Vision Correction Treatment
The Underlying Problem: Focusing
Contrary to what we may see in the pervasive everyday advertising, refractive surgery is not the cure for all visual problems. It is, however, an excellent option that through today’s technology of computers and lasers can often significantly improve visual problems that are related to focusing errors of the eye. It is critical to understand that one’s ocular status must first be thoroughly examined to rule out other medical problems that may be leading to loss or blurring of vision.
Assuming that there are no other medical eye problems such as cataracts, glaucoma, retinal problems, etc., the most common cause of blurred vision is a refractive error. A refractive error is a condition of the eye where the various components are not proportioned perfectly to allow pinpoint focusing of objects upon the retina.
Light focuses through cornea, lens and other structures, is focused precisely on the retina. The retina changes the light energy into electrical energy that travels up the optic nerve to the brain to allow the sensation of vision.
Myopic (Shortsighted) Eye
As a result of too long an eye, or too steep a cornea, light comes to focus at a point in front of the retina, so that when it reaches the retina, it is no longer in focus.
As a result of too short an eye, or too flat a cornea, light comes to focus at a point that is in back of the retina, so that light that actually reaches the retina is out of focus.
Because the cornea has a warped shape like a football, instead of a spherical shape like a baseball, light will bend more in one meridian than another, and cannot precisely focus at one spot on the retina, thus leading to blurred vision.
These focusing problems can technically be compensated for through the use of glassesor contact lenses. Although glasses and contact lenses are helpful for many people, others have need for permanent surgical alternatives. Some reasons that glasses and/or contacts may not work for some people:
- glasses may be too thick and cumbersome for many people and constantly falling out of adjustment
- the thicker the glasses get, the more an individual will be bothered by distortions in the peripheral vision
- concerns over emergency situations or getting up at night and always having to stumble to look for their glasses
- occupations or lifestyle interests that are incompatible with glasses (imagine trying to water-ski and not being able to see the boat pulling you because you can’t focus without your glasses!)
- inability to wear contact lenses – often because the eyes won’t tolerate them
- occupations and lifestyles that are also incompatible with contact lenses
- individuals who lack the dexterity to handle contacts or have very sensitive eyes that blink and water any time contact lens insertions are attempted
Options For Refractive Surgery
Refractive surgery is simply a term used for a group of various surgical procedures that alter the relationships of the focusing components of the eye in an attempt to bring light to a sharp focal point on the retina without the assistance of the crutches of glasses or contact lenses. Possible options for refractive surgery (several of which have become outdated now) are as follows:
- Radial Keratotomy (RK) and its related procedures of Astigmatic Keratotomy (AK) and Hexagonal Keratotomy (HK) – procedures that are now passing on to extinction
- Automated Lamellar Keratoplasty (ALK) – also extinct.
- Excimer Laser – Photo-Refractive Keratectomy (PRK)
- Laser Assisted In-situ Keratomileusis (LASIK)
- EpiLasik, Sub-Bowman’s Keratomileusis (SBK)
- Intacs corneal ring segments
Radial Keratotomy (RK)
In radial keratotomy, the cornea is flattend by multiple radial incisions on the corneal surface.
In astigmatism, only the steeper plane of the cornea is flattened with arc shaped corneal incisions – which round the ‘football” shape into a rounder “baseball” shape.
Radial Keratotomy (RK) has been used in the United States since the 1970′s. The procedure involves the use of a diamond blade to fashion radial shaped incisions in the mid periphery and peripheral portions of the cornea to approximately 90% depth. This flattens the central cornea by weakening the peripheral corneal supporting structures and thus theoretically reduces or eliminates myopia.
In Astigmatic Keratotomy (AK), astigmatism can be addressed through peripheral tangential or accurate partial incisions to flatten preferentially the steeper meridian of a warped cornea and thus attempt to make it more spherical. Hexagonal Keratotomy (HK), is mentioned here only for historical interest: it was a procedure that attempted to correct far-sighted (hyperopia) through a hexagon shaped incision to be the opposite of RK – but it failed miserably and fortunately is no longer a recognized procedure.
Automated Lamellar Keratoplasty
- In the older technique of keratomileusis, which led up to later development of LASIK, a corneal flap is lifted.
- Under the flap, a corneal button is removed (with a blade instead of laser) thus flattening the cornea.
- The flap is placed back over the corneal bed, leaving the cornea flattened and thus neutraliing the myopia.
- It costs less
- It works reasonably well in selected individuals with low amounts of myopia (up to 3 diopters)
- Some individuals do not respond according to predictions – can be somewhat unpredictable
- High incidence of undercorrections – with the need for multiple enhancements
- Overcorrections in some individuals – leading to far sightedness
- Glare at night very common for 6 months to 18 months
- Fluctuation in vision for up to 6 months
- Can form abnormal scar tissue
- Problems wearing contact lenses afterwards if it is not successful
- Structural integrity of cornea weakened – more potential for problems in injury
- Serious blurring of vision at high altitudes (e.g. mountain climbers)
- BECAUSE OF THESE DISADVANTAGES, AND THE PROGRESS IN LASERS, RK IS NO LONGER PERFORMED
This procedure has been performed since 1949, but only in recent years has it improved as a result of advancements in equipment used. To treat myopia, a sophisticated instrument referred to as an automated microkeratome is used to fold back a thin superficial layer of the cornea (called the ‘corneal cap’). Under this cap, a predetermined amount of corneal tissue is removed from the center of the cornea with the microkeratome. The cap is then folded back into place without the need for sutures. The removal of this central corneal tissue allows flattening of the cornea, thus reducing nearsightedness.
- Higher amounts of myopia (up to approx. 18 diopters) and some cases of moderate hyperopia could be treated
- Return to normal activities relatively quickly
- Technical problems with the removal of corneal tissue mechanically lead to some unpredictable results and scarring
- The technique has been essentially replaced with the newer Excimer laser procedures
Excimer Laser Photorefractive Keratectomy(PRK)
In the PRK procedure, the laser removes microscopic amounts of corneal tissue on the front surface of the central cornea, making this central area flatter and thus allowing the light to focus more accurately on the retina. Since the front surface of the cornea is reshaped, the superficial corneal covering letter (epithelium) must grow back across the surface before the eye is comfortable and starts to see well.
In the United States, PRK has been approved to treat low to moderate amounts of nearsightedness as long as the procedure is performed with an FDA approved Laser instrument.
- Works best for low amounts of myopia (up to approx 3 diopters)
- Relatively precise – low incidence of need for enhancements – felt to be more accurate than RK
- Procedure is very short – patient can return to normal activities within 2 to 3 days
- For degrees of myopia greater than approx 4 diopters, the recovery of vision can be slow because of scar tissue than can persist for many weeks
- Because epithelium is removed by the laser, the eye can be watery and light sensitive for 2 to 3 days until it heals
- Cortisone eye drops must be used for approx. 8 weeks to minimize scarring during the healing phase