What are 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 they can focus on the retina, the layer of light-sensitive cells lining 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 due to its shape, your eye doesn’t refract the light properly, so the image you see is blurred. Although refractive errors are called eye disorders, they are not diseases. In a normal eye, the cornea and lens focus light rays on the retina. Retina Lens Cornea Light rays Light is focused onto the retina
What are the different types of refractive errors ?
A myopic eye is longer than normal or has a cornea that is too steep. As a result, light rays focus in front of the retina instead of on it. Close objects look clear but distant objects appear blurred. Myopia is inherited and is often discovered in children when they are between ages eight and 12 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. In myopia, the eye is too long or the cornea is too steep. Distant objects appear blurry because images focus in front of the retina instead of on it.
If you have high myopia, you have a higher risk of detached retina. Your ophthalmologist should discuss the warning signs of retinal detachment with you if you are in this risk 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.
A hyperopic eye is shorter than normal or has a cornea that is too flat. As a result, light rays focus beyond the retina instead of on it. Most children are farsighted, yet they do not experience blurry vision. With focusing (accommodation), children’s eyes are able to bend the light rays and place them directly on the retina. As long as the farsightedness is not too severe, hyperopic children will have clear vision for seeing objects at a distance and up close. As we get older we slowly lose our ability to focus, and adults with hyperopia may experience increased difficulties with reading or other tasks up close. Like nearsightedness, farsightedness is usually inherited. Babies and young children tend to be slightly hyperopic. As the eye grows and becomes longer, hyperopia lessens. In hyperopia, the eye is too short. Close objects appear blurry because images focus beyond the retina.
astigmatism (distorted vision)
The cornea is the clear front window of the eye. A normal cornea is round and smooth, like a basketball. If 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’s almost like looking into a fun-house mirror in which you appear too tall, too wide or too thin. It is possible to have astigmatism in combination with myopia or hyperopia.
presbyopia (aging 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 did, it is more difficult to read at close range.
This normal condition is called presbyopia. You can also have presbyopia in combination with myopia, hyperopia or astigmatism. In astigmatism, the cornea is uneven. Images focus in front of and beyond the retina, causing both close and distant objects to appear blurry.
Retina Lens Cornea Light rays Light is focused in front of and behind the retina
How are refractive errors corrected ?
Eyeglasses or contact lenses 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. There is not adequate scientific evidence to suggest that eye exercises, vitamins or pills can prevent or cure refractive errors.
Wearing eyeglasses is 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 lens coating that screens out UV light is available. Glasses or goggles made of protective lens material (polycarbonate) should be used for sports and all hazardous activities. All children and adults who have one poorly seeing eye should wear protective polycarbonate lenses at all times to protect their “good” eye.
Bifocals are eyeglasses used to correct presbyopia. They have a correction for reading on the bottom half of the lens and another for seeing at a 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 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.
There is now a wide variety of contact lenses available. The type best suited for you depends on your refractive error and your lifestyle. If you want to wear contact lenses, discuss the various options with your ophthalmologist.
LASIK and Epi-LASIK
Laser in situ keratomileusis (LASI K) With LASI K, an instrument called a microkeratome is used to create a thin flap in the cornea. This flap is folded back, and an excimer laser sculpts the exposed corneal tissue to reshape it. The flap is then repositioned and allowed to heal naturally. No stitches are necessary to hold the flap in place
Epi-LASIK uses a special device called an epikeratome to separate the thin epithelial sheet — much thinner than a LASIK flap — from the rest of the cornea. An excimer laser is then used to reshape the cornea. Afterwards, the epithelial sheet is repositioned (or in some cases, is simply removed). A bandage contact lens is applied and left in place for a few days to help the epithelium heal. Advanced Surface Ablation (PRK and LASEK) Photorefractive keratectomy (PRK) With PRK, the surgeon removes the epithelium from the cornea. An excimer laser is then used to reshape the cornea. A bandage contact lens is applied and left in place for a few days to help the epithelium heal.
With LASEK, a dilute alcohol solution is used to separate the epithelium and an instrument is used to gently scroll it to the side.
An excimer laser is then used to reshape the cornea. Then, the epithelial sheet is repositioned (or in some cases, is simply removed). A bandage contact lens is applied and left in place for a few days to help the epithelium heal. complications and side effects Because LASIK, epi-LASIK and advanced surface ablation are surgical procedures, it is very important to make an informed decision when deciding whether refractive surgery is the right choice for you. Complications and side effects from these procedures may include:
Blurry and fluctuating vision;
Glare and halos;
Undercorrection or overcorrection;
Poor night vision;
Permanent vision loss.
HAVING HAD ANY FORM OF REFRACTIVE SURGERY USUALLY PRECLUDES ONE FROM HAVING A REFRACTIVE LENS EXCHANGE LATER ON IN LIFE!
PATIENTS RENDERED NORMAL SIGHTED WITH REFRACTIVE SURGERY WILL NEED READING GLASSES ONCE THEY BECOME PRESBYOPIC AT ABOUT AGE 45.
THUS THE GLASSES FREE INTERVAL FOR PATIENTS IS AT BEST ONLY BETWEEN AGES 25 – 45.
Refractive lens exchange, particularly for those with cataracts is the preferred method of reducing dependence on spectacles. With multifocal lenses, studies show a 70% success rate of making people glasses free –permanently .
· The risks and costs of intra-ocular surgery.
· Many patients are excluded on ground of pre-existing eye pathology limiting contrast sensitivity, irregular astigmatism, occupations, prolonged need to night drive ( e.g. taxi drivers) , occupation and need for fine acuity ( watchmakers, astronomers) , temperament and unrealistic expectations.
Its non-reversibility .
Intraocular lens exchange is a much riskier operation than
cataract surgery or primary lens exchange
. Intraocular lens exchange is a much riskier operation than cataract surgery or primary lens exchange
· Glare and haloes at night ( usually mild and improves over time)
· Milky or blurred vision ( often permanent)
· For mild myopes, for reading, the quality of post – surgical vision may compare poorly with native myopic unaided reading vision.
What is the best method of correcting refractive errors ?
There is no best method for correcting refractive errors. The most appropriate correction for you depends on your eyes and your lifestyle. You should discuss your eye condition and your lifestyle with your ophthalmologist to decide which correction may be most effective for you.