The use of long-term contact lenses can cause changes 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 appearance of polymegethism in the corneal endothelium. Decreased corneal sensitivity, vision loss, and photophobia have also been observed in patients who have been wearing contact lenses for long periods of time. Many of the lens changes induced in the corneal structure are reversible when contact lenses are released for extended periods of time.
Knowledge of the shape and function of the cornea and the various types of contact lenses and their common complications is important to understand this article.
Video Effects of long-term contact lens wear on the cornea
Changes in function and morphology
The effects of extended contact lens wear on the cornea have been studied extensively and well documented. When determining the effect of long-term contact lens on the cornea, many studies do not distinguish between hard and soft contact lens users, while studies that have made this differentiation have found similar results. This may be because most of the changes caused by the lens on the cornea are caused by hypoxia, which occurs as long as there is a physical barrier on the surface of the cornea. In certain cases, hard contact lenses are shown to cause similar changes in the structure of the cornea as soft contact lenses, although these changes are more dramatic because rigid lenses are capable of causing greater trauma to the eye.
Maps Effects of long-term contact lens wear on the cornea
Structural changes
The 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 after the use of long-term contact lenses. A vacuole is a fluid-filled space that begins to appear one week after the use of extended contact lenses begins; their numbers increase over time with extended contact lens wear. Microcysts tend to appear three months after wearing contact lenses started and increase in amount over time during wearing resume contact lenses. On average, more than five times more epithelial microcysts than normal have been observed in long-term contact lens wearers.
Among patients who had been wearing soft hydrogel contact lenses for more than a year, significant reductions in epithelial oxygen deposition, epithelial thickness, and stromal thickness were noted, while increased endothelial polymegetism was found. In patients who have contact lenses for about five years or more, a 30 to 50 m reduction in central and peripheral corneal thickness has been noted. In addition, the reduction is more pronounced in patients who wear hard contact lenses than in patients who wear soft contact lenses. Increased endothelial polymegethism is also found in long-term users of rigid gas permeable lenses shortly after a week of contact lens wear begins. This change is indicated by a significant increase in Max/Min cell size ratio in contact lens wearers. Endothelial pleiomorphism is another factor arising from the long-term use of rigid gas permeable lenses; Significant reductions in hexagonal cells were recorded after one year, accompanied by an increase in the number of cells other than six sides.
Increased corneal curvature is another known change arising from the use of long-term contact lenses; increased corneal curvature can be as much as 0.5 dioptri larger than normal. Corneal surface instability and asymmetry are also caused by long-term contact lens wear; this problem is sometimes correlated with astigmatism in contact lens wearers and is thought to be caused by hypoxia, surface printing, and chronic and mild trauma to the cornea from the use of contact lenses.
Long-term use of PMMA or thick hydrogel contact lenses has been found to cause corneal warangel (shape distortion).
There is some evidence to suggest that rigid permeable gas contact lenses are able to slow nearsight development after long-term use. This same effect is not found in patients who have been wearing soft contact lenses for long periods of time. Greater corneal thickness is found in patients who wear soft contact lenses than in patients who wear rigid gas permeable contact lenses, suggesting that the latter may slow the development of myopia by leveling the cornea.
Functional changes
Corneal sensitivity is significantly reduced after extended contact lens wear (five years or more). However, this difference in sensitivity does not correlate with changes in the number of bundles of nerve fibers in the subcasmic plexus of the cornea. Long-term use of PMMA or thick hydrogel contact lenses has been found to cause increased eye irritability, photophobia, blurred vision, and persistent halo.
The long-term use of rigid gas permeable contact lenses has been associated with a much slower nearsightedness.
Variable unchanged
The number of corneal keratocytes in the epithelial stroma has not been found to change with the use of long-term contact lenses. The density of endothelial cells also does not change with the use of long-term contact lenses. No strong association was found between the use of long-term contact lenses and corneal astigmatism.
Reversibility of damage
Epithelial oxygen absorption has been rediscovered to normal levels one month after discontinuation of contact lens wear. Epithelial thickness has been rediscovered to normal levels immediately after one week after discontinuation of contact lens wear. However, endothelial polymegetism does not seem to return to normal levels even long after discontinuation of contact lens wear. Even after a six-month period in which contact lenses are not used, polymegetism seems to remain. The thickness of the stroma does not return to its normal level even after a full month in which contact lens wear is stopped. The density of microcysts also remains for one month after contact lenses are removed, and microcysts do not disappear completely until two to three months after wearing contact lenses completely discontinued.
Reduced absorption and epithelial oxygen thickness are thought to be caused by hypoxia due to long-term contact lens wear, which inhibits metabolism and epithelial mitosis. Recovery of normal epithelial oxygen absorption may occur if contact lens wear is completely discontinued for one month. Because long-term contact lenses are correlated with extended hypoxia, cellular growth and epithelial metabolism after removal of contact lenses (and therefore, enhanced oxygen circulation) leads to an early increase in microcysts containing cellular debris. Over time, however, microcysts will be lost if contact lenses are not worn.
The corneal sensitivity has been found to decrease significantly after the use of long-term contact lenses. However, this difference in sensitivity does not correlate with changes in the number of nerve fibers in the corneal subclassal plexus, suggesting that reduced corneal sensitivity after prolonged periods of contact wear is not due to a reduction in nerve fibers but may be a functional change. One or two years of hard contact lenses have not been shown to affect corneal sensitivity, but noticeable changes are observed after five years of hard contact lenses. However, a significant reduction in the sensitivity of this cornea appears reversible. After discontinuation of the use of hard contact lenses, corneal sensitivity has been fully revived after several months: patients who have been wearing hard contact lenses for a decade or longer can regain normal corneal sensitivity after four months of not using contact lenses at all..
Long-term use of PMMA or thick hydrogel contact lenses has been found to cause corneal damage (form of distortion), increased eye irritability, photophobia, blurred vision, and persistent halo. Collectively, these symptoms are the Corneal Fatigue Syndrome (CES), which is associated with corneal endothelium abnormalities including edema, polymegethism, irregular mosaics, and pigment deposition. Patients with CES suffer from corneal endothelium that is compromised due to hypoxia and chronic acidosis. These problems can be reduced by providing the patient with a lens that allows greater oxygen permeability.
Cause
Increased corneal curvature is thought to be caused by corneal corneal induced ectasia.
Two explanations have been proposed for thinning of the stroma caused by contact lenses. It is thought that edema caused by contact lenses can inhibit the synthesis of stromal tissue. Alternatively, hypoxia caused by the lens can trigger a buildup of lactic acid that causes erosion of stromal tissue. The mechanism behind contact lens-induced polymegethism is unknown, although it is also considered a by-product of corneal edema and epithelial hypoxia.
It is thought that the adhesion of contact lenses to the cornea can cause adaptation to mechanical stimuli, thereby reducing the corneal sensitivity to tactile stimuli. The proposed explanation for reducing sensitivity is the calm induced free nerve endings after long-term corneal exposure to contact lenses.
See also
- Contact lens
- Keratitis
- Cornea
- Pollution of fungi on contact lenses
References
Source of the article : Wikipedia
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