Effects of long-term contact lens wear on the cornea: Difference between revisions
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{{short description|Overview of the effects of long-term contact lens wear on the cornea}} |
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{{Update|inaccurate=yes|talk=Not up-to-date and not neutral|reason=The hypoxia issues raised by this article might not apply to today's [[Contact lens#Soft lenses|Silicone Hydrogel]] contact lenses|date=October 2021}} |
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| ⚫ | Long-term [[contact lens]] use can lead to alterations in corneal thickness, stromal thickness, curvature, corneal sensitivity, cell density, and epithelial oxygen uptake, etc. Other changes may include the formation of epithelial [[vacuole]]s and [[Microcyst|microcysts]] (containing cellular debris) as well as the emergence of [[polymegethism]] in the [[corneal endothelium]]. Decreased corneal sensitivity, vision loss, and [[photophobia]] have also been observed in patients who have worn contact lenses for an extended period of time. Many contact lens-induced changes in corneal structure are reversible if contact lenses are removed for an extended period of time. |
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Knowledge concerning the form and function of the [[cornea]] and the various types of [[Contact lens#Types|contact lenses]] and their common [[Contact lens#Complications|complications]] is important to understanding this article. |
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== Form and function of the cornea == |
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[[File: Gray871.png|thumb|'''Layers of the Cornea''' |
[[File: Vertical section human cornea-Gray871.png|thumb|'''Layers of the Cornea''' |
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(1) [[Corneal epithelium|Epithelium]] |
(1) [[Corneal epithelium|Epithelium]] |
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(2) [[Anterior elastic lamina]] |
(2) [[Anterior elastic lamina]] |
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(5) [[corneal endothelium|Endothelium]] of the [[anterior chamber]]]] |
(5) [[corneal endothelium|Endothelium]] of the [[anterior chamber]]]] |
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The cornea is the clear, outermost layer of the eye that overlays the [[Anterior chamber of eyeball|anterior chamber]], [[Iris (anatomy)|iris]], and [[Pupil (eye)|pupil]]. It functions primarily as (1) a protective barrier to the rest of the eye by shielding against dust, bacteria, and other foreign agents, and (2) an exterior lens that helps focus entering light onto the retina. Additionally, the cornea serves to filter out certain harmful UV rays. Absolute transparency is a critical feature of the cornea and so it is an avascular region that relies on [[tears]] around its anterior face and [[aqueous humor]] around its posterior face for nourishment and protection against infection. Furthermore, because of its lack of blood vessels, oxygen is directly absorbed by the cornea rather than delivered through hemoglobin. Each of the five layers (in order from outermost to innermost) of the cornea serves an essential function.<ref name="Facts About The Cornea and Corneal Disease">{{cite web|title=Facts About The Cornea and Corneal Disease|url=http://www.nei.nih.gov/health/cornealdisease/|publisher=National Institutes of Health}}</ref> |
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'''[[Corneal epithelium|Epithilium]]''' – Blocks entry of foreign particles such as dust, debris, and bacteria. Absorbs and distributes oxygen and nutrients from tears to the rest of the cornea. The epithelial cells attach to and organize themselves around a basement membrane. |
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'''[[Bowman's membrane|Bowman’s Layer]]''' – Primarily composed of [[collagen]], which helps the cornea maintain its shape. |
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'''[[Substantia propria|Stroma]]''' – Constitutes 90% of the cornea’s thickness. Its main components are water and collagen and it helps maintain the cornea’s shape and elasticity. The arrangement of collagen within the stroma plays an important role in maintaining transparency and assisting in light conduction. |
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'''[[Descemet's membrane|Descemet’s Membrane]]''' – Composed primarily of collagen fibers and protects against injury and infection. |
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'''[[corneal endothelium|Endothelium]]''' – Extremely thin, innermost layer that is essential for maintaining corneal transparency. It pumps excess fluid out of the corneal stroma to prevent swelling, which would interfere with the clarity of vision. |
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== Contact lens types == |
