Effects of Contact lens on corneal Physiology

Information about Effects of Contact lens on corneal Physiology

Published on August 6, 2014

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Effects of Contact lens on Corneal Physiology: Effects of Contact lens on Corneal Physiology Resource Person Mr. Niraj Dev Joshi Presenter Prativa Devkota B.Optom 3 rd year Anatomy: Anatomy PowerPoint Presentation: thinner in the center, 544 ± 34 μm (range: 440–650 μm ) increases in thickness in the periphery approximately 700 μm as it reaches the limbus studies over a 30-year time period showed that no significant age-related change in central corneal thickness change in thickness occurred beyond the infant years Blood Supply of Oxygen: Blood Supply of Oxygen Nerve supply of Cornea: Nerve supply of Cornea PowerPoint Presentation: Corneal Physiology is primarily concerned with: The source of energy which fuel the cornea’s metabolic activity. Corneal transparency and its maintenance. Source of Nutrients: Source of Nutrients OXYGEN • Atmosphere • Ophthalmic artery anterior chamber limbus palpebral conjunctiva Supply of Oxygen : Supply of Oxygen Bulbar conjunctival vessels may also contribute in a small way, since they can also be observed to be dilated at initial eye opening following a period of sleep Atmospheric source of oxygen is indirect since atmospheric oxygen must first dissolve into the tear film before it becomes available for use in the metabolic activities of even the most superficial layers of the cornea During eye closure, the vasculature of the palpebral conjunctiva plays an important role SOURCES OF CORNEAL OXYGEN : SOURCES OF CORNEAL OXYGEN LAYER OPEN EYE CLOSED EYE Epithelium atmosphere palpebral conjunctiva aqueous humor ? bulbar conjunctiva ? aqueous humor ? Stroma aqueous humor aqueous humor atmosphere ? Endothelium aqueous humor aqueous humor The highest level of oxygen level across the cornea is at the anterior surface. : The highest level of oxygen level across the cornea is at the anterior surface. Other source of Nutrients: Other source of Nutrients Glucose Glycolysis pathway Hexose Monophosphate Pathway Amino acid Supplied through aqueous humour through passive diffusion Corneal Transparency: Corneal Transparency Attributable to both the lattice-like arrangement of collagen fibrils in the corneal stroma and the transparency of cells that reside in the cornea Tear Osmolarity: Tear O smolarity the salt content of the tear film is about 10% greater than that of freshly produced tears due to evaporation. When the eye is closed during sleep, there is a shift in tear tonicity from the open eye value of about 0.97% NaCl to 0.89% NaCl following 6 hours sleep Corneal Temperature: 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 cornea’s rate of metabolic activity (Freeman and Fatt , 1973). Therefore, the use of contact lenses in the closed eye presents a physiological challenge to the cornea largely not lens related a routine factor in eye closure exacerbated by the presence of contact lenses PowerPoint Presentation: 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 Oxygen Permeability: Oxygen Permeability referred to as the Dk. D is the diffusion coefficient - a measure of how fast dissolved molecules of oxygen move within the material k is a constant representing the solubility coefficient or the number of oxygen molecules dissolved in the material defined as 'the rate of oxygen flow under specified conditions through unit area of contact lens material of unit thickness when subjected to unit pressure differences' Contd.. : Contd.. Dk value is a physical property of a contact lens material It is not a function of the shape or thickness of the material sample, but varies with temperature 10~n cm2/s ml O2/ml X mmHg (usually referred to as Fatt units), now also known as barrers . Oxygen Transmissibility: Oxygen Transmissibility The Dk /t for a particular lens under specified conditions defines the ability of the lens to allow oxygen to move from anterior to posterior surface. value of t is generally an average lens thickness for powers between ±3.00 dioptres (D). not a physical property of a contact lens material, but is a specific characteristics related to the sample thickness Dk /t, with Fatt units of 10~9cm/s ml O2/ml X mmHg and ISO units of cm/s ml O2/ml hPa Corneal Oxygen Requirements SCL Daily Wear: Corneal Oxygen Requirements SCL Daily Wear For zero daytime swelling: • Dk / t = 24.1 ± 2.7 x 10-9 • EOP of 9.9% Corneal Oxygen