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What Are Refractive Errors?
Refractive errors are the most common type of eye disorders. They occur when the shape of the eye does not bend light correctly, which keeps the light rays from focusing properly on your retina. The result is blurred vision.
What Causes Refractive Errors?
Refractive errors can be caused by:
- Eyeball length (too long or too short)
- Problems with the shape of the cornea (outer layer of the eye)
- Aging of the eye’s lens
Refractive errors get passed on through genetics. Research shows that environmental factors also play a role. Children and adults who read and look at screens more have higher chances of developing refractive errors.
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What Are The Symptoms Of Refractive Errors?
The most common symptom of refractive errors is blurred vision. Other symptoms include:
- Double vision
- Hazy vision
- Halos or glares around lights
- Headaches or migraines
- Eye strain (soreness or fatigue in your eyes)
- Difficulty focusing (especially when reading or looking at a screen)
4 Types Of Eye Refractive Errors
Refractive errors can be diagnosed easily by an eye doctor during a normal eye examination. There are four common types of refractive errors:
Astigmatism is when your eye isn’t completely round. The most common type of astigmatism is corneal astigmatism, which means that your cornea (the clear outer layer of your eye) is egg-shaped. The other type is lenticular astigmatism, which means that the lens of the eye (located behind your colored iris) is the wrong shape.
Astigmatism often occurs with myopia (nearsightedness) or hyperopia (farsightedness). Symptoms include:
- Blurry vision far away or close up
- Fluctuating vision
- Difficulty seeing at night
- Glare and starburst patterns around lights.
Astigmatism can be treated by an optometrist using eyeglasses, contact lenses, or an ophthalmologist performing an eye surgery such as refractive lens exchange, phakic intraocular lens implant, PRK, or LASIK.
Nearsightedness makes distant objects appear blurry. Often your near vision will be fine, but your ability to see far away is poor. It occurs when the eyeball grows too long from front to back. Extra curvature of the cornea or a lens that is too thick can also be a cause.
Myopia usually starts developing during childhood and can progress slowly or rapidly. The most common symptoms include:
- Blurry vision when looking at distant objects
- Frequent eye rubbing
- Inability to see the front of the classroom, television, etc.
- Excessive blinking
- Difficulty seeing while driving (especially at night)
- Unawareness of distant objects
Myopia can be treated by an eye care professional. They will usually prescribe eyeglasses or contact lenses. Refractive surgery, such as PRK or LASIK, can also treat myopia.
Farsightedness is very common, with about 25 percent of the population being affected by it. Hyperopia causes close objects to appear blurry, but distant objects to appear clearly. It occurs when the eyeball is shorter than it should be. If you find yourself needing reading glasses, you may have early signs of hyperopia.
Hyperopia is usually present at birth. Some children grow out of it, with the curvature of the eye lengthening to normal, while others stay at the same prescription or develop worse vision. Common symptoms of hyperopia include:
- Trouble focusing on close items
- Blurry vision
- Fatigue after close-up tasks like reading or computer work
Contact lenses, eyeglasses, or corrective surgery such as PRK or LASIK can treat farsightedness.
Presbyopia comes from the hardening of the lens of your eye occurring during middle or old age. Common symptoms of presbyopia include:
- Holding reading material or screens farther away
- Blurred vision at normal reading distance
- Eye strain or headaches after close up work such as reading or computer work
- These symptoms get worse in dim lighting
Presbyopia can be treated with eyeglasses, including bifocals, prescription reading glasses, trifocals, or progressives. Bifocal or monovision contact lenses are also an option. Refractive surgery such as LASIK, PRK, and conductive keratoplasty are safe and effective treatment options as well.
Additional Refractive Errors
Other refractive errors occur as symptoms of eye disorders and eye diseases or independently. These include:
Contrast sensitivity is the ability to tell the difference between similar shades and patterns. It is a symptom of several conditions, including cataracts, glaucoma, macular degeneration, and diabetes.
If you have poor contrast sensitivity, you might be unable to tell the difference between an item and its background. People with poor contrast sensitivity may experience difficulty:
- Driving at night or in poor visibility conditions
- Locating objects against a similarly colored background
- Reading items with poor contrast, such as a newspaper
- Going up or down steps or curbs
- Telling the difference between facial features of others
Contrast sensitivity treatment depends on the cause of this symptom. Eye drops, eye injections, or surgery are the most common treatment options.
Anisometropia is when two eyes have unequal refractive power, making it difficult to focus. This occurs when your eyeballs, corneas, or lenses are different in each eye. Simple anisometropia is when one eye has a refractive error. Compound anisometropia is when both eyes have refractive errors. Mixed anisometropia is when one eye is myopic (nearsighted), and the other is hyperopic (farsighted). Symptoms include:
- Blurred vision
- Poor depth perception
- Amblyopia (lazy eye)
Anisometropia is treated with corrective lenses, contact lenses, or laser eye surgery.
Aniseikonia is when the size of items is different in each eye. This can occur as a result of different refractive power in each eye or refractive error correction. Symptoms include:
- Balance disorders
Aniseikonia is treated with corrective eyeglasses or contact lenses.
A lazy eye may be the result of a refractive error in one eye but not the other or opposing refractive errors in the eyes.
Refractive Error Treatment Options
During routine eye exams, doctors can find and diagnose refractive errors. They will then list and recommend your treatment options. Untreated refractive errors can lead to worsening vision, amblyopia (lazy eye), and in extreme cases, visual impairment or blindness.
The most common refractive error treatment options are:
Prescription eyeglasses can treat most refractive errors. The main types of eyeglasses are:
Single vision lenses correct one field of vision, near or far. Doctors prescribe them to nearsighted or farsighted patients.
Bifocal lenses have separate sections for near and far sight correction. They are usually prescribed to patients who have presbyopia and an additional refractive error such as myopia, hyperopia, or astigmatism.
Trifocals have sections for near, intermediate, and far distant correction. The drawback of trifocals and bifocals is that there are lines between each section of vision correction, making it difficult for the eye to adjust at first.
Progressive, or multifocal, lenses provide the same correction as bifocal and trifocals, but without the lines separating the sections of distance correction.
Contact lenses can also treat refractive errors. There is a large variety of contact lenses available, the most common being:
- Rigid gas-permeable (RGP)
- Daily-wear soft lenses
- Extended-wear disposable
- Planned replacement
Contact lenses can be made for monovision, bifocal, or multifocal vision correction, depending on the type of refractive error they are treating.
Refractive Error Correction Surgery
Refractive surgery is becoming increasingly popular as technology advances, and people are learning more about how safe and effective it can be. It often offers a permanent or more long-term solution than glasses or contacts.
Refractive surgery improves the refractive state of the eye and decreases or eliminates dependency on glasses or