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Contact lenses can be roughly classified under two main categories: soft contact lenses and hard contact lenses. Soft contact lenses are generally made using a flexible [[polymer]]-plastic material with water, allowing for oxygen permeability. Additionally, some are capable of providing UV protection. Soft contact lenses often come in the form of daily disposables or extended wear disposables (made of silicone hydrogel and usable for up to 30 days). In contrast, rigid gas permeable contact lenses are much more durable than their soft counterparts and may require daily wear in order to adjust to. Today, rigid gas permeable lenses are primarily made of silicone polymers, which are conducive to oxygen circulation.<ref name="Eye Health and Contact Lenses">{{cite web|title=Eye Health and Contact Lenses|url=http://www.webmd.com/eye-health/contact-lenses-colored-soft-hard-toric-bifocal|publisher=WebMD}}</ref> |
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PMMA lenses ([[polymethyl methacrylate]]) are another form of rigid contact lenses that are impermeable to oxygen. When PMMA lenses are worn, oxygen is delivered to the eye only after being dissolved into tears. Developed in the 1960s, PMMA lenses are rarely prescribed today because of the ubiquity of both soft and hard contact lens alternatives that offer greater comfort. Nevertheless, they are still favored by some people for their durability and cheap price.<ref name="Contact Lenses">{{cite web|title=Contact Lenses|url=http://www.kellogg.umich.edu/patientcare/conditions/contact.lenses.html|publisher=University of Michigan Kellogg Eye Center}}</ref> Other subcategories of contact lenses include bifocal (for correcting [[presbyopia]]), [[Toric lens|toric]] (for correcting astigmatism) and corneal reshaping contact lenses.<ref name="Eye Health and Contact Lenses" /> |
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== General risks == |
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[[File:Clare-314.jpg|thumb|'''[[Keratitis]]''', or an inflammation of the cornea]] |
[[File:Clare-314.jpg|thumb|'''[[Keratitis]]''', or an inflammation of the cornea]] |
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In general, contact lens use is thought to be safe as long as the proper precautions are adhered to. The most prominent risks associated with contact lens wear include increased epithelial permeability, bacterial adherence, microcysts, corneal [[edema]], and endothelial polymegathism.<ref name="What's the Best Prescription for Healthy Contact Lens Wear?">{{cite web|title=What's the Best Prescription for Healthy Contact Lens Wear?|url=http://www.clspectrum.com/article.aspx?article=12590|publisher=Contact Lens Spectrum}}</ref> Mishandling of contact lenses can also cause [[corneal abrasion]]s. When induced by contact lenses, corneal abrasions can progress to bacterial keratitis and cause corneal perforations, scarring, and vision impairment.<ref name="Corneal Abrasion in Emergency Medicine">{{cite web|title=Corneal Abrasion in Emergency Medicine|url=http://emedicine.medscape.com/article/799316-overview#a0199|publisher=Medscape Reference}}</ref> Furthermore, decreased corneal sensitivity following extended contact lens wear may increase a person’s susceptibility to becoming infected without being aware of it.<ref name="Effect of Long-term Wear of Hard Contact Lenses on Corneal Sensitivity" /> |
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Increased [[myopia]] has also been observed in patients following long-term contact lens wear.<ref name="What's the Best Prescription for Healthy Contact Lens Wear?" /> |
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== Changes in function and morphology == |
== Changes in function and morphology == |
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The effects of extended contact lens wear on the cornea have been studied extensively and are well-documented. When determining the effects of long-term contact lens use on the cornea, many studies do not differentiate between users of hard and soft contact lenses, while studies that have made this differentiation have found similar results. This is probably because most contact lens-induced changes to the cornea are caused by [[Hypoxia (medical)|hypoxia]], which occurs as long as any physical barrier to the surface of the cornea is present. In certain instances, hard contact lenses were shown to cause the same changes in corneal structure as soft contact lenses, though these changes were more dramatic because rigid lenses are capable of inflicting greater trauma on the eyes.<ref name="The effects of long-term contact lens wear on corneal thickness, curvature, and surface regularity">{{cite journal|last=Liu|first=Z.| |
The effects of extended contact lens wear on the cornea have been studied extensively and are well-documented. When determining the effects of long-term contact lens use on the cornea, many studies do not differentiate between users of hard and soft contact lenses, while studies that have made this differentiation have found similar results. This is probably because most contact lens-induced changes to the cornea are caused by [[Hypoxia (medical)|hypoxia]], which occurs as long as any physical barrier to the surface of the cornea is present. In certain instances, hard contact lenses were shown to cause the same changes in corneal structure as soft contact lenses, though these changes were more dramatic because rigid lenses are capable of inflicting greater trauma on the eyes.<ref name="The effects of long-term contact lens wear on corneal thickness, curvature, and surface regularity">{{cite journal|last=Liu|first=Z.|author2=Pflugfelder, S.|title=The effects of long-term contact lens wear on corneal thickness, curvature, and surface regularity|journal=Ophthalmology|date=January 2000|volume=107|issue=1|pages=105–111|doi=10.1016/S0161-6420(99)00027-5|pmid=10647727}}</ref> |