Requirements SCL Extended Wear: Corneal Oxygen Requirements SCL Extended Wear For overnight oedema = 4.0% • Dk / t = 87.0 ± 3.3 x 10-9 • EOP of 17.9% Corneal Oxygen Requirements SCL Extended Wear: Corneal Oxygen Requirements SCL Extended Wear For zero residual swelling at eye closure: • Dk / t = 34.3 ± 5.2 x 10-9 • EOP of 12.1% HYPOXIA AND HYPERCAPNIA FROM CONTACT LENS WEAR: HYPOXIA AND HYPERCAPNIA FROM CONTACT LENS WEAR The Po2 required by the human cornea for normal function is considered to be at least 75 mm Hg If the oxygen decreases below a critical level anaerobic glycolysis using the Embden -Meyerhof pathway Glucose pyruvate lactate HYPOXIA AND HYPERCAPNIA FROM CONTACT LENS WEAR: HYPOXIA AND HYPERCAPNIA FROM CONTACT LENS WEAR Because lactate does not diffuse rapidly out of the cornea, the consequence of decreased aerobic metabolism is stromal lactate accumulation. Hypoxia thus creates : lowered epithelial metabolic rate an increase in epithelial lactate production an acidic shift in stromal pH. Contd..: Contd.. After prolonged corneal hypoxia, depletion of the glycogen reserves of the cornea diminished adenosine triphosphate (ATP) and ultimately a slowing of the water transport system in the endothelium. Accumulation of lactic acid in the stroma a decrease in the pumping action of the endothelium Corneal Edema Acute physiologic changes to the cornea: Acute physiologic changes to the cornea epithelial thinning hypoesthesia superficial punctate keratitis epithelial abrasions stromal edema endothelial blebs. Chronic Changes: Chronic Changes corneal neovascularization stromal thinning corneal shape alterations endothelial cell polymegathism pleomorphism (signs of endothelial cell stress) Contd..: Contd.. Hard contact lenses cause the same acute and chronic physiologic changes to cornea as soft contact lenses while corneal shape alternations due to mechanical pressure on the anterior corneal surface is more observed Effect of Contact Lens Materials on Tear Physiology : Effect of Contact Lens Materials on Tear Physiology The contact lens causes CLDE by interrupting tear film reformation rupturing the lipid layer increases in tear film evaporation, disrupting normal tear physiology through thinning and break up of the tear film (in the case of soft lenses) by per-evaporation of fluid from the corneal tissue . CLDE was ranked the third major problems after lens deposits and the patient compliance Effect of Contact Lens Materials on Tear Physiology LEE CHOON THAI, BSc , MCOptom , ALAN TOMLINSON, DSc , FCOptom , FAAO, and MARSHALL G. DOANE, PhD Department of Vision Sciences, Glasgow Caledonian University, Scotland (LCT, AT), Schepens Institute, Boston, Massachusetts (MGD Contd..: Contd.. each blink only exchanged about 1.1% of the tear volume when SCL were worn. Polse (1979) On the other hand, RGPs exchanged some 10 to 20 times this amount It is estimated that 10-20% of the tear film behind an RGP lens is exchanged for fresh, oxygenated tears following a blink smaller SCLs (TD: 12mm) resulted in greater exchange rates but even these lenses only gave ‘modest’ results when compared with RGP contact lenses TEAR FILM EFFECTS OF CONTACT LENSES : TEAR FILM EFFECTS OF CONTACT LENSES • Evaporation rates: SCLs ≈ RGPs • CLs reduce BUT • BUT: - RGPs - 4 to 6 s SCLs - 4 to 10 s Tissue fragility : Tissue fragility Reduced epithelial adhesion is found following contact lens wear. reduced numbers of hemidesmosomes due to loss of basal cell shape and chronic corneal hypoxia following contact lens wear. Tissue fragility : Tissue fragility Manifested decreases in corneal electrical potential punctate staining epithelial abrasion an increased risk of microbial infection Epithelial Mitosis: Epithelial Mitosis The hypoxia causes a decrease in the level of metabolic activity including the rate of cell mitosis. Cell life span increases and those at the anterior surface of the epithelium may not retain normal functional resistance. As a result of these changes, the overall resistance of the epithelium is lowered and the risk of infection increased. • 9% oxygen required to prevent: - suppression of mitosis - accumulation of lactate in the anterior chamber Epithelial Thinning: Epithelial Thinning The thickness of the epithelium is found to be reduced as cell production rate and wastage reach a new equilibrium. decrease in the number of cell layers and the appearance of cuboidal rather than columnar basal cells no intracellular edema, and the epithelium actually thins by