The human eye works like an optical system where light rays reflected from objects around us pass through clear cornea, lens and other structures to form a sharp focus on the retina. The images from the retina are transmitted via the optic nerve to the brain, which interprets our vision. When the rays of light are not sharply focused on the retina, a blurred or hazy image is formed resulting in a condition termed as Refractive Error. “Refractive Error is not a disease in the true sense, but a defect in the optical system of the eye which becomes a frequent reason for reduced visual acuity,” says Dr. Marmamula.
With expert inputs from Dr. Srinivas Marmamula, Associate Optometrist & Public Health Specialist, International Centre for Advancement of Rural Eyecare, L V Prasad Eye Institute, Hyderabad.
A refractive error is a very common eye disorder, which is sometimes so severe that it causes visual impairment. The common refractive errors are:
- Myopia (near-sightedness): Difficulty in seeing distant objects clearly which can be corrected using minus lenses.
- Hyperopia (far-sightedness): Difficulty in seeing close objects clearly and eye strain. It can be corrected using plus lenses.
- Astigmatism: Distorted vision resulting from an irregularly curved cornea, the clear front part of the eyeball. This can be corrected using cylindrical lenses.
- Presbyopia: The natural ability of the lens in the eye to change its power/ focus for distance and near vision, like a zoom lens camera, diminishes with age, which leads to difficulty in reading or seeing at arm's length. It differs from the other disorders - it is linked to ageing and occurs almost universally after 40 – 45 years of age irrespective of presence of other refractive errors. This is corrected using bifocal lenses or progressive addition lenses.
Testing for refractive errors involves vision testing and retinoscopy by an optometrist to measure the amount of refractive error that is present and prescribing spectacles for correction. Refractive errors can also be corrected with contact lenses or refractive surgery. While spectacles offer a safe, simple and relatively inexpensive mode of correction, contact lenses are cosmetically more acceptable, but require care for safe and infection free usage. Refractive surgery is now gaining popularity in urban India.
Refractive errors (other than presbyopia) can occur at any age. If children are found to be holding the reading material at a closer distance than normal and/or preferring to watch television from a closer distance, then presence of refractive errors should be suspected and they should consult an optometrist for refraction.
*Image courtesy: © Thinkstock photos/ Getty Images
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