentiate and gradually emerging at the corneal surface

The differentiation process requires 7-14 days

Function:

Provide a barrier to external stimuli

Maintain the trilayered structure of the tear film

Corneal stroma

Largest portion of the cornea; 90% of the corneal thickness

uniform arrangement of collagen fiber +

The mean diameter of collagen fibers & distance between such fibers – less than half of the wavelength of visible light

Allowing light to pass through the cornea

Function

Maintain corneal strength, stability of shape, and transparency

Endothelium

A single layer of corneal endothelial cells

Thickness: 5um

Shape: hexagonal

Contain a large nucleus and abundant mitochondria: metabolically active

Function

Endothelial pump: active transport of ion/water

Maintain the stromal deturgescence(relatively dehydrated)

Stromal transparency

Oxygen and nutrient supply

Corneal epithelial and endothelial cells are metabolically active

Glucose and oxygen are essential to maintain the normal metabolic functions of the cornea

Glucose

Diffusion from the aqueous humor

Oxygen

Diffusion from tear fluid, which absorbs oxygen from the air

Direct exposure of tear fluid to the atmosphere is thus essential for oxygenationof the cornea

Physiologic changes due to prolonged eyelid closure

In the closed-eye environment:

o Disruption of the oxygen supply to the cornea

o Oxygen at corneal surface from 21% (at a partial pressure of 155mmHg) to 8% (at 55 mmHg)

o Increase carbon dioxide : acidic pH

o Decrease tear volume

o Corneal edema

o Corneal endothelial bleb response

o Decrease corneal sensitivity

o Increase the microbial load on the conjunctiva and lid margins

Changes in the cornea caused by contact lens

A contact lens acts as a barrier to the supply of oxygen to the cornea

According to the structures affected:

Tear film

Epithelium

Stroma

Endothelium

According to the causes:

Hypoxia-mediated events

Immune events

Mechanical events

Hypoxia from contact lens wear

Reduction in oxygen supply to the cornea 8 –15% depending on the gas permeability of the lens material used

Oxygen permeability (Dk) = rate of oxygen flow through a given area of the material

D = the diffusion coefficient of the material

k = the solubility coefficient of the material

Oxygen transmissibility (Dk/L) = the rate of flow and relation of the lens thickness

L = the thickness of the lens

Unit = number x 10-9 (cm x ml O2)/(s x ml x mmHg)

In the open-eye conditions, the corneal oxygen demand requires at least 20 Dk/L

Daily-wear soft contact lenses should have a Dk/L of 20 to 34 to avoid inducing edema

Holden BA, Mertz GW. Invest Ophthalmol Vis Sci 1984; 25::1161-7

Harvitt DM, Bonanno JA. Optom Vis Sci 1999; 76: 712-29

The oxygen transmissibility necessary to avoid hypoxia in the closed eye is at least 75 Dk/L

Extended-wear soft contact lenses need a Dk/L of 75 to 89 to avoid inducing edema

Holden BA, Mertz GW. Invest Ophthalmol Vis Sci 1984; 25::1161-7

Harvitt DM, Bonanno JA. Optom Vis Sci 1999; 76: 712-29

Tear film effects of hypoxia

Tear film complements and pH change

• Increased secretory immunoglobulin A, albumin

• Increase number of polymorphonuclearleukocytes which are actively phagocytic

Tear production change

• Decrease in the tear breakup times

Epithelial effects of hypoxia

Epithelial metabolic rate reduction

• Metabolism is reduced because of a 15% decrease in oxygen uptake

• Cell synthesis is reduced

Epithelial morphology changes

• Epithelial thinning

• Epithelial cell size increase

• Epithelial microcysts

• Fewer microvilli

• Desquamation of corneal epithelial cells

Effect on vision

Epithelial defects

• Loosening the epithelial tight junctions

• Decrease in hemidesmosome synthesis

Separating the corneal epithelial cells

Enhancing the risk of infection

Neovascularization

• The progression of limbal hyperemia and the penetration of vessels into the cornea

• Several factors;

o Metabolic factors; hypoxia, lactic acid, edema

o Angiogenic suppression

o Vasostimulation

o Neural control

If neovascularization is extensive;

Corneal scarring

Lipid deposition

Intracorneal hemorrhage

Corneal hypoesthesia

• A decrease in corneal sensation

• Adaptation to chronic hypoxia

Stromal effects of hypoxia

Stromal acidosis

• Corneal metabolism changes from aerobic to anaerobic

consequent accumulation of lactic acid

Stromal edema

• Due to

A break in epithelial and endothelial barriers

A reduction in pump function

Increase osmotic activity of the stroma

Diameter and distance between collagen fibers becomes heterogeneous

Corneal edema

Then the cornea loses its transparency

Stromal thinning

• A chronic pathophysiologic change in patients who have worn contact lenses for years

• Correlated with degeneration and death of stromal keratocytes

Corneal shape alterations

• Result in corneal distortion or warpage

• More commonly associated with hard lens

• Contact lens with high oxygen transmissibility induce little warpage

Central irregular astigmatism

Radial asymmetry

Changes in the axis of astigmatism

Reversal of the normal pattern of progressive flattening from the center to the periphery

Resolve after discontinues wearing the lens

Endothelial effects of hypoxia

Endothelial bleb

• Appear as black, nonreflecting areas in the endothelial mosaic and as an increase in separation between cell

Polymegethism

• A greater-than-normal variation of corneal endothelial cell size

• Reduction in endothelial cell density

• Only the silicone elastomer contact lens, which has high gas permeability, does not lead to significant endothelial polymegetism

Endothelial function change

• Long term contact lens wear reduces endothelial functional reserve

• Correlates with the duration and transmissibility of the contact lens worn

The pump function is lost

The corneal stroma swells

Irregularity of the interfiber distance

Results in scattering of incident light

The cornea hazy

Symptoms of corneal hypoxia

Red eyes

Eye irritation

Tearing

Sensitive to light

Vision change and unstable

Management;

Discontinuing lens use

Refitting with a lens of higher Dk

Reducing hours of lens use

Immune-events from contact lens wear Allergic conjunctivitis

Giant papillary conjunctivitis

Corneal infection

• Rare , but potentially serious and vision threatening

Related to;

• Improper contact lens care/hygiene

• A poor lens fit

Reduce risk;

Fitted properly

Use contact lens care systems

Follow-up care

Patients should understand the signs and symptoms

Use of disposable lens

Better patient education

More convenient care systems

Use of more oxygen-permeable lens materials

Sterile infiltrates

Seen in the peripheral cornea

Often more than one spot

The epithelium over the spots is intact

Mechanical events from contact lens wear

Corneal abrasions

Result from;

Foreign bodies under a lens

A poor insertion/ removal technique

A damaged contact lens

Punctate keratitis

Related to;

A poor lens fit

A toxic reaction to lens solutions

Dry eye

Most of problems can be treated in one of the following ways;

Discontinuing lens use

Refitting at a later date after changing lens parameters, material, and Dk

Switching to disposable lenses

Decreasing lens wear

Contact lens materials and manufacturing

Contact lens parameters;

• Wettability

• Oxygen permeability

• Lens deposition

Material choice will affect;

• Flexibility

• Contact lens comfort

• Stability

• Quality of vision

Silicon monomers;

• Bulky molecular structure

Create a more open polymer architecture

Fluorine

Increase the gas solubility of polymers

Counteract the tendency of silicon to bind hydrophobic debris to the contact lens surfaces

Thank you for your attention