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== Structural change == |
== Structural change == |
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Long-term use of soft hydrogel contact lenses has been shown to alter the following in the cornea: epithelial oxygen uptake, epithelial thickness, stromal thickness, and corneal endothelial morphology. Furthermore, the formation of epithelial vacuoles and microcysts has been observed following long-term contact lens wear.<ref name="Effects of long-term extended contact lens wear on the human cornea.">{{cite journal|last=Holden|first=B.A.| |
Long-term use of soft hydrogel contact lenses has been shown to alter the following in the cornea: epithelial oxygen uptake, epithelial thickness, stromal thickness, and corneal endothelial morphology. Furthermore, the formation of epithelial vacuoles and microcysts has been observed following long-term contact lens wear.<ref name="Effects of long-term extended contact lens wear on the human cornea.">{{cite journal|last=Holden|first=B.A.|author2=Sweeney, B.F. |author3=Vannas, A. |author4=Nilsson, K.T. |author5=Efron, N. |title=Effects of long-term extended contact lens wear on the human cornea.|journal=Invest. Ophthalmol. Vis. Sci.|date=November 1985|volume=26|issue=11|pages=1489–1501|pmid=3863808 }}</ref> Vacuoles are fluid-filled chambers that begin to appear one week after extended contact lens use begins; their number increases over time with extended contact lens wear. Microcysts tend to appear three months after contact lens wear begins and increase in number over time as long as contact lens wear resumes.<ref name="Epithelial and endothelial effects from the extended wear of contact lenses.">{{cite journal |last1=Holden|first1=BA |last2=Vannas|first2=A |last3=Nilsson|first3=K |last4=Efron|first4=N |last5=Sweeney|first5=D |last6=Kotow|first6=M |last7=La Hood|first7=D |last8=Guillon|first8=M |title=Epithelial and endothelial effects from the extended wear of contact lenses.|journal=Curr. Eye Res.|date=June 1985|volume=4|issue=6|pages=739–42|doi=10.3109/02713688509017678|pmid=2992884 }}</ref> On average, over five times as many epithelial microcysts than normal have been observed in long-term contact lens wearers.<ref name="Effects of long-term extended contact lens wear on the human cornea." /> |
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Among patients who have worn soft hydrogel contact lenses for over a year, significant reductions in epithelial oxygen uptake, epithelial thickness, and stromal thickness have been recorded, while an increase in endothelial |
Among patients who have worn soft hydrogel contact lenses for over a year, significant reductions in epithelial oxygen uptake, epithelial thickness, and stromal thickness have been recorded, while an increase in endothelial polymegethism was found.<ref name="Effects of long-term extended contact lens wear on the human cornea." /> In patients who had worn contact lenses for approximately five years or more, a 30 to 50 μm reduction in central and peripheral corneal thickness has been recorded. Furthermore, the reduction was more pronounced in patients wearing hard contact lenses than in patients wearing soft contact lenses. Increased endothelial polymegethism is also found in long-term wearers of rigid gas permeable lenses as soon as one week after contact lens wear begins. This change is indicated by significant increases in Max/Min cell size ratio in contact lens wearers.<ref name="Corneal Endothelial Polymegethism and Pleomorphism Induced by Daily-Wear Rigid Gas-Permeable Contact Lenses">{{cite journal|last=Esgin|first=H.|author2=Erda, N.|title=Corneal Endothelial Polymegethism and Pleomorphism Induced by Daily-Wear Rigid Gas-Permeable Contact Lenses|journal=CLAO Journal|date=January 2002|volume=28|issue=1|pages=40–43|pmid=11838988 }}</ref> Endothelial pleiomorphism is another factor that arises from long-term use of rigid gas permeable lenses; significant decreases in hexagonal cells are noted after one year, accompanied by increased numbers of cells of other than six sides.<ref name="Corneal Endothelial Polymegethism and Pleomorphism Induced by Daily-Wear Rigid Gas-Permeable Contact Lenses" /> |
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Increased corneal curvature is yet another change known to arise from long-term contact lens wear;<ref name="The effects of long-term contact lens wear on corneal thickness, curvature, and surface regularity" /> this increase in corneal curvature can be as much as 0.5 diopters greater than normal.<ref name="Contact Lens-Induced Corneal Curvature and Thickness Changes">{{cite journal|last=Miller|first=D.|title=Contact Lens-Induced Corneal Curvature and Thickness Changes|journal=Arch Ophthalmol.| |