approximately 6% The epithelial thickness is reduced to nearly one third its normal thickness in areas of contact lens bearing . Corneal Hypothesia: Corneal Hypothesia Important and the first effects of hypoxia, of which the patient is unaware, is a drop in corneal sensitivity Corneal Hypothesia: Corneal Hypothesia 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. the oxygen level required to maintain the nerve fibres in their normal state is 9–10%. Hamano (1985) PowerPoint Presentation: 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 acetycholine Corneal sensation may be a more sensitive test than refraction, keratometry , or pachometry for monitoring the status of corneal health during contact lens wear. Epithelial Metabolic Rate Reduction: Epithelial Metabolic Rate Reduction With extended-wear soft contact lenses, the epithelial metabolism is reduced because of a 15% decrease in oxygen uptake With decreased pumping ability, increased permeability of the epithelial cells can result in dehydration. Epithelial Morphology Changes: Epithelial Morphology Changes With extended-wear soft contact lenses, the mean corneal epithelial cell size is affected most. mature cells have fewer microvilli and less mucin , more sites are available for possible bacterial adhesion Epithelial Microcysts : Sign: Epithelial Microcysts : Sign appear as small (10 - 50 μm , average 20 μm ), usually circular , occasionally irregular, translucent, refractile dots’ sign of altered epithelial metabolism. usually located in the central and paracentral corneal regions differentiated from other dot-like corneal features by virtue of their location, i.e. epithelial as opposed to stromal or deeper locations). Epithelial microcysts: Epithelial microcysts Can occur in: dystrophies inflammations infections chronic hypoxia • Common in SCL EW • Also occurs commonly in non-wearers • Low count regarded as ‘acceptable’ critical levels are between 30 (Gottschalk, 1988) and 50(Zantos,1984B ). Microcyst :Contd…: Microcyst : Contd … microcysts are usually asymptomatic. Visual acuity is unaffected unless the number of microcysts approaches 200 ( Efron , 1999). Zantos and Holden (1978) reported one case in which visual acuity decreased by one line due to the presence of a large number of microcysts . Microcyst :Contd…: Microcyst : Contd … Pathologic examination of microcysts shows degenerated epithelial cells (apoptotic cells), probably from dysfunction of the basal cells of the epithelium, with cellular degeneration and lysis . Microcyst :Contd…: Microcyst : Contd … Stromal Effects of hypoxia: Stromal Effects of hypoxia Stromal acidosis Stromal edema Stromal thining Neovascularisation Corneal shape alteration Stroma Acidosis: Stroma A cidosis hypercapnia accounts for about 30% of the total pH drop accumulation of stromal lactic acid during anaerobic metabolism. Respiratory acidosis is caused by the accumulation of carbon dioxide ( hypercapnia ) because the gasimpermeable contact lens precludes normal efflux of carbon dioxide. Under open-eye conditions, the human stromal pH increases by 0.15 to 7.55. decrease by as much as 0.25 during wear of soft contact lens of nearly zero oxygen transmissibility. Corneal Swelling: Corneal Swelling during sleep because the oxygen available to the cornea is reduced by approximately two-thirds by the coverage of the lids. overnight swelling range from 2.9% (du Toit et al., 1998) or 3% (Sweeney, 1991) to 5.5% (Harper et al., 1996). such oedema is greater centrally than peripherally, and occurs in an anterior-posterior Stromal Edema: Stromal Edema break in epithelial or endothelial barriers, reduction in pump function (mainly endothelial), or An increase in osmotic activity ( imbibition pressure) of the stromal Corneal Swelling: Corneal Swelling Generally asymptomatic unless corneal swelling is significant • Reduced vision: - diffusive ‘spectacle’ blur - haziness, haloes, coloured haloes - little or no Rx change detectable SCL Overnight Wear : SCL Overnight Wear Material (8 hrs wear) Swelling (%) Low Water 12 Mid Water 10 High Water 11 Siloxane Elastomer 2.5 RGP CORNEAL SWELLING : RGP CORNEAL SWELLING Material (8 hrs wear) Swelling (%) Low Dk 10-13 Mod Dk 7-9 High Dk 5-6 Corneal Swelling with RGP lenses : 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 significant exchange of tears that takes place with each blink Corneal swelling: Corneal swelling With extended-wear soft contact lenses, the increase in stromal thickness occurs significantly more