contact lens

1. CORNEAL RESPONSE TO
ANOXIA STRESS FROM
CONTACT LENS WEAR
Asst.Prof.Lt.Col.Theeratep Tantayakom, MD
Cornea and Refractive Surgery specialist
Phramongkutklao hospital

2. Corneal physiology and
metabolism

3. Epithelium
Tear film
Bowman’s layer
Stroma
Descemet’s
membrane
Endothelium

4. Tear film
 Cover the corneal surface
 Volume 6.5uL, thickness 7um

5. Consist of 3 layers;
 A superficial lipid layer;
0.1um
 An aqueous layer;
7um
 A mucinous layer;
0.05um

6. Component:
 More than 98% is water
 Also contains many biologically ions and
molecules

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

11. Corneal stroma
 Largest portion of the cornea;
90% of the corneal thickness

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

13. Function
 Maintain corneal strength, stability of shape,
and transparency

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

29.  Tear production change
• Decrease in the tear breakup times

30. Epithelial effects of hypoxia
 Epithelial metabolic rate reduction
• Metabolism is reduced because of a 15%
decrease in oxygen uptake
• Cell synthesis is reduced

31.  Epithelial morphology changes
• Epithelial thinning
• Epithelial cell size increase

32. • Epithelial microcysts
• Fewer microvilli
• Desquamation of corneal epithelial cells
 Effect on vision

33.  Epithelial defects
• Loosening the epithelial tight junctions
• Decrease in hemidesmosome synthesis
 Separating the corneal epithelial cells
 Enhancing the risk of infection

34.  Neovascularization
• The progression of limbal hyperemia and the
penetration of vessels into the cornea

35. • Several factors;
o Metabolic factors; hypoxia, lactic acid, edema
o Angiogenic suppression
o Vasostimulation
o Neural control

36.  If neovascularization is extensive;
 Corneal scarring
 Lipid deposition
 Intracorneal hemorrhage

37.  Corneal hypoesthesia
• A decrease in corneal sensation
• Adaptation to chronic hypoxia

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

48. Symptoms of corneal hypoxia
 Red eyes
 Eye irritation

49.  Tearing
 Sensitive to light
 Vision change and unstable

50.  Management;
 Discontinuing lens use
 Refitting with a lens of higher Dk
 Reducing hours of lens use

51. Immune-events from contact
lens wear
 Allergic conjunctivitis
 Giant papillary conjunctivitis

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

59. Contact lens materials and
manufacturing
 Contact lens parameters;
• Wettability
• Oxygen permeability
• Lens deposition

60.  Material choice will affect;
• Flexibility
• Contact lens comfort
• Stability
• Quality of vision

61.  Silicon monomers;
• Bulky molecular structure
 Create a more open polymer architecture

62.  Fluorine
 Increase the gas solubility of polymers
 Counteract the tendency of silicon to bind
hydrophobic debris to the contact lens
surfaces

64. Thank you for
your attention