Laser

Laser treatment

Based on the suspicion that the ultraviolet light exposed by the excimer laser is capable of changing the shape of the cornea, the method Photo Refractive Keratectomy (PRK) was invented around 1980. Initially the lasers were manufactured by 2 companies; Summit and Visx. Later others joined; Nidek, Chiron and Lasersight.

Technique: After removing the epithelium the laser shoots a crater-shape wound in the Bowman layer and the underlying corneal stroma. The energy of each laser output is quickly absorbed in the corneal tissue. This brings the temperature of the water contained in the cells to boiling. The cell bursts and evaporates. Through this the cornea flattens, which will cause the light beams not to curve as much and moves the burning point of the light beams further back. 

Undercorrection occurs when not enough tissue has been removed, overcorrection means too much has been removed. In the last case the burning point is behind the retina, so to speak. 

In a case of astigmatism the cornea is curved unevenly. This can be cured by lasering two kidney-shaped areas situated opposite each other. This evens out the corneal tissue. The first lasers lacked the function to treat astigmatism and farsightedness. Although the lasers nowadays are capable of treating both myopia and hyperopia, treating farsighted people is still a lot more complicated than treating shortsighted people. A thin layer of cells will start to grow over the lasered wound and its bare nerves after about three days. The pain will lessen and the chance of infection minimised. Because the Bowman membrane was damaged, a scar will form after 3 months. This can cause blurry vision. 
The scale as to which the severity of this is judged is from 0-5, zero being clear cornea, good vision and five being severe scar forming and worsened eyesight. The blur, generally speaking, usually stays present for a few months. Unfortunately it can also be permanent. A blur of scale 1 usually doesn't cause any visual problems. It can, however, cause some light effects.

Advantages:

• Emotionally easier than other techniques that are more of a "serious medical procedure". May people don't consider this treatment to be surgery. The laser is judged as a modern technique that can't go wrong.

• In comparison with RK the chance of under or overcorrection is much smaller. Also chance of regression (returning to visual problems) is much smaller.

Disadvantages:

• Pain. The treatment itself is painless, but as soon as the anaesthetic is worn off nearly all patients experience some irritation and pain during a few hours after the treatment. In lasek this pain will probably remain the same for 36 hours.

• Blurry vision. Nearly all patients will have blurry vision or slightly blurry vision during a few months (6-9). This is because the Bowman membrane is damaged. The more correction needed, the higher and more energy the laser has to produce which increases the amount of damage done. The chance of blurry vision for a long period of time will increase (1-3% of treated eyes).

• The healing process can vary a lot, especially when old lasers are used. There is also a possibility of decentration when manually-operated lasers are used. Also the edges of the wounds won't be even when old laser are used (for example the Summit and Visx 20/20). All these aspects lead to worsened vision and possible optical side effects such as starbursts, light circles and problems with night vision.

• A droopy eyelid is quite common during the healing process. This will steadily disappear.

• Uneven images if only one eye is treated at one time. This can be solved by wearing a contact lens in the untreated eye.

• The chance of losing an eye is not ruled out. However, the chance is considerably smaller than through the wearing of glasses.