in the center than in the periphery because of hypoxia. With current soft contact lenses and RGP lenses, unadapted patients usually have daytime corneal edema of 1% to 6% and nighttime edema of 10% to 15%, as measured on awakening. With extended-wear lenses, overnight edema averages 10% to 12% Corneal swelling of 5% or less seems to indicate that the cornea can tolerate 7 nights of extended wear Contd…: Contd … The level of anterior corneal hypoxia induced by the new siloxane hydrogels appears to be less than with conventional hydrogels ( Fonn et al., 1999B, Levy et al., 2000, Sweeney et al., 2000). Overnight oedema is also significantly less than with conventional hydrogels and approximates that of no lens wear ( Fonn et al., 1999B, Sweeney, 1999). Corneal swelling : Striae: Corneal swelling : Striae striae appear as fine, whispy , greyish , whitish or translucent corneal lines in the central to mid-peripheral, posterior Stroma Striae were postulated to be the result of stromal oedema (Wechsler, 1974) Corneal swelling : striae: Corneal swelling : striae The level of corneal swelling required to produce striae is of the order of 4-6% (Holden and Swarbrick , 1989). They found that a count of 10 striae represented 11% ±2% corneal swelling .La Hood and Grant (1990) The advent of siloxane hydrogels has lowered further the incidence of striae in contact lens wearers Corneal Oedema: Folds and Black Lines: Corneal Oedema : Folds and Black Lines The level of corneal swelling required to produce folds and possibly black lines is7% to 12%. Black lines should be regarded as a clear sign of oedema exceeding clinically acceptable levels (Holden and Swarbrick,1989). significant stresses generated within the cornea result in folds appearing in the posterior stroma adjacent to Descemet’s membrane Endothelial Fold: Endothelial Fold The black lines seen with the slit-lamp represent a decreasing and/or buckling of the posterior stroma , Descemet’s membrane, and the endothelium. The stresses producing the buckling in the deeper corneal layers are the result of the stroma being pushed in a posterior direction by significant corneal oedema . The stresses, plus the restrictions on expansion imposed laterally on the cornea by the sclera at the limbus , mean that the cornea must fold or buckle to accommodate its swollen self Stromal Thinning: Stromal Thinning Whereas stromal thinning is regarded as a chronic response to corneal oedema , stromal swelling is regarded as an acute response chronic oedema that may lead to the dissolution of polysaccharide( glycosaminoglycans ) ground substance in the stroma ( Efron , 1999). Stromal Thinning: Stromal Thinning Holden et al. 1985 reported a thinning of 11µm over an approximately 5 year period, i.e. about 2µ m per year of lens wear Thinning of the stroma has been demonstrated to occur in long-term, SCL, extended wear patients Stromal Thinning: Stromal Thinning Thinning by 2% may be a sequelae of chronic stromal edema correlated with degeneration and possible death of stromal keratocytes A study with the Orbscan ( Orbscan Inc., Salt Lake City, UT) topography system showed that the mean corneal thickness in the center and in eight peripheral areas was significantly reduce by approximately 30 to 50µm in long-term soft contact lens wearers compared with noncontact lens wearing control subjects Corneal Shape Alterations: Corneal Shape Alterations Topographic abnormalities were detected in 75% of corneas with PMMA lens wear, 57% with RGP lens wear, 31%with daily-wear soft lenses, and 23% with extended-wear soft lenses, compared with 8% of normal corneas without contact lens wear. These changes can cause spectacle blur or contact lens decentration . The curvature changes are more apparent in the horizontal than in the vertical meridian Changes are less marked with soft contact lenses, which induce a slight corneal flattening in the first 2 or 3 weeks, followed by a period of relative steepening. Corneal Neovascularisation: Corneal Neovascularisation produced as a response to metabolic or an angiogenic factor by mature existing blood Vessels It is especially common with large and thick contact lenses and results in development of new corneal vessels in up to 20% of wearers Steps in the Neovascularization process: Steps in the Neovascularization process (1) Limbal hyperemia, a dilatation of existing limbal capillaries, is reversible and is common with hydrogel soft contact lenses worn overnight but can also occur with any tightly fitting