Refractive surgery

What is refractive surgery?

Refractive surgery is a procedure that changes the shape of the cornea to correct visual impairment such as myopia (near-sightedness), hypermetropia (far-sightedness), and some types of astigmatism (irregular curvature of the cornea).

The eye functions on the same principles as a camera. Vision is clear if the cornea and the lens combine to focus an image precisely on to the retina, the inner layer of the eye that senses light and makes it possible to see. Vision is blurred if the cornea, the lens and the length of the eyeball place the image in front or behind the retina. These errors can be corrected non-surgically by using glasses or contact lenses. However, refractive surgery can permanently eliminate or reduce the need for glasses or contact lenses. Refractive surgery changes the curvature of the cornea, thus changing the way images focus on the retina.

What are the different types of procedures?

There are several surgical procedures that help correct the various refractive errors. These differ in how they actually change the shape of the cornea. They are:

• Radial keratotomy (RK)
  
• Photorefractive keratotomy (PRK)
  
• Laser in-situ Keratomileusis (LASIK)
  
• Intracorneal rings
  
• Epikeratophakia
  
• Keratophakia
  
• Keratoplasty

Radial keratotomy (RK): This procedure reshapes the cornea by creating linear incisions or cuts in the cornea with the help of a surgical knife. These cuts relax the walls of the cornea, resulting in flattening of the curvature of the central cornea.

Photorefractive keratotomy (PRK) or Excimer PRK: In this procedure the surface layer of the cornea is removed but it grows back in place within a few days after the surgery. An excimer laser uses an ultraviolet wavelength to deliver pulses of energy that removes a small disc shaped part of the central cornea. The procedure may result the following effects:

• Flattening of the eye for correction of near-sightedness is achieved by removing more tissue from the centre of the cornea than the outer region.
  
• Flattening only along one axis corrects astigmatism.
  
• Removing more tissue from the outer region than the centre corrects far-sightedness.

The total time required for this procedure is approximately 20 minutes. The process is computer controlled by a shutter that opens and closes to control the power of the laser beam.

Once the procedure is completed, anti-inflammatory drugs and antibiotics are applied to promote healing and prevent pain and infection. A bandage soft contact lens is placed on the eye, which acts as a patch to allow the surface cells to heal. Patients may experience no discomfort, or may have mild burning, pain, foreign body sensation, itchiness, light sensitivity, swelling of the lids and/or burning of varying degrees during the first week. Vision remains poor until the corneal surface has re-smoothened itself. Then it gradually improves over one week to 6 months.

Laser In-situ Keratomileusis (LASIK): Excimer LASIK is a surgical procedure that also uses the excimer laser. In this, a special surgical blade is used to create a flap that can be lifted up to expose the part of the cornea that is treated with the laser. After the procedure has been performed, the flap is laid back down on the cornea. This flap is made by the help of an instrument called keratome. The corneal flap protects the area treated by the laser, and so patients do not usually need a contact lens to reduce discomfort or promote healing. Also, since the outer surface of the flap has not been treated and is smooth and clear, patients have a clear vision sooner than they would following PRK.

Intracorneal rings are surgical implants that can be inserted into the peripheral cornea in order to change its shape.

Who requires refractive surgery?

One should consult an ophthalmologist or an eye surgeon before considering refractive surgery. A comprehensive eye examination should be done to determine if the procedure is suitable for an individual. However, the following list of conditions can be used to as general guideline. A patient should:

• be at least 18 years of age or older.
  
• have stable vision for at least one year prior to surgery.
  
• have healthy eyes, free from retinal detachment, corneal scars, and other diseases.
  
• have a refractive problem for which the treatment is required.

What is the effectiveness of the different procedures?

Radial keratotomy: This is the oldest and the cheapest of all the procedures, and is effective in improving the vision in people who have mild near-sightedness.

PRK: This procedure is widely used for treating more severe forms of near-sightedness, as well as far-sightedness. Almost 80-90% of people who require regular glasses are eligible for this treatment. It is an effective procedure that helps 75-95% of treated patients. The results may not be as good among patients with more extreme forms of refractive errors. The visual improvement appears to be stable after complete healing.

LASIK: The two primary advantages of LASIK over PRK are quicker visual recovery time and minimal discomfort after operation.

