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What are refractive errors?
The light rays that enter our eyes are bent or "refracted", so that they focus onto the retina. The retina is the light-sensitive layer of the eye that transmits the image of the visual world to the brain. This bending of light occurs across all the transparent structures of the eye but in particular, across the cornea and lens. If the degree of bending is not correct, then the image that is formed on the retina is not focused and the picture appears blurred. This is the basis of a refractive error and may require eye glasses or other optical correction.
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What are the types of refractive errors?
Myopia (short sight or nearsightedness) - is a condition in which the eye is longer than usual, i.e. the distance from the cornea to the retina is too great. When a distant scene is viewed by a short-sighted eye, light entering the eye converges too greatly for its length, so that it is focused in front of the retina. This results in a blurred image on the retina itself.
Short sight can be corrected optically with a concave spectacle lens which diverges the rays of light. With such a correction in place, the image of a distant scene is sharply focused on the retina and the details can be seen without any effort on the part of the individual The same effect can be achieved with a contact lens.
Because light arising from a near object is already diverging, there is no need for a short-sighted person to use eye glasses for near. The short-sighted eye is already 'focussed for near' and a newspaper can be read without refractive help. This is the basis for the term 'short sight'.
Myopia is commonly inherited and appears during teenage years when the body grows rapidly. There is very little change from 20-40 years. Patients who have a more severe degree of myopia (high myopia) have a greater likelihood of developing retinal detachment and must be evaluated from time to time by an ophthalmologist.
Hyperopia (long sight or farsightedness) - is a condition in which the eye is shorter than usual so that rays of light arising from a distance are insufficiently bent as they enter the eye. Light cannot therefore be focussed on the retina without a focusing effort, because the point of focus is behind the eye. The retinal image is blurred. Fortunately, young, long-sighted individuals can bring the image into focus, simply by the effort of focusing, (or accommodation), which involves a change in shape of the lens of the eye. This shape change converges the rays of light so that they can be focussed sharply onto the retina. Distant objects are therefore seen clearly - hence the term 'long sight'.
During the process of accommodation, the front surface lens becomes more curved and the lens becomes optically more powerful, converging the rays of light more strongly. The extent to which the shape of the lens can be changed is, however, limited. Since the long-sighted patient must already make a focussing effort to see clearly in the distance, there is less reserve to focus for near. For this reason the long-sighted individual while seeing clearly in the distance, may notice blurring for near and need optical help.
This difficulty for near increases with age, since the our ability to change the shape of the eye-lens, decreases as we get older. For this reason, a long-sighted person needs reading spectacles at a younger age than someone who has no optical problem. Long sight can be corrected optically with a convex lens which converges the light's rays.
Astigmatism - is an asymmetry of the optics of the eye. Normally, the front surface of the cornea is rounded and it bends the light rays uniformly so that they are brought to a point focus on the retina. In some instances, the cornea is more curved in one plane than another so that the image of an object can never be brought to a point. This results in blurred vision for both distant and close objects.
Astigmatism may occur on its own, or accompany either myopia or hyperopia. It may be corrected with a spectacle lens which compensates for the asymmetry of corneal curvature using a cylindrical lens.
Presbyopia (far-sightedness) - is a condition of advancing age. As we get older, the lens of the human eye gradually loses its ability to change shape. This leads to a progressive loss of the ability to focus for near which reaches its peak at about the age of 45 years. At this age we lose our ability to focus for near, completely. A patient who is normal sighted (i.e., is neither long- nor short-sighted) will continue to see distant objects clearly, but needs optical help for near i.e., reading glasses. As noted above, the onset of presbyopia occurs sooner in long-sighted people. -
How can refractive errors be corrected?
Some of this has been touched on above. It is important to first identify the nature of the refractive error, which the ophthalmologist (eye specialist) can easily do. There are several options available. These are - eye glasses, contact lenses and corrective(refractive) surgery.
Eyeglasses - this is the traditional method for correcting refractive errors. Glasses can also protect the eyes against harmful ultraviolet rays. Glasses may need to be changed at regular intervals if the refractive error changes. Bifocal glasses are those that have two lenses- the one on top for distant vision and a segment below for reading and other near tasks.
Contact lenses - A wide variety of lenses are now available and provide a good alternative for the traditional glasses. The various options must be discussed with the prescribing doctor so that the lens that suits an individual's life style and other requirements can be prescribed. Contact lenses may not suit some persons, particularly those exposed to dusty and dry conditions and others with allergies. Careful hygiene is necessary in handling contact lenses to reduce the risks of infection.
Refractive surgery - These surgical procedures are not for every person with a refractive error. Eyeglasses or contact lenses are a better and safer option. Refractive surgery is sometimes advised for patients who have difficulties with the other methods of correction or where their life style so demands. These procedures include radial keratotomy (RK), photorefractive (excimer laser) keratoplasty (PRK), LASIK and intra-ocular corneal rings. In these procedures cuts are made in the cornea using a laser, sometimes combined with other techniques, to change its shape and allow the light to be more accurately focused on the retina. -
What are the complications of surgery for refractive errors?
Complications of surgery are uncommon but can be serious. There may be infection, temporary pain, glare and fluctuating vision for the first few months after surgery. The weakened cornea may be more prone to injury and may rupture. There may be a need for additional surgery.
