Refractive Errors
What are Refractive Errors ?
The eye functions like a camera. Light rays entering the eye are focused by the cornea and the lens of the eye onto the light sensitive retina (which is like the film of a camera) at the back of the eye. The retina then transmits a clear image, or photo, to the brain. In people with refractive error (or poor vision, such as nearsightedness or farsightedness), the light rays do not get focused on to the retina and blurred images are formed. These can be measured as aberration patterns.
Treatment of Refractive Errors
Myopia and treatment options
Myopia, commonly known as near-sightedness or short-sightedness, refers to a condition where the patient is unable to see distant objects. The word 'myopia' is derived from the Greek root myein - meaning 'to close', and ops - meaning 'eye', probably referring to the tendency of near-sighted individuals of partially closing the eye or squinting to improve the sharpness of distant objects.
Myopia is rare before the school-going age, gradually increasing during school life and reaching its peak during the intense study years of college. Its etiology is multi-factorial, with both genes and environment playing important roles. Though it is believed that prolonged reading harms the eyes, attempts to reduce accommodative fatigue (like introducing pauses during reading and teaching, eye exercises, etc.) have not helped to reduce myopia among children.
Myopia is classified into low (-3 diopters), moderate (-3 to -6 diopters), and high (more than -6 diopters). Persons with severe myopia are liable to have vision-threatening complications and should have a dilated eye examination at least once a year. (A diopter is the power of a lens for focussing light rays.)
The treatment options for myopia are spectacles, contact lenses, and refractive surgery. Spectacles are inexpensive and safe and offer clear vision, relief from eyestrain and headaches, and reduced risk of developing a squint. Patients with high power are advised to use high index glasses, which are not very thick. Plastic glasses are lighter, but get scratched easily.
Those who do not want to wear spectacles can opt for contact lenses, which are mainly of two types: soft and semi-soft or rigid gas permeable. Soft contact lenses may be conventional or disposable. Soft lenses do not allow enough oxygen to reach the eye and, on a long-term basis, may lead to complications like blood vessels on the cornea, inflammation and dry eyes. Semi-soft lenses or rigid gas permeable lenses are better, because they correct more astigmatism as compared to soft lenses. For high errors of astigmatism, surgeons may suggest toric contact lenses, or soft and semi-soft lenses.
Disposable lenses (daily or weekly) are discarded as indicated, while extended wear lenses can be worn while sleeping also, though this is not recommended. Sleeping or swimming with the lenses on increases the risk of corneal ulcers and infections, which are sight-threatening complications. All types of lenses should be removed at the end of the day, cleaned and soaked in the specified solution.
Hyperopia and treatment options
Hyperopia refers to the condition known commonly as long-sightedness, wherein the patient has difficulty seeing objects or reading at close distances without the aid of glasses.
In general, LASIK is recommended for patients with up to +4 to +5 diopters spherical of hyperopia or far-sightedness. It is relatively easier to flatten the cornea for myopia patients, but it is more difficult to steepen the cornea in hyperopia. If we steepen the cornea too much, there will be a gradual regression of the power and a risk of the patient losing the best corrected vision. If the error is more than +5 diopters, in young patients contact lenses are preferable to surgery. For patients above the age of 40, refractive lens exchange (along with the removal of the clear lens) with a regular monofocal/multifocal lens is recommended.