Increased corneal curvature is yet another change known to arise from long-term contact lens wear;<ref name="The effects of long-term contact lens wear on corneal thickness, curvature, and surface regularity" /> this increase in corneal curvature can be as much as 0.5 diopters greater than normal.<ref name="Contact Lens-Induced Corneal Curvature and Thickness Changes">{{cite journal|last=Miller|first=D.|title=Contact Lens-Induced Corneal Curvature and Thickness Changes|journal=Arch. Ophthalmol.|date=October 1968|volume=80|issue=4|pages=430–432|doi=10.1001/archopht.1968.00980050432004|pmid=5674798 }}</ref> Corneal surface irregularity and asymmetry are also caused by long-term contact lens wear; these problems are sometimes correlated with astigmatism in contact lens wearers and are thought to be caused by hypoxia, surface molding, and chronic and mild trauma to the cornea from contact lens use.<ref name="The effects of long-term contact lens wear on corneal thickness, curvature, and surface regularity" /> |
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Long-term use of PMMA or thick hydrogel contact lenses have been found to cause corneal warpage (shape distortion).<ref name="Corneal Exhaustion Syndrome with Long-Term Wear of Contact Lenses">{{cite journal|last=Sweeney|first=D.|title=Corneal Exhaustion Syndrome with Long-Term Wear of Contact Lenses|journal=Optometry and Vision Science| |
Long-term use of PMMA or thick [[hydrogel]] contact lenses have been found to cause corneal warpage (shape distortion).<ref name="Corneal Exhaustion Syndrome with Long-Term Wear of Contact Lenses">{{cite journal|last=Sweeney|first=D.|title=Corneal Exhaustion Syndrome with Long-Term Wear of Contact Lenses|journal=Optometry and Vision Science|date=August 1992|volume=69|issue=8|pages=601–608|doi=10.1097/00006324-199208000-00002|pmid=1513555 |s2cid=42597709 }}</ref> |
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There is some evidence to show that rigid gas permeable contact lenses are capable of slowing myopic progression after long-term wear. This same effect was not found in patients who had worn soft contact lenses for an extended period of time. Greater corneal steepening was found in patients wearing soft contact lenses than in patients wearing rigid gas permeable contact lenses,<ref name="A Randomized Trial of the Effects of Rigid Contact Lenses on Myopia Progression">{{cite journal|last=Walline|first=J.| |
There is some evidence to show that rigid gas permeable contact lenses are capable of slowing myopic progression after long-term wear. This same effect was not found in patients who had worn soft contact lenses for an extended period of time. Greater corneal steepening was found in patients wearing soft contact lenses than in patients wearing rigid gas permeable contact lenses,<ref name="A Randomized Trial of the Effects of Rigid Contact Lenses on Myopia Progression">{{cite journal|last=Walline|first=J.|author2=Jones, L. |author3=Mutti, D. |author4=Zadnik, K. |title=A Randomized Trial of the Effects of Rigid Contact Lenses on Myopia Progression|journal=Arch. Ophthalmol.|date=December 2004|volume=122|issue=12|pages=1760–1766|doi=10.1001/archopht.122.12.1760|pmid=15596577 }}</ref> suggesting that the latter may slow the progression of myopia by flattening the cornea. |
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== Functional change == |
== Functional change == |
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Corneal sensitivity is significantly diminished after extended contact lens wear (five or more years). However, this difference in sensitivity is not correlated with a change in the number of [[nerve fiber]] bundles in the subbasal plexus of the cornea.<ref name="Confocal Microscopy In Vivo in Corneas of Long-Term Contact Lens Wearers">{{cite journal|last=Patel|first=S.| |
Corneal sensitivity is significantly diminished after extended contact lens wear (five or more years). However, this difference in sensitivity is not correlated with a change in the number of [[nerve fiber]] bundles in the subbasal plexus of the cornea.<ref name="Confocal Microscopy In Vivo in Corneas of Long-Term Contact Lens Wearers">{{cite journal|last=Patel|first=S.|author2=McLaren, J. |author3=Hodge, D. |author4=Bourne, W. |title=Confocal Microscopy In Vivo in Corneas of Long-Term Contact Lens Wearers|journal=Invest. Ophthalmol. Vis. Sci.|date=April 2002|volume=43|issue=4|pages=995–1003|pmid=11923239 }}</ref> Long-term use of PMMA or thick hydrogel contact lenses have been found to cause increased eye irritability, photophobia, blurred vision, and persistent haloes.<ref name="Corneal Exhaustion Syndrome with Long-Term Wear of Contact Lenses" /> |
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Long-term use of rigid gas permeable contact lenses has been associated with slower myopic progression |
Long-term use of rigid gas permeable contact lenses has been associated with slower myopic progression.<ref name="A Randomized Trial of the Effects of Rigid Contact Lenses on Myopia Progression" /> |
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== Unchanged variables == |
== Unchanged variables == |
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== Reversibility of damage == |
== Reversibility of damage == |
