Effects of Long-Term Contact Lens Wear on the Cornea
Encyclopedia
Long-term contact lens use leads to alterations in corneal thickness, stromal thickness, curvature, corneal sensitivity, cell density, and epithelial oxygen uptake, etc. Other changes include the formation of epithelial vacuole
s and microcysts (containing cellular debris) as well as the emergence of polymegathism in the corneal endothelium. Decreased corneal sensitivity, vision loss, and photophobia
have also been observed in patients who have worn contact lens for an extended period of time. Surprisingly, many contact lens-induced changes in corneal structure are reversible if contact lenses are removed for an extended period of time.
, and 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.
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.
Bowman’s Layer
– Primarily composed of collagen
, which helps the cornea maintain its shape.
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.
Descemet’s Membrane
– Composed primarily of collagen fibers and protects against injury and infection.
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.
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. 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. Other subcategories of contact lenses include bifocal (for correcting presbyopia), toric (for correcting astigmatism), and corneal reshaping contact lenses.
, and endothelial polymegathism. 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. Furthermore, decreased corneal sensitivity following extended contact lens wear may increase a person’s susceptibility to becoming infected without being aware of it.
Increased myopia
has also been observed in patients following long-term contact lens wear.
, 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.
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 polymegathism was found. 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 polymegathism 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. 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.
Increased corneal curvature is yet another change known to arise from long-term contact lens wear; this increase in corneal curvature can be as much as 0.5 diopters greater than normal. 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.
Long-term use of PMMA or thick hydrogel contact lenses have been found to cause corneal warpage (shape distortion).
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, suggesting that the latter may slow the progression of myopia by flattening the cornea.
bundles in the subbasal plexus of the cornea. Long-term use of PMMA or thick hydrogel contact lenses have been found to cause increased eye irritability, photophobia, blurred vision, and persistent haloes.
Long-term use of rigid gas permeable contact lenses has been associated with slower myopic progression
.
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 polymegathism does not seem to return to normal levels even long after the cessation of contact lens wear. Even after a six month period in which contact lenses are not worn, polymegathism seems to remain. Stromal thickness does not return to a normal level even after an entire month in which contact lens wear is halted. The density of microcysts also remains as long as one month after contact lenses are removed, and microcysts do not disappear completely until two to three months after contact lens wear is completed halted.
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. Recovery of normal epithelial oxygen uptake can occur if contact lens wear is completely halted for one month. 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.
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. One of 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.
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
, polymegathism, 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.
Two explanations have been proposed for contact lens-induced stromal thinning. It is thought that contact lens-induced edema may inhibit stroma tissue synthesis. Alternatively, contact lens-induced hypoxia may trigger a lactic acid buildup that leads to the erosion of stromal tissue. The mechanism behind contact lens-induced polymegathism is unknown, though it is also thought to be a byproduct of corneal edema and epithelial hypoxia.
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.
Vacuole
A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...
s and microcysts (containing cellular debris) as well as the emergence of polymegathism in the corneal endothelium. Decreased corneal sensitivity, vision loss, and photophobia
Photophobia
Photophobia is a symptom of abnormal intolerance to visual perception of light. As a medical symptom photophobia is not a morbid fear or phobia, but an experience of discomfort or pain to the eyes due to light exposure or by presence of actual physical photosensitivity of the eyes, though the term...
have also been observed in patients who have worn contact lens for an extended period of time. Surprisingly, many contact lens-induced changes in corneal structure are reversible if contact lenses are removed for an extended period of time.
Form and function of the cornea
The cornea is the clear, outermost layer of the eye that overlays the anterior chamber, irisIris (anatomy)
The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...
, and 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
Tears
Tears are secretions that clean and lubricate the eyes. Lacrimation or lachrymation is the production or shedding of 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.
Epithilium
Corneal epithelium
The corneal epithelium is made up of epithelial tissue and covers the front of the cornea. It acts as a barrier to protect the cornea, resisting the free flow of fluids from the tears, and prevents bacteria from entering the epithelium and corneal stroma.The corneal epithelium consists of several...
– 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.
Bowman’s Layer
Bowman's membrane
The Bowman's membrane is a smooth layer in the eye. It is located between the superficial epithelium and the stroma in the cornea. It is composed of strong collagen fibers and helps the cornea maintain its shape...
– Primarily composed of collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
, which helps the cornea maintain its shape.
Stroma
Substantia propria
The substantia propria is fibrous, tough, unyielding, and perfectly transparent.It is composed of about 200 flattened lamellæ , superimposed one on another. They are each about 1.5-2.5 micrometres in thickness. These fibrils run at different angles between the limbi...
– 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.
