Refractive Errors

In order for our eyes to be able to see, light rays must be bent or refracted by the cornea and the lens so that they can focus on the retina, the nerve layer that lines the back of the eye.

The retina receives the picture formed by these light rays and sends the image to the brain through the optic nerve.

A refractive error means that the shape of your eye doesn't refract the light properly, so that the image you see is blurred. Although refractive errors are called eye disorders, they are not diseases.

 

Types of Refractive Errors

MYOPIA (NEARSIGHTEDNESS): A myopic eye is longer than normal, so that the light rays focus in front of the retina. Close objects look clear but distant objects appear blurred.

Myopia is inherited and is often discovered in children when they are between eight and twelve years old. During the teenage years, when the body grows rapidly, myopia may become worse. Between the ages of 20 and 40, there is usually little change.

If the myopia is mild, it is called low myopia, Severe myopia is known as high myopia.

If you have high myopia, you have a higher risk of detached retina of detached retina. Your ophthalmologist would discuss the warning signs of retinal detachment with you if you fall in this category. If the retina does detach, a surgical procedure is the only way to repair it. It is important to have regular eye examinations by an ophthalmologist to watch for changes in the retina.

HYPEROPIA (FARSIGHTEDNESS): A hyperopic eye is shorter than normal. Light from close objects such as page of a book, cannot focus clearly on the retina.

Like nearsightedness, farsightedness is usually inherited. Babies and young children tend to be slightly hyperopic. As the eye grows and becomes longer, hyperopic lessens.

ASTIGMATISM: The Cornea is the clear front window of the eye, A normal cornea is round and smooth like a basketball. When you have astigmatism, the cornea curves more in one direction than in the other, like a football.

Astigmatism distorts or blurs vision for both near and far objects. It might make them look too tall, too wide or too thin. It is possible to have astigmatism in combination with myopia or hyperopia.

PRESBYOPIA (AGEING EYES): When you are young, the lens in your eye is soft and flexible. The lens of the eye changes its shape easily, allowing you to focus on objects both close and far away.

After the age of 40, the lens becomes more rigid. Because the lens can't change shape as easily as it once could, it is more difficult to read at close range.  You can also have presbyopia in combination with myopia, hyperopia or astigmatism.

HOW ARE REFRACTIVE ERRORS CORRECTED?

EYEGLASSES

Eyeglasses are the most common methods of correcting refractive errors. They work by refocusing light rays on the retina, compensating for the shape of your eye. Refractive surgery is also an option to correct or improve your vision. These surgical procedures are used to adjust your eye's focusing ability by reshaping the cornea, or front surface of your eye.

Glasses are an easy method to correct refractive errors. They can also help protect your eyes from harmful light rays, such as ultraviolet (UV) light rays.

A special coating that screens out UV light is available when you order your glasses.

CONTACT LENSES 

Contact lenses are also one of the most common methods of correcting refractive errors. It also works by refocusing light rays on the retina. There is a method referred to as orthokeratology which involves the use of a series of hard contact lenses to gradually flatten the cornea and reduce the refractive error.

Improvement of sight from orthokeratology is temporary. After use of the lenses is discontinued, the cornea returns to its original shape and myopia returns.

BIFOCALS

Bifocals are glasses used to correct presbyopia. They have a correction for reading on the bottom half of the lens and another for seeing far distance on the top. Trifocals are lenses with three different lens corrections in one set of eyeglasses.

If you don't need correction for seeing at a distance, you can't receive a prescription for reading glasses or buy them over the counter to correct presbyopia.

No exercise or medication can reverse presbyopia. You will probably need to change your prescription from time to time between the ages of 40 and 60, because your lens will continue to lose flexibility. 

REFRACTIVE SURGERY

Below are a number of surgeries that could be done to correct refractive errors:

RADIAL KERATOTOMY (RK): This involves using radial eight incisions on the cornea to flatten cornea and correct myopia.

ASTIGMATIC KERATOTOMY (AK): This is a microsurgical procedure whereby the doctor makes deep incisions in the cornea (usually one or two) in a curvillinear pattern. The incisions flatten the areas of the cornea that are too steeply curved to correct myopia and astigmatism.

PHOTOREFRACTIVE KERATOTOMY(PRK): This involves the use of an excimer laser to reduce myopia and astigmatism. Using an invisible, high-energy light, the laser sculpts the cornea. No surgical blades are used here.

LASER IN SITU KERATOMILEUSIS (LASIK): LASIK is a combined microsurgical and excimer laser procedure to correct myopia, hyperopia and astigmatism. In LASIK, a highly specialized instrument (microkeratome) is used to cut a thin flap in the cornea. This flap is folded back, and the excimer laser sculpts the exposed corneal tissue to reshape it. The flap is then replaced and not allowed to heal back into position. Stitches are not used in this procedure.

PS: There is no best method for correcting refractive errors. The most appropriate correction for you is dependent on your eyes and lifestyle. Your ophthalmologist will decide which correction will be most effective for you based on these.