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| ⚫ | Epithelial oxygen uptake has been found to return to normal levels one month after cessation of contact lens wear. Epithelial thickness has been found to return to a normal level as soon as one week following the cessation of contact lens wear. However, endothelial polymegethism does not seem to return to normal levels even long after the cessation of contact lens wear.<ref name="Effects of long-term extended contact lens wear on the human cornea." /> Even after a six-month period in which contact lenses are not worn, polymegethism seems to remain.<ref name="Epithelial and endothelial effects from the extended wear of contact lenses." /> Stromal thickness does not return to a normal level even after an entire month in which contact lens wear is halted.<ref name="Epithelial and endothelial effects from the extended wear of contact lenses." /> The density of microcysts also remains as long as one month after contact lenses are removed,<ref name="Effects of long-term extended contact lens wear on the human cornea." /> and microcysts do not disappear completely until two to three months after contact lens wear is completely halted.<ref name="Epithelial and endothelial effects from the extended wear of contact lenses." /> |
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Many of the observed changes appear to be reversible. |
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| ⚫ | Epithelial oxygen uptake has been found to return to normal levels one month after cessation of contact lens wear. Epithelial thickness has been found to return to a normal level as soon as one week following the cessation of contact lens wear. However, endothelial |
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Reductions in epithelial oxygen uptake and thickness are thought to be caused by long-term contact lens wear-induced hypoxia, which hinders epithelial metabolism and mitosis.<ref name="Effects of long-term extended contact lens wear on the human cornea." /> Recovery of normal epithelial oxygen uptake can occur if contact lens wear is completely halted for one month.<ref name="Epithelial and endothelial effects from the extended wear of contact lenses." /> Because long periods of contact lens wear are correlated with extended hypoxia, the resurgence of cellular growth and epithelial metabolism following contact lens removal (and hence, improved oxygen circulation) leads to an initial, increased resurgence of microcysts containing cellular debris. Over time, however, microcysts will disappear if contact lenses are not worn.<ref name="Effects of long-term extended contact lens wear on the human cornea." /> |
Reductions in epithelial oxygen uptake and thickness are thought to be caused by long-term contact lens wear-induced hypoxia, which hinders epithelial metabolism and mitosis.<ref name="Effects of long-term extended contact lens wear on the human cornea." /> Recovery of normal epithelial oxygen uptake can occur if contact lens wear is completely halted for one month.<ref name="Epithelial and endothelial effects from the extended wear of contact lenses." /> Because long periods of contact lens wear are correlated with extended hypoxia, the resurgence of cellular growth and epithelial metabolism following contact lens removal (and hence, improved oxygen circulation) leads to an initial, increased resurgence of microcysts containing cellular debris. Over time, however, microcysts will disappear if contact lenses are not worn.<ref name="Effects of long-term extended contact lens wear on the human cornea." /> |
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Corneal sensitivity has been found to be significantly diminished following long-term contact lens wear. However, this difference in sensitivity is not correlated with a change in the number of nerve fiber bundles in the subbasal plexus of the cornea, suggesting that diminished corneal sensitivity following extended periods of contact lens wear is not caused by a reduction in nerve fiber bundles but possibly a change in functionality.<ref name="Confocal Microscopy In Vivo in Corneas of Long-Term Contact Lens Wearers" /> One |
Corneal sensitivity has been found to be significantly diminished following long-term contact lens wear. However, this difference in sensitivity is not correlated with a change in the number of nerve fiber bundles in the subbasal plexus of the cornea, suggesting that diminished corneal sensitivity following extended periods of contact lens wear is not caused by a reduction in nerve fiber bundles but possibly a change in functionality.<ref name="Confocal Microscopy In Vivo in Corneas of Long-Term Contact Lens Wearers" /> One or two years of hard contact lens wear has not been shown to affect corneal sensitivity, but real changes are observed following five years of hard contact lens wear. However, this significant decrease in corneal sensitivity appears to be reversible. Following cessation of hard contact lens usage, corneal sensitivity has been shown to be fully regained after several months: patients who had worn hard contact lenses for a decade or longer were able to regain normal corneal sensitivity after four months of not wearing contact lenses at all.<ref name="Effect of Long-term Wear of Hard Contact Lenses on Corneal Sensitivity">{{cite journal|last=Millodot|first=M.|title=Effect of Long-term Wear of Hard Contact Lenses on Corneal Sensitivity|journal=Arch. Ophthalmol.|date=July 1978|volume=96|issue=7|pages=1225–1227|doi=10.1001/archopht.1978.03910060059011|pmid=666631}}</ref> |
