Refractive Errors

Diseases & Conditions

Like any other part of the body, the eye too is susceptible to disease. Here is a list of conditions that can affect our eyes.

Cataract
Conjunctivitis
Diabetic Retinopathy
Glaucoma
Refractive Errors
Retinal Detachment
Corneal Blindness

Refractive Errors
For clear eye sight, the cornea, which is the clear front window of the eye, and the lens, behind the pupil, must focus properly or refract light rays on the retina, which is the light sensitive screen lining the inner back wall of the eye.

The retina receives the picture formed by the light rays refracting from an object and sends the image to the brain through the optic nerve. This enables you to see the object. Myopia or short sightedness, hypermetropia or long sightedness and astigmatism or distorted vision, are caused by differences in the length or the shape of the eye. Presbyopia or aging eye occurs when the lens inside the eye loses the elasticity and focussing ability for near vision.

These conditions are called refractive errors because the shape of the eye affects the way the eye refracts or bends light and focuses it on the retina.

Causes and Types
Myopia
Myopia is quite common in children between eight to 12 years. Typically, the condition gets worse as the body grows. Changes in the length of the eye may require new glasses as frequently as every six months, while the child is growing. A myopic eye is more oval than round, longer from front to back than the normal eye. The lens is unable to change its shape sufficiently to focus light from distant objects clearly on the retina. Sometimes myopia can be caused by a change in the shape of the cornea or the lens.

Myopia is best treated with spectacles or contact lenses. Neither will reverse or cure the condition, but they will compensate for the elongated shape of the eye by refracting light rays to come into focus on the retina.

Surgical treatment of myopia has a number of forms.

In Radial Keratotomy, a number of precise incisions are made into the cornea to change its shape. The amount of refractive change is not completely predictable and can fluctuate on a daily basis. Glasses may still be necessary for the clearest distant vision or for reading. This procedure is no longer very popular since the introduction of laser to correct myopia.

Laser is being used to correct myopia by a process called Photorefractive Keratectomy. The laser precisely sculpts the surface of the cornea using invisible, high energy light. LASIK (Laser In-Situ Keratomileusis) is a further advancement in the correction of myopia and gives good results.

Hypermetropia
Also an inherited trait, it occurs because the eyeball is shorter than usual, making it difficult for the light to focus on the retina. Young children tend to be hypermetropic to some degree. As the eye grows and becomes longer, hypermetropia lessens. Sometimes hypermetropia is associated with crossed eyes in children, since the muscles of the eye have to contract tightly, to see up close. Hypermetropia is usually corrected with spectacles and contact lenses. Among children, only those with severe hypermetropia or crossed eyes need to wear spectacles.

Astigmatism
For normal undistorted vision, the cornea should be smooth and equally curved in all directions. When an individual has astigmatism, the cornea is 'warped', which means it curves more in one direction than in the other. In other words, the cornea is shaped more like the side of an egg than a ball. Severe astigmatism is usually inherited, maybe present at birth, and frequently remains unchanged throughout life. Mild astigmatism is very common and may be acquired any time in life. Astigmatism is not difficult to correct with spectacles if the distortion proceeds in a regular direction across the cornea. A similar but opposite curve can be ordered in prescription glasses to neutralise or offset the distortion.

If the cornea is injured or scarred, the distortion may be irregular and this can be corrected only by means of a hard contact lens or by replacing the diseased cornea by a donated human corneal tissue.

Hard or gas-permeable contact lenses improve astigmatism better than soft contact lenses, but special soft contact lenses that correct astigmatism are sometimes helpful. Very severe astigmatism may require special contact lenses called toric contact lenses.

Presbyopia
After the age of forty, many people experience difficulty in focussing the eye for reading and close work. This is caused purely by the aging process. Presbyopia is corrected by increasing the power of the reading spectacles as the person ages. No known treatment, including diet or exercise can halt the progress of presbyopia. Bifocal or trifocal lenses may be prescribed for people with presbyopia who also have other error of refraction such as myopia, hypermetropia or astigmatism.

Signs and Symptoms:

  • Unclear vision – in myopia, distant objects appear blurred, while in hypermetropia, nearer objects appear blurred. In astigmatism there is distortion in sight.
  • You may develop eye-strain and lose interest in reading.
  • Eye discomfort and headaches may become frequent or severe.
  • Symptoms disappear on using appropriate glasses or contact lenses.