Descemet’s Membrane
Descemet's membrane
Descemet's membrane is the basement membrane that lies between the corneal proper substance, also called stroma, and the endothelial layer of the cornea. It is composed of a different kind of collagen than the stroma. The endothelial layer is located at the posterior of the cornea...
– Composed primarily of collagen fibers and protects against injury and infection.
Endothelium
Corneal endothelium
The corneal endothelium is a single layer of cells on the inner surface of the cornea. It faces the chamber formed between the cornea and the iris....
– 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.
Contact lens types
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
-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. 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. Other subcategories of contact lenses include bifocal (for correcting presbyopia), toric (for correcting astigmatism), and corneal reshaping contact lenses.
General risks
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 edemaEdema
Edema or oedema ; both words from the Greek , oídēma "swelling"), formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin or in one or more cavities of the body that produces swelling...
, and endothelial polymegathism. Mishandling of contact lenses can also cause corneal abrasion
Corneal abrasion
Corneal abrasion is a medical condition involving the loss of the surface epithelial layer of the eye's cornea.-Symptoms and signs:Symptoms of corneal abrasion include pain, photophobia, a foreign-body sensation, excessive squinting, and a reflex production of tears...
s. When induced by contact lenses, corneal abrasions can progress to bacterial keratitis and cause corneal perforations, scarring, and vision impairment. Furthermore, decreased corneal sensitivity following extended contact lens wear may increase a person’s susceptibility to becoming infected without being aware of it.
Increased myopia
Myopia
Myopia , "shortsightedness" ) is a refractive defect of the eye in which collimated light produces image focus in front of the retina under conditions of accommodation. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in...
has also been observed in patients following long-term contact lens wear.
Changes in function and morphology
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 hypoxiaHypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
, 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.
Structural change
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. 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. On average, over five times as many epithelial microcysts than normal have been observed in long-term contact lens wearers.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 polymegathism was found. 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 polymegathism 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. 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.
Increased corneal curvature is yet another change known to arise from long-term contact lens wear; this increase in corneal curvature can be as much as 0.5 diopters greater than normal. 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.
Long-term use of PMMA or thick hydrogel contact lenses have been found to cause corneal warpage (shape distortion).
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, suggesting that the latter may slow the progression of myopia by flattening the cornea.
Functional change
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 fiberNerve fiber
A nerve fiber is a threadlike extension of a nerve cell and consists of an axon and myelin sheath in the nervous system. There are nerve fibers in the central nervous system and peripheral nervous system. A nerve fiber may be myelinated and/or unmyelinated. In the central nervous system , myelin...
bundles in the subbasal plexus of the cornea. Long-term use of PMMA or thick hydrogel contact lenses have been found to cause increased eye irritability, photophobia, blurred vision, and persistent haloes.
Long-term use of rigid gas permeable contact lenses has been associated with slower myopic progression
Unchanged variables
The number of corneal keratocytes in the epithelial stroma has not been found to change with long-term contact lens wear. Endothelial cell density also does not change with long-term contact lens wear. No strong relationship has been found between long-term contact lens wear and corneal astigmatismAstigmatism (eye)
Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism: regular and...
.
Reversibility of damage
Many of the observed changes appear to be reversible.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 polymegathism does not seem to return to normal levels even long after the cessation of contact lens wear. Even after a six month period in which contact lenses are not worn, polymegathism seems to remain. Stromal thickness does not return to a normal level even after an entire month in which contact lens wear is halted. The density of microcysts also remains as long as one month after contact lenses are removed, and microcysts do not disappear completely until two to three months after contact lens wear is completed halted.
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. Recovery of normal epithelial oxygen uptake can occur if contact lens wear is completely halted for one month. 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.
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. One of 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.
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
Edema
Edema or oedema ; both words from the Greek , oídēma "swelling"), formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin or in one or more cavities of the body that produces swelling...
, polymegathism, irregular mosaic, and pigment deposition. Patients with CES suffer from compromised corneal endothelium resulting from chronic hypoxia and acidosis
Acidosis
Acidosis is an increased acidity in the blood and other body tissue . If not further qualified, it usually refers to acidity of the blood plasma....
. These problems can be alleviated by providing a patient with lenses that allow for greater oxygen permeability.
Etiology
Increases in corneal curvature are thought to be caused by corneal thinning-induced ectasia.Two explanations have been proposed for contact lens-induced stromal thinning. It is thought that contact lens-induced edema may inhibit stroma tissue synthesis. Alternatively, contact lens-induced hypoxia may trigger a lactic acid buildup that leads to the erosion of stromal tissue. The mechanism behind contact lens-induced polymegathism is unknown, though it is also thought to be a byproduct of corneal edema and epithelial hypoxia.
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.