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Long-term use of PMMA or thick hydrogel contact lenses has been found to cause corneal warpage (shape distortion), increased eye irritability, photophobia, blurred vision, and persistent haloes. Collectively, these symptoms constitute Corneal Exhaustion Syndrom (CES), which is associated with corneal endothelium abnormalities including [[edema]], |
Long-term use of PMMA or thick hydrogel contact lenses has been found to cause corneal warpage (shape distortion), increased eye irritability, photophobia, blurred vision, and persistent haloes. Collectively, these symptoms constitute Corneal Exhaustion Syndrom (CES), which is associated with corneal endothelium abnormalities including [[edema]], polymegethism, irregular mosaic, and pigment deposition. Patients with CES suffer from compromised corneal endothelium resulting from chronic hypoxia and [[acidosis]]. These problems can be alleviated by providing a patient with lenses that allow for greater oxygen permeability.<ref name="Corneal Exhaustion Syndrome with Long-Term Wear of Contact Lenses" /> |
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== Cause == |
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Increases in corneal curvature are thought to be caused by corneal thinning-induced [[ |
Increases in corneal curvature are thought to be caused by corneal thinning-induced [[Keratoconus|ectasia]].<ref name="The effects of long-term contact lens wear on corneal thickness, curvature, and surface regularity" /> |
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Two explanations have been proposed for contact lens-induced stromal thinning. It is thought that contact lens-induced edema may inhibit stroma tissue synthesis.<ref name="Effects of long-term extended contact lens wear on the human cornea." /> Alternatively, contact lens-induced hypoxia may trigger a lactic acid buildup that leads to the erosion of stromal tissue.<ref name="Effects of long-term extended contact lens wear on the human cornea." /> The mechanism behind contact lens-induced |
Two explanations have been proposed for contact lens-induced stromal thinning. It is thought that contact lens-induced edema may inhibit stroma tissue synthesis.<ref name="Effects of long-term extended contact lens wear on the human cornea." /> Alternatively, contact lens-induced hypoxia may trigger a lactic acid buildup that leads to the erosion of stromal tissue.<ref name="Effects of long-term extended contact lens wear on the human cornea." /> The mechanism behind contact lens-induced polymegethism is unknown, though it is also thought to be a byproduct of corneal edema and epithelial hypoxia.<ref name="Effects of long-term extended contact lens wear on the human cornea." /> |
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It is thought that constant adhesion of contact lenses to the cornea may lead to adaptation to mechanical stimuli, thus decreasing corneal sensitivity to tactile stimuli. A proposed explanation for the reduced sensitivity is the induced quiescence of free nerve endings following long term corneal exposure to contact lenses.<ref name="Effect of Long-term Wear of Hard Contact Lenses on Corneal Sensitivity" /> |
It is thought that constant adhesion of contact lenses to the cornea may lead to adaptation to mechanical stimuli, thus decreasing corneal sensitivity to tactile stimuli. A proposed explanation for the reduced sensitivity is the induced quiescence of free nerve endings following long term corneal exposure to contact lenses.<ref name="Effect of Long-term Wear of Hard Contact Lenses on Corneal Sensitivity" /> |
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*[[Keratitis]] |
*[[Keratitis]] |
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*[[Cornea]] |
*[[Cornea]] |
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*[[Fungal contamination of contact lenses]] |
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== References == |
== References == |
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Latest revision as of 03:40, 8 April 2024
 | This article's factual accuracy may be compromised due to out-of-date information. The reason given is: The hypoxia issues raised by this article might not apply to today's Silicone Hydrogel contact lenses. (October 2021) |
Long-term contact lens use can lead to alterations in corneal thickness, stromal thickness, curvature, corneal sensitivity, cell density, and epithelial oxygen uptake, etc. Other changes may include the formation of epithelial vacuoles and microcysts (containing cellular debris) as well as the emergence of polymegethism in the corneal endothelium. Decreased corneal sensitivity, vision loss, and photophobia have also been observed in patients who have worn contact lenses for an extended period of time. Many contact lens-induced changes in corneal structure are reversible if contact lenses are removed for an extended period of time.
Knowledge concerning the form and function of the cornea and the various types of contact lenses and their common complications is important to understanding this article.
