Laser eye surgery and in particular LASIK is a bad idea if you have keratoconus because further thinning of the cornea and resulting weakening of the cornea can lead to rapid progression of keratoconus. The fact is that not all people know they have keratoconus and some do have LASIK. These patients are at a higher risk of post-LASIK ectasia. So why should you consider laser eye surgery at the time of corneal collagen cross-linking for keratoconus?
Keratoconus causes progressive bulging of part of the cornea which leads to increasing astigmatism and worsening vision. The purpose of corneal collagen cross-linking in keratoconus patients is to strengthen the cornea and prevent further worsening of vision. Whilst some patients notice an improvement in their vision after cross-linking and a reduction in astigmatism, this is usually not very marked. This improvement in vision and reduction of astigmatism occur due an overall flattening of the cornea after cross-linking. By using a laser to remove the epithelial layer before performing cross linking it is possible to preferentially flatten the bulging part of the cornea, thereby causing a greater reduction in astigmatism after the cross-linking procedure. This leads to a better correction of vision. It is not possible to do this by simply scraping the epithelium away which is the common practice.
The excimer laser is able to preferentially flatten the bulge in the cornea caused by keratoconus because the epithelium in this area tends to be thinner. Hence if we assume that the normal epithelium is 55 microns thick and that the epithelium over the bulge is 30 microns thick, if the surgeon programs the laser to remove 55 microns of tissue it will laser through the thinner area of epithelium faster and then remove a further 25 microns of tissue from the layer underneath (total 55 microns). The normal epithelium will also be removed (55 microns) but not the underlying layer. This preferentially flattens the bulging part of the cornea, achieving a greater reduction in post operative astigmatism.
The supporting evidence for this approach comes from a number of published studies that have shown greater reduction of astigmatism when collagen cross-linking is combined with laser removal of epithelial tissue.
Taking this one step further it is possible to eliminate some of the optical aberrations of the keratoconic cornea by using the laser to reshape the cornea to an ideal shape. This is called topography guided trans-PRK.
Unlike standard laser eye surgery in patients who simply want to eliminate glasses, the aim of laser surgery in keratoconus patients is to make their corneal shape more normal and to reduce astigmatism. By doing so the patient is likely to achieve a better level of vision with glasses or contact lenses. Laser surgery in keratoconus patients is always combined with cross-linking or performed after collagen cross-linking. Whether you can have laser surgery at the time of collagen cross-linking will depend on your corneal thickness.
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