Corneal physiology in relation to contact lens wear

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  • 1.Corneal physiology in relation to contact lens wearResource person Gauri Shankar ShresthaPresenter Hira Nath Dahal 3rd year Optometry

2. Corneal anatomy Consists of 5 layers Epithelium Bowmansmembrane Stroma (Substantia propria) Descemets membrane (Posterior elastic lamina) Endothelium 3. Blood supply to the cornea 4. Nerve supply to the cornea 5. Corneal physiology Primarily concerned with 1. Source of energy of cornea that fuel the corneas metabolic activity 2. Corneal transparency and its maintenance 6. Corneal transparency Attributable to 1. Avascularity 2. Uniform index of refraction 3. Tight barrier of epithelium and endothelium 4. Lattice like arrangement of collagen fibrils in the corneal stroma 7. Corneal metabolism Structural integrity of the cornea is maintained by an active fluid transport system which depends on corneal metabolism. Normal metabolic process are essential for cell growth, replacement and in the case of the corneal epithelium and endothelium, for the maintenance of ionic pump mechanism, which is responsible for maintaining the state of corneal hydration. 8. The main nutrient needed for corneal metabolic activity is glucose, which is primarily sourced from aqueous humour. Amino acids, vitamins and minerals are also derived from this source Energy in the form of ATP is generated by the breakdown of glucose Glucose is the main substrate in carbohydrate metabolism, but a small supplementary carbohydrate supply is stored in the cornea by conversion of glucose to glycogen. 9. Aqueous: imp source for glucose also negligible amount of glucose enters the cornea from the tear film and by diffusion from the peri limbal capillaries Metabolism in the cornea involves three alternative pathways: Embden Meyerhof pathway(Glycolysis) Krebs tricarboxylic acid cycle Hexose -Monophosphate shunt (HMP) 10. Glycolysis is an oxygen free method of producing energy, but the yield is low. Also, the by product lactate is not so easily dispersed as the water and carbon dioxide. The eye uses this pathway under situations where oxygen availability is reduced The Krebs tricarboxylic acid cycle produces a comparatively high yield of energy and the byproducts of H20 and CO2 11. In the corneal epithelium, glucose is also metabolized through the HMP, but without the gain in ATP Purpose is the production of NADPH which is utilized in the biosynthesis of lipids by corneal epithelium. The ribose produced by the pentose shunt may be used to build nucleic acids, DNA & RNA 12. Oxygen required for essential metabolism of the cornea is primarily derived from the atmosphere via the tears and diffusion across the corneas anterior surface. Each layer of the cornea consumes oxygen at its own rate Epithelium: 40% Stroma: 39% Endothelium: 21% of total oxygen consumption of the cornea On the basis of volume of oxygen per unit volume of tissue, epithelial oxygen utilisation is about ten times that of the stroma and approx. 0.2 that of the endothelium 13. Disturbance of corneal metabolism Corneal Thickness Corneal Transparency Corneal Vascularization Mechanical or chemical damage and causes metabolic stress in the cornea 14. Corneal Hydration Water content of normal cornea is approx. 80% (highest water content of any connective tissue in the body) The health of a tissue depends upon the mechanical integrity of its component cells. In the case of the cornea, when these cells malfunction there is loss in transparency with the consequent reduction in visual performance and an increase in corneal thickness Impaired function of epithelium and endothelium results in a n increase in corneal hydration Much greater corneal hydration will result from damage of the endothelium, since this provides the facility of the ionic pump mechanism and the mechanical barrier to the flow of fluid from the aqueous to the cornea. 15. Sources of corneal oxygen LAYEROPEN EYECLOSED EYEEpitheliumAtmosphere Aqueous humourPalpebral conjunctiva Bulbar conjunctiva Aqueous humourStromaAqueous humour AtmosphereAqueous humourendotheliumAqueous humourAqueous humour 16. Oxygen tension levels vary across the cornea and within the cornea. The highest level is at the anterior surface. 17. Reduction in normal supply of oxygen to the corneal cells will result in significant alteration to the physiology of the cornea If there is not enough oxygen available to convert the glucose, by means of glycolysis, into sufficient energy and allow the waste product, lactic acid, to diffuse quickly out of the tissue, the less energy is available to the cellular activity. This results in too much lactic acid being produced, which builds up in the stroma, and so is implicated in the cause of corneal oedema by causing an osmotic imbalance 18. Corneal Temperature normal corneal temperature - 33-36 C corneal temperature has been shown to rise in the closed eye by about 3 C. Elevated corneal temperatures have been associated with increases in the anterior corneas rate of metabolic activity (Freeman and Fatt, 1973). 