Effects of long-term contact lens wear on the cornea
Functional changes from long-term contact lens use include decreased corneal sensitivity, vision loss, and photophobia.
[2] The hypoxic environment the cornea experiences when hydrogel contact lenses are worn is conducive to forming microcysts and vacuoles, most likely due to suppression of corneal endothelium metabolism.
[4] 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.
Contact lens-induced hypoxia triggers the cornea to undergo anaerobic respiration, resulting in a buildup of lactic acid that osmotically draws water into corneal cells, causing edema.
[4] 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.
[1] Long-term use of contact lenses has been associated with an increased risk of corneal neovascularization, an irreversible sight threatening condition in which new blood vessels grow in the avascular cornea as a response to prolonged hypoxia.
[15] Research subjects who wore silicon hydrogel contact lenses on an extended (overnight) basis for 9 months showed no signs of neovascularization, whereas subjects who wore hydrogel contact lenses showed signs of moderate neovascularization after only 3 months of extended wear.
The surface of cosmetic contact lenses are often rougher than their prescription counterparts, and this increases the risk of mechanical trauma to both the inside of the eyelid as well as the cornea.
Surface roughness may also facilitate eye secretion buildup, microorganism proliferation, and corneal infection, especially if cosmetic contacts are stored unused in lens solution for prolonged periods.
[22] A proposed explanation for the reduced sensitivity is the induced quiescence of free nerve endings following long term corneal exposure to contact lenses.
[23] Long-term use of PMMA or thick hydrogel contact lenses have been found to cause increased eye irritability, photophobia, blurred vision, and persistent haloes.
[18] There is some evidence to show that rigid gas permeable contact lenses are capable of slowing myopic progression after long-term wear.
[1] Epithelial oxygen uptake has been found to return to normal levels one month after cessation of contact lens wear.
[2] Recovery of normal epithelial oxygen uptake can occur if contact lens wear is completely halted for one month.
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.
[23] 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.