contact lens (2) superficial neovascularization ( pannus ) is the progression of limbal hyperemia and the penetration of vessels into the superficial cornea (3) deep stromal neovascularization results from chronic hypoxia that may progress to an active inflammatory or fibrovascular deep pannus (4) there may be an intracorneal Prevalence of Neovascularization: Prevalence of Neovascularization The prevalence of neovascularization is low with RGP or PMMA contact lenses more common with daily-wear soft contact lenses, higher with extended-wear soft contact lenses, and very high with aphakic extended-wear lenses Polymegathism: Polymegathism derivative of the Greek words ‘many’ (poly) ‘size’ ( megethos ) Thus, literally means ‘many sizes while some cells get smaller, others enlarged to leave the average largely unaltered Polymegathism : Aetiology: Polymegathism : Aetiology occurs with increasing age the ongoing wear of contact lenses with low oxygen transmissibilities , corneal surgical procedures involving significant corneal incisions, i.e. corneal trauma Prevalence of Neovascularization : contd..: Prevalence of Neovascularization : contd.. Polymegethism is one of the features of the corneal exhaustion or fatigue syndrome Recovery from contact lens-induced endothelial polymegethism is slow, and the condition may be irreversible, even after cessation of contact lens wear Endothelial polymegethism places the cornea at greater risk for surgical complications Endothelial Bleb: Endothelial Bleb Blebs appear as very small, circumscribed, irregularly-shaped, black zones obscuring the cellular mosaic when viewed with a slit-lamp using specular reflection Blebs form within minutes (certainly within 10 minutes) of the application of a lens (especially if it has a relatively low transmissibility) response peaks after about 20 to 30 minutes Endothelial Blebs: Endothelial Blebs Pathologic examination of blebs shows edema of the nuclear endothelial cells, with intracellular fluid vacuoles and fluid space between cells blebs occur with conventional and disposable contact lenses of similar oxygen transmissibility, but their occurrence is minimal or absent with silicone elastomer contact lenses. blebs are asymptomatic and are thought to be of little clinical significance; they represent a short-term as well as long-term adaptation of the endothelium Corneal Exhaustion syndrome: Corneal Exhaustion syndrome Result of long-term wear of lenses with no, or low,O2transmissibility - PMMA (most likely) - low water, toric SCLs - low water, high BVP spherical SCLs • Sudden development of lens intolerance • All layers of the cornea affected symptoms: symptoms • Altered/irregular refraction • Distorted keratometer mires • Acute oedema • Posterior stromal haze/opacities • Endothelial changes - polymegethism - distortion/bumpiness Symptoms: Symptoms • Reduced lens comfort • Decreased wearing time • Blurred or fluctuating vision Corneal Response to Orthokeratolgy: Corneal Response to Orthokeratolgy After 1 day, statistical significance flattening was noted which progressed to reach 0.22 + 0.07mm (1.19 + 0.38 D) at 28 days Central epithelial thinning was apparent reaching statistical significance on day,(7.11 + 7.1µm; 9.6%) mid –peripheral corneal thickening ,2.5 mm form central cornea was statistical significant by day 13.0 + 11.1µm Conclusion: Conclusion Very thin, high-water content hydrogel soft contact lenses provide improved oxygen transmissibility but not to the level required to maintain normal epithelial aerobic metabolism. can induce corneal desiccation, have inadequate durability, and are difficult to handle Silicone elastomer contact lenses have yet to attain successful clinical performance in terms of surface chemistry, comfort, and maintenance of lens movement for any group of patients except aphakic infants and children Contd..: Contd.. New lenses such as the silicone hydrogel and fluorosilicone hydrogel hybrid lenses are in trial and have the potential to overcome some of these physiologic limitations True daily-wear disposable contact lenses may also overcome other issues with regard to contact lens safety but will remain expensive for many patients References: References Physiologic Changes of the Cornea with Contact Lens Wear Thomas J. Liesegang , M.D. The IACLEContact Lens Course MODULE 6 The Cornea in Contact Lens Wear The IACLEContact Lens Course MODULE 7 Contact Lens-Related Complications , First Edition

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