19. With SCLs, the lens anterior surface is about 0.5 C cooler than the cornea underneath. With an RGP lens, the anterior surface is slightly cooler still as a result of the lens lower thermal conductivity 20. What is EOP?? quantifies the corneal environment under a contact lens by ascertaining what oxygen concentration produces an identical corneal response to that produced by the lens. The oxygen pressure in the air corresponds to about 20.9% (or about 159 mmHg; that value is actually close to 155 mmHg because of the presence of water vapour) and each percentage point is equal to a pressure of about 7.4 mmHg. 21. It is an in vivo measurement made on living tissue and therefore provides a more accurate indication of corneal function. The results of the corneal oxygen consumption measurement provide an accurate indication of the oxygen need, or thirst, of the cornea. Any type of lens can be fitted and an assessment made of the effect on the corneal oxygen consumption rate. This permits a direct comparison of SCL and RGP lenses. 22. P02 in open and closed eye Open eye: Under the lids: 33.7 to 61.4 mm Hg Under contact lenses: 0 to 82.3 mm Hg In the anterior chamber: 40 to 59.7 mm Hg In the anterior chamber, contact lenses on: 25 to 75 mm Hg Ruben, 1975, Benjamin, 1994, Thiel, 1967, Fatt and Ruben, 1993 23. Closed-eye: At the central cornea: 50 to 67 mm Hg In the anterior chamber: 55 mm Hg Under contact lenses: 0 to 35 mm Hg Ruben, 1975, ONeal et al., 1983, Ichijima et al., 1998 24. To understand the effects of contact lenses on the structure and function of the cornea, it is necessary to consider its normal oxygen requirements under a variety of conditions both with and without lenses. 25. What is the minimum level of oxygen required by the cornea to maintain normal metabolic activity? Holden and Mertz attempted to define the minimum contact lens oxygen transmissibility required to meet the needs of various modes of contact lens wear. Physiologically, the ideal SCL, when worn on a daily wear (DW) basis, should cause zero corneal oedema (swelling). 26. To minimize its impact on the average cornea, the oxygen transmissibility of a SCL should be 24.1 2.7 x 10 -9 units based on the study of Holden and MertzFor a SCL to induce zero swelling during DW, the EOP value should be 9.9% for the average cornea 27. Oxygen Requirements During SCL Extended Wear: zero residual swellingFollowing overnight wear of an SCL, the cornea swells by an amount which is related to the oxygen transmissibility of the material. After eye opening, the cornea begins to thin as its metabolic activity increases due to the higher availability of oxygen. 28. According to Holden and Mertz (1984), the oxygen transmissibility of an SCL must be 34.3 x 10-9 units or an EOP of 12.1%. This Dk/t value limits the permissible overnight corneal swelling to approximately 8.0%. 29. Corneal oxygen requirements SCL Extended wear For overnight oedema = 4.0% Dk/t= 87.0 3.3 x 10-9 EOP of 17.9% 30. Minimum oxygen requirement CriterionMinimum 02 (%)Corneal swellingDW: 9.9%, EW: 17.9%Epithelial mitosis13.2%Epithelial healing10.4%Corneal sensitivity7.7%Glycogen depletion5%Endothelial blebs15-16.6% 31. Ocular environment in open and closed eye VariablesClosed eyeOpen eyeCornea (pH)7.397.55Tears (pH)7.257.45Temperature36.2034.50Tonicity (% NaCl)0.890.97O2 (mmHg)61.00155.00CO2 (mmHg)55.000.00 32. Cornea and contact lens Contact lens effectively occludes the cornea from its surrounding environment of oxygen, tears and ocular secretions. The effect depends upon lens thickness, size, method of fitting and material 33. Any reduction in the amount of oxygen available to a metabolically active tissue can significantly alter the physiological equilibrium of the component cells and therefore, the tissue itself. The cornea is no exceptionAn adequate supply of oxygen to the cornea is vital to its metabolic processes and the maintenance of its structural integrity. For successful contact lens wear, the lenses fitted must supply at least the minimum level of oxygen the cornea requires. 34. If the oxygen decreases below the critical level anaerobic glycolysis using the EmbdenMeyerhof pathway converts: Glucosepyruvate lactate Because lactate doesnt diffuse rapidly out of the cornea, the consequence of decreased aerobic metabolism is stromal lactate accumulation 35. Hypoxia thus creates: Loweredepithelial metabolic rate An increase in epithelial lactate production An acidic shift in pHAfter prolonged hypoxia: Depletion of the glycogen reserves of the cornea Diminished Adenosine Triphosphate (ATP) & ultimately a slowing of water transport system in the endothelium 36. A decrease in the pumping action of the endotheliumleading to corneal edema 37. Acute physiological changes to the corneaEpithelial thinning Hypoesthesia Superficial puncate keratitis Epithelial abrasions Stromal edema Endothelial blebs 38. Chronic changes Corneal