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corneal response to anoxia stress from contact lens wear
1. CORNEAL RESPONSE TO
ANOXIA STRESS FROM
CONTACT LENS WEAR
Asst.Prof.Lt.Col.Theeratep Tantayakom, MD
Cornea and Refractive Surgery specialist
Phramongkutklao hospital
7. Function
Protect the cornea from dehydration
Maintain the smooth epithelial surface
Source of nutrients for the corneal epithelium
8. Corneal epithelium
Nonkeratinized, stratified, squamous cells
Thickness 50um (10% of total thickness)
Consists of 5-6 layers of epithelial cells
9. 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
10. Function:
Provide a barrier to external stimuli
Maintain the trilayered structure of the tear
film
12. 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
14. Endothelium
A single layer of corneal
endothelial cells
Thickness: 5um
Shape: hexagonal
Contain a large nucleus and
abundant mitochondria:
metabolically active
15. Function
Endothelial pump: active transport of
ion/water
Maintain the stromal deturgescence
(relatively dehydrated)
Stromal transparency
16. 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
17. Glucose
Diffusion from the aqueous humor
Oxygen
Diffusion from tear fluid, which absorbs
oxygen from the air
18. Direct exposure of tear fluid to the
atmosphere is thus essential for oxygenation
of the cornea
19. 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)
20. 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
21. Changes in the cornea caused by
contact lens
A contact lens acts as a barrier to the supply
of oxygen to the cornea
22. According to the structures affected:
Tear film
Epithelium
Stroma
Endothelium
23. According to the causes:
Hypoxia-mediated events
Immune events
Mechanical events
24. 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
25. 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)
26. 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 OphthalmolVis Sci 1984; 25::1161-7
Harvitt DM, Bonanno JA. OptomVis Sci 1999; 76: 712-29
27. 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 OphthalmolVis Sci 1984; 25::1161-7
Harvitt DM, Bonanno JA. OptomVis Sci 1999; 76: 712-29
28. Tear film effects of hypoxia
Tear film complements and pH change
• Increased secretory immunoglobulin A,
albumin
• Increase number of polymorphonuclear
leukocytes which are actively phagocytic
30. Epithelial effects of hypoxia
Epithelial metabolic rate reduction
• Metabolism is reduced because of a 15%
decrease in oxygen uptake
• Cell synthesis is reduced
38. Stromal effects of hypoxia
Stromal acidosis
• Corneal metabolism changes from aerobic to
anaerobic
consequent accumulation of lactic acid
39. Stromal edema
• Due to
A break in epithelial and endothelial barriers
A reduction in pump function
Increase osmotic activity of the stroma
40. Diameter and distance between collagen
fibers becomes heterogeneous
Corneal edema
Then the cornea loses its transparency
41. Stromal thinning
• A chronic pathophysiologic change in
patients who have worn contact lenses for
years
• Correlated with degeneration and death of
stromal keratocytes
42. Corneal shape alterations
• Result in corneal distortion or warpage
• More commonly associated with hard lens
• Contact lens with high oxygen
transmissibility induce little warpage
43. 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
44. Endothelial effects of hypoxia
Endothelial bleb
• Appear as black, nonreflecting areas in the
endothelial mosaic and as an increase in
separation between cell
45. 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
46. Endothelial function change
• Long term contact lens wear reduces
endothelial functional reserve
• Correlates with the duration and
transmissibility of the contact lens worn
47. The pump function is lost
The corneal stroma swells
Irregularity of the interfiber distance
Results in scattering of incident light
The cornea hazy
52. Corneal infection
• Rare , but potentially serious and vision
threatening
Related to;
• Improper contact lens care/hygiene
• A poor lens fit
53. Reduce risk;
Fitted properly
Use contact lens care systems
Follow-up care
Patients should understand the signs and
symptoms
54. Use of disposable lens
Better patient education
More convenient care systems
Use of more oxygen-permeable lens
materials
55. Sterile infiltrates
Seen in the peripheral cornea
Often more than one spot
The epithelium over the spots is intact
56. Mechanical events from
contact lens wear
Corneal abrasions
Result from;
Foreign bodies under a lens
A poor insertion/ removal technique
A damaged contact lens
57. Punctate keratitis
Related to;
A poor lens fit
A toxic reaction to lens solutions
Dry eye
58. 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