Intracorneal rings: The advantage of this procedure is that the rings can be removed surgically if the results are not satisfactory. The other three procedures mentioned above are irreversible. Also, since the intracorneal procedure is performed on the peripheral cornea, it has the advantage of not altering the clarity of the central cornea, which is the most important part for clear vision.

What are the disadvantages and complications?

Radial keratotomy: The surgical cuts made in the cornea have the undesirable affect of weakening the eye and making it more susceptible to severe injury. The effects of surgery may not remain stable over a period of time, causing a need for glasses or contact lenses. These limitations make this procedure undesirable, especially in view of the advantages offered by newer forms of refractive surgery.

PRK: There appear to be minimal risks associated with excimer laser surgery on the cornea. However, there are a number of side effects that patients should be aware of prior to surgery. Some of these are:

• Delayed healing: The outer layer of the cornea is removed during this procedure. Normally, this layer replaces itself within 2-3 days. Rarely, this may be delayed and special medications or treatments may be needed to promote healing.
  
• Light sensitivity: Most eyes are sensitive to light after surgery because of the disturbance in the tissues of the eyes. Sunglasses should be worn in bright sunlight for a few days to weeks after surgery.
  
• Corneal haze: During post-operative healing, different degrees of haze may develop in the cornea. This occurs most commonly in those cases where large of correction is required. If superficial corneal haze occurs, it may cause glare at night from bright lights, which may or may not interfere with vision. This generally diminishes and disappears within 6-9 months, but may not disappear in all cases.
  
• Initial over-correction: The initial goal of treatment is over-correction because the cornea tends to regress to its original shape as it heals. The eye stabilizes to near predicted results within 3 months. Rarely, cases may remain overcorrected.
  
• Under-correction: This is a more common complication than over-correction, and is based on each individual’s rate of healing. Little under-correction do not affect the vision seriously but some cases require further surgery for clearer vision. Repeat laser procedures can be done after six months if under- correction remains a problem.
  
• Glare and halos: After this procedure, some patients may experience glare due to scattering of light or see halos around points of light. These are most common at night and tend to lessen as the eye heals.

Complications of LASIK: The risks or complications following LASIK appears to be very few. Some of the complications are:

• Flap complications: Very rarely, the corneal flap might be very thin and may have a hole. In such cases, the final surface of the cornea might be irregular resulting in poor vision. If this occurs, the procedure is stopped and the flap is allowed to heal first. The LASIK can then be attempted again after 6-12 later. Sometimes, the flap may not stick down tightly after the surgery, or become infected. Cells from the outer surface of the cornea might grow under the flap causing the cornea to become cloudy and irregular. An extremely rare but serious complication occurs when the flap is cut too deeply in the cornea, causing the eye to rupture.
  
• Over-correction or under-correction: Due to variations in healing and corneal response, some patients may be under corrected or over-corrected following the LASIK. Some patients may need to use glasses or contact lenses. Additional laser treatments are also performed in some cases.
  
• Glares and halos: After refractive surgery, some patients might experience glare and halos around points of light. Most patients do not feel that these symptoms impair them, and they tend to lessen as the eye heals.

Intracorneal rings: There is very little long-term follow-up available for this procedure. The number of patients that would be suitable candidates for this procedure is quite limited as it can only correct relatively low grades of near-sightedness without astigmatism. Although it is a reversible procedure, it may not always be possible to return to the pre-operative state.

What are the benefits?

The benefits of refractive surgery are many:

• Physiological: Some refractive problems cannot be corrected with conventional glasses or contact lenses. For these, refractive surgery provides clear vision without using these visual aids.
  
• Occupational: There are some jobs like defence services and police department work that require workers to obtain a high standard of visual ability without visual aids. Refractive surgery would allow such people freedom to pursue the career of their choice without being restricted by their vision. It is also beneficial for sports person especially for swimmers and flyers.
  
• Cosmetic: many people feel that glasses distract from their physical appearance. Refractive surgery is certainly a solution to this problem. However, this surgery is generally not encouraged for purely cosmetic reasons.
  
• Psychological: Many people feel handicapped and restricted by their visual aids. Some feel a low sense of self-esteem because the use of visual aid make them feel clumsy, unattractive and left out of some activities. By reducing or eliminating dependence on visual aids, many psychological problem may be reduced.