neovascularisation Stromal thinning Corneal shape alterations Endothelial cell polymegathism Pleomorphism (signs of endothelial cell stress) 39. Corneal Swelling with Soft Contact Lenses All conventional soft contact lenses act as physical barriers to the supply of oxygen from the atmosphere and reduce the level of oxygen available to the cornea. Higher the water content the greater the oxygen permeability of the material. For a given water content, the thinner the lens, the greater the transmission of oxygen to the cornea. There is only minimal exchange of tears behind an SCL with each blink. This exchange therefore contributes a negligible proportion of the oxygen supplied to the cornea during SCL wear. 40. Contd.... As the thickness of a lens increases, its oxygen transmissibility decreases and a higher level of corneal swelling results. Over an eight-hour period of wear, an ultra-thin HEMA lens may induce only minimal swelling. A thicker lens such as a toric or a plus power (typical tc = 0.13 mm) induces about 8% corneal swelling. Centre thickness (tc)Swelling (%)0.1380.0750.031 41. In the case of high water content material (75%) High water content (75%), 8 hours weartc (mm)Swelling (%)0.320.150.5 42. Corneal swelling with SCL overnight wear Material (8 hrs wear)Swelling (%)Low Water12Mid Water10High Water11Siloxane Elastomer2.5 43. Corneal Swelling with RGP lenses RGP lenses do not impede the supply of oxygen to the cornea as significantly as do conventional SCLs. This is due to: Higher oxygen permeability of RGP materials. Lens design features, such as a smaller total diameter. Fitting characteristics, such as greater movement over the eye. 44. Effects of Corneal Hypoxia Any reduction in the supply of oxygen to the cornea can have significant effects on normal metabolic activity. The effects are numerous and they range from mild to severe in their impact on the cornea Reduced aerobic glycolysis Lactate accumulation (stroma) Stromal acidosis Osmotic imbalance Oedema (swelling) Structural changes 45. Other effects from reduced oxygen supply to the cornea include changes to the: Epithelial mitotic rate. Density of nerve fibre endings. Sensitivity of the cornea. Corneal pH. 46. Corneal Sensitivity and Contact Lens Wear The cornea is the most densely innervated tissue in the body. This innervations protects the eye by making it highly sensitive and responsive to foreign bodies, abrasion, etc. A minimum of 8% oxygen is required to maintain corneal sensitivity at or near a normal level for the average person (Millodot and OLeary, 1980) 47. Change in sensitivity d/t contact lens wear Corneal hypoesthesia is the first effect of hypoxia, of which patient is unaware about Epithelial acetylcholine is a neurotransmitter to corneal nerves and is decreased in hypoxia Decreased sensation is milder with soft contact lenses and the return of sensation is more rapid, compared with PMMA lenses. Oxygen level required to maintain the nerve fibres in their normal state is 910% Hamano (1985) 48. Corneal hypoesthesia is thought to be an adaptation to chronic hypoxia, to decreased corneal pH, or to mechanical stimulation and is correlated with levels of acetycholineCorneal sensation may be a more sensitive test than refraction, keratometry, or pachometry for monitoring the status of corneal health during contact lens wear. 49. Corneal pH and Contact Lens Wear Bonanno and Polse (1987) demonstrated that the corneal stromal environment becomes more acidic (lowered pH) in a range of circumstances including contact lens wear.It is postulated that a reduction (Holden et al., 1985) or changes (Williams, 1986) in corneal pH cause the endothelial bleb response. 50. Hypoxia and structural changes to the cornea Both short-term and chronic corneal hypoxia can result in significant changes in the structure and function of corneal tissue In most cases, even subtle changes are visible with the slit-lamp bio microscope. Epithelial and stromal oedema loss of transparency Microcysts and vacuoles Striae Folds Endothelial blebs Endothelial polymegethism Vascularization 51. Corneal tissue fragility Reduced epithelial adhesion is found following contact lens wear. It is related to the reduced numbers of hemidesmosomes, which is due to loss of basal cell shape and chronic corneal hypoxia following contact lens wear. 52. The hypoxia causes a decrease in the level of metabolic activity including the rate of cell mitosis. Cell life increases and those at anterior surface of the epithelium may not retain normal functional resistance. This will have the effect of compromising the epithelium with the increased likelihood of microbial penetration and subsequent infection. 53. Tear Film Osmolality Normal osmolality: 294-334 mOsm/litre (0.911.04%)Contact lens effects: Initialhard lens wear produces decreased tear osmolality brought about by reflex tearing. The cornea swells (mainly stromal) some 2-4%. Initial soft contact lens wear increases tear o...

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