Changes in function and morphology[edit]
The effects of extended contact lens wear on the cornea have been studied extensively and are well-documented. When determining the effects of long-term contact lens use on the cornea, many studies do not differentiate between users of hard and soft contact lenses, while studies that have made this differentiation have found similar results. This is probably because most contact lens-induced changes to the cornea are caused by hypoxia, which occurs as long as any physical barrier to the surface of the cornea is present. In certain instances, hard contact lenses were shown to cause the same changes in corneal structure as soft contact lenses, though these changes were more dramatic because rigid lenses are capable of inflicting greater trauma on the eyes.[1]
Structural change[edit]
Long-term use of soft hydrogel contact lenses has been shown to alter the following in the cornea: epithelial oxygen uptake, epithelial thickness, stromal thickness, and corneal endothelial morphology. Furthermore, the formation of epithelial vacuoles and microcysts has been observed following long-term contact lens wear.[2] Vacuoles are fluid-filled chambers that begin to appear one week after extended contact lens use begins; their number increases over time with extended contact lens wear. Microcysts tend to appear three months after contact lens wear begins and increase in number over time as long as contact lens wear resumes.[3] On average, over five times as many epithelial microcysts than normal have been observed in long-term contact lens wearers.[2]
Among patients who have worn soft hydrogel contact lenses for over a year, significant reductions in epithelial oxygen uptake, epithelial thickness, and stromal thickness have been recorded, while an increase in endothelial polymegethism was found.[2] In patients who had worn contact lenses for approximately five years or more, a 30 to 50 μm reduction in central and peripheral corneal thickness has been recorded. Furthermore, the reduction was more pronounced in patients wearing hard contact lenses than in patients wearing soft contact lenses. Increased endothelial polymegethism is also found in long-term wearers of rigid gas permeable lenses as soon as one week after contact lens wear begins. This change is indicated by significant increases in Max/Min cell size ratio in contact lens wearers.[4] Endothelial pleiomorphism is another factor that arises from long-term use of rigid gas permeable lenses; significant decreases in hexagonal cells are noted after one year, accompanied by increased numbers of cells of other than six sides.[4]
Increased corneal curvature is yet another change known to arise from long-term contact lens wear;[1] this increase in corneal curvature can be as much as 0.5 diopters greater than normal.[5] Corneal surface irregularity and asymmetry are also caused by long-term contact lens wear; these problems are sometimes correlated with astigmatism in contact lens wearers and are thought to be caused by hypoxia, surface molding, and chronic and mild trauma to the cornea from contact lens use.[1]
Long-term use of PMMA or thick hydrogel contact lenses have been found to cause corneal warpage (shape distortion).[6]
There is some evidence to show that rigid gas permeable contact lenses are capable of slowing myopic progression after long-term wear. This same effect was not found in patients who had worn soft contact lenses for an extended period of time. Greater corneal steepening was found in patients wearing soft contact lenses than in patients wearing rigid gas permeable contact lenses,[7] suggesting that the latter may slow the progression of myopia by flattening the cornea.
Functional change[edit]
Corneal sensitivity is significantly diminished after extended contact lens wear (five or more years). However, this difference in sensitivity is not correlated with a change in the number of nerve fiber bundles in the subbasal plexus of the cornea.[8] Long-term use of PMMA or thick hydrogel contact lenses have been found to cause increased eye irritability, photophobia, blurred vision, and persistent haloes.[6]
Long-term use of rigid gas permeable contact lenses has been associated with slower myopic progression.[7]
Unchanged variables[edit]
The number of corneal keratocytes in the epithelial stroma has not been found to change with long-term contact lens wear.[8] Endothelial cell density also does not change with long-term contact lens wear.[2] No strong relationship has been found between long-term contact lens wear and corneal astigmatism.[1]
Reversibility of damage[edit]
Epithelial oxygen uptake has been found to return to normal levels one month after cessation of contact lens wear. Epithelial thickness has been found to return to a normal level as soon as one week following the cessation of contact lens wear. However, endothelial polymegethism does not seem to return to normal levels even long after the cessation of contact lens wear.[2] Even after a six-month period in which contact lenses are not worn, polymegethism seems to remain.[3] Stromal thickness does not return to a normal level even after an entire month in which contact lens wear is halted.[3] The density of microcysts also remains as long as one month after contact lenses are removed,[2] and microcysts do not disappear completely until two to three months after contact lens wear is completely halted.[3]
