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by Dr. Merrill J. Allen, Dr. Steven M. Beresford, Dr. Francis A. Young.
UNDERSTANDING REFRACTIVE ERRORS The eye’s ability to focus light onto the retina is determined by the length of the eyeball, the curvature of the cornea, and the power of the inner lens. If the light isn’t precisely focused, the image on the retina is blurred. The blurring is called a refractive error and is caused by imperfections in the eyeball, and/or the cornea, and/or the inner lens. There are three major types of refractive error: 1) Myopia (nearsightedness). Near objects can be seen clearly but distant objects are blurred. In extreme cases of myopia, objects can be seen clearly only if they are very close to the eye, sometimes just a few inches away. 2) Hyperopia (farsightedness). Far objects can be seen clearly but near objects are blurred. In extreme cases of hyperopia, objects at all distances are blurred. 3) Astigmatism. Objects are blurred regardless of their distance from the eye. Astigmatism can occur alone or in combination with myopia or hyperopia. There’s also the condition known as presbyopia, which is loss of focusing power due to the aging process. Presbyopia is not really a refractive error and usually shows up over the age of forty. It affects people who previously had good vision as well as people with refractive errors. Research has shown that optical defects in the eye known as “spherical and chromatic aberrations” and “light scattering and diffraction” make the image on the retina somewhat blurred and distorted, even in perfectly normal eyes. The reason we are able to see clearly is because the retinal image is enhanced by the brain’s visual cortex in a process known as “hyperacuity”. In fact, what we actually see is up to six times clearer than the image on the retina! LASIK SURGERY: SCULPTING THE CORNEA Lasik stands for ‘Laser Assisted In Situ Keratomileusis’ and is an operation on the cornea to alter its shape. Here’s how it works: The operation is performed on an outpatient basis with the patient sedated and awake. Eye drops are used to numb the surface of the eye. The operation is painless and usually takes 10 to 15 minutes. Both eyes are usually operated on at the same time but sometimes the doctor waits to see the result on the first eye before operating on the second eye. Lasik is performed in two steps. First, an instrument called a microkeratome is used to remove the outer layer of the cornea in the form of a flap, which is folded back to expose the underlying tissue (the stroma). The second step uses an excimer laser to reshape the stroma and modify the cornea’s focusing power. The laser is guided by a sophisticated computerized system that tracks the position of the eye several thousand times per second and sculpts the cornea by precisely vaporizing the tissues. The doctor then replaces the flap, which adheres to the eyeball through natural adhesion. No stitches are used. A clear plastic or perforated metal shield is placed over the eye to protect the flap. There is almost no post-operative pain. The flap remains in position while healing takes place. The first few days are especially critical and great care must be taken not to rub the eye or the flap can be dislodged. After about 3 days, the flap begins to bond to the stroma. After a couple of weeks, the flap is quite secure but several months may be needed before healing is complete. Patients can usually see the results of Lasik within a day or two and a skilled doctor can often produce amazing improvements. Several million people have had Lasik since it was first performed in the mid 1990s and it is considered to be a safe and reliable procedure. However, there are risks and complications that you should be aware of: 1) Not everybody can see 20/20 after Lasik and you may need to have additional treatment or you may still need corrective lenses, although the prescription will be weaker than before. 2) A small percentage of patients have postoperative complications such as corneal scarring or infection, irregular astigmatism (permanent warping of the cornea), dry eye syndrome, glare or haloes around lights, inability to wear contact lenses, and so on. The incidence of these complications is estimated at 3% to 6%. 3) If you are myopic, you may need to wear glasses for reading after Lasik even if you can see distant objects clearly. The reason is that although Lasik changes the shape and refractive power of the cornea, it doesn’t increase the focusing power of the inner lens. So what you gain in distance vision, you may lose in near vision. THERAPEUTIC EYE EXERCISES AND LASIK Lasik treats the symptoms – the blurred image on the retina – but ignores the underlying cause of the blurred vision, which may be nearpoint stress, poor diet, or the aging process. If these factors are causing your eyes to malfunction, your vision may eventually deteriorate even if the surgery is successful. For this reason, we recommend using therapeutic eye exercises after Lasik to make your eyes healthier, increase the focusing power of the inner lens, and maintain or enhance the results from the surgery. For example, if the surgery has brought you down to 20/30, therapeutic eye exercises may yield further improvement. Likewise, if your near vision is worse after surgery, therapeutic eye exercises will probably improve it. Of course, therapeutic eye exercises should only be done when your eyes have fully recovered from the surgery. |