Reductions in epithelial oxygen uptake and thickness are thought to be caused by long-term contact lens wear-induced hypoxia, which hinders epithelial metabolism and mitosis.[2] Recovery of normal epithelial oxygen uptake can occur if contact lens wear is completely halted for one month.[3] Because long periods of contact lens wear are correlated with extended hypoxia, the resurgence of cellular growth and epithelial metabolism following contact lens removal (and hence, improved oxygen circulation) leads to an initial, increased resurgence of microcysts containing cellular debris. Over time, however, microcysts will disappear if contact lenses are not worn.[2]
Corneal sensitivity has been found to be significantly diminished following long-term contact lens wear. However, this difference in sensitivity is not correlated with a change in the number of nerve fiber bundles in the subbasal plexus of the cornea, suggesting that diminished corneal sensitivity following extended periods of contact lens wear is not caused by a reduction in nerve fiber bundles but possibly a change in functionality.[8] One or two years of hard contact lens wear has not been shown to affect corneal sensitivity, but real changes are observed following five years of hard contact lens wear. However, this significant decrease in corneal sensitivity appears to be reversible. Following cessation of hard contact lens usage, corneal sensitivity has been shown to be fully regained after several months: patients who had worn hard contact lenses for a decade or longer were able to regain normal corneal sensitivity after four months of not wearing contact lenses at all.[9]
Long-term use of PMMA or thick hydrogel contact lenses has been found to cause corneal warpage (shape distortion), increased eye irritability, photophobia, blurred vision, and persistent haloes. Collectively, these symptoms constitute Corneal Exhaustion Syndrom (CES), which is associated with corneal endothelium abnormalities including edema, polymegethism, irregular mosaic, and pigment deposition. Patients with CES suffer from compromised corneal endothelium resulting from chronic hypoxia and acidosis. These problems can be alleviated by providing a patient with lenses that allow for greater oxygen permeability.[6]
Cause[edit]
Increases in corneal curvature are thought to be caused by corneal thinning-induced ectasia.[1]
Two explanations have been proposed for contact lens-induced stromal thinning. It is thought that contact lens-induced edema may inhibit stroma tissue synthesis.[2] Alternatively, contact lens-induced hypoxia may trigger a lactic acid buildup that leads to the erosion of stromal tissue.[2] The mechanism behind contact lens-induced polymegethism is unknown, though it is also thought to be a byproduct of corneal edema and epithelial hypoxia.[2]
It is thought that constant adhesion of contact lenses to the cornea may lead to adaptation to mechanical stimuli, thus decreasing corneal sensitivity to tactile stimuli. A proposed explanation for the reduced sensitivity is the induced quiescence of free nerve endings following long term corneal exposure to contact lenses.[9]
See also[edit]
References[edit]
- ^ a b c d e Liu, Z.; Pflugfelder, S. (January 2000). "The effects of long-term contact lens wear on corneal thickness, curvature, and surface regularity". Ophthalmology. 107 (1): 105–111. doi:10.1016/S0161-6420(99)00027-5. PMID 10647727.
- ^ a b c d e f g h i j k Holden, B.A.; Sweeney, B.F.; Vannas, A.; Nilsson, K.T.; Efron, N. (November 1985). "Effects of long-term extended contact lens wear on the human cornea". Invest. Ophthalmol. Vis. Sci. 26 (11): 1489–1501. PMID 3863808.
- ^ a b c d e Holden, BA; Vannas, A; Nilsson, K; Efron, N; Sweeney, D; Kotow, M; La Hood, D; Guillon, M (June 1985). "Epithelial and endothelial effects from the extended wear of contact lenses". Curr. Eye Res. 4 (6): 739–42. doi:10.3109/02713688509017678. PMID 2992884.
- ^ a b Esgin, H.; Erda, N. (January 2002). "Corneal Endothelial Polymegethism and Pleomorphism Induced by Daily-Wear Rigid Gas-Permeable Contact Lenses". CLAO Journal. 28 (1): 40–43. PMID 11838988.
- ^ Miller, D. (October 1968). "Contact Lens-Induced Corneal Curvature and Thickness Changes". Arch. Ophthalmol. 80 (4): 430–432. doi:10.1001/archopht.1968.00980050432004. PMID 5674798.
- ^ a b c Sweeney, D. (August 1992). "Corneal Exhaustion Syndrome with Long-Term Wear of Contact Lenses". Optometry and Vision Science. 69 (8): 601–608. doi:10.1097/00006324-199208000-00002. PMID 1513555. S2CID 42597709.
- ^ a b Walline, J.; Jones, L.; Mutti, D.; Zadnik, K. (December 2004). "A Randomized Trial of the Effects of Rigid Contact Lenses on Myopia Progression". Arch. Ophthalmol. 122 (12): 1760–1766. doi:10.1001/archopht.122.12.1760. PMID 15596577.
- ^ a b c Patel, S.; McLaren, J.; Hodge, D.; Bourne, W. (April 2002). "Confocal Microscopy In Vivo in Corneas of Long-Term Contact Lens Wearers". Invest. Ophthalmol. Vis. Sci. 43 (4): 995–1003. PMID 11923239.
- ^ a b Millodot, M. (July 1978). "Effect of Long-term Wear of Hard Contact Lenses on Corneal Sensitivity". Arch. Ophthalmol. 96 (7): 1225–1227. doi:10.1001/archopht.1978.03910060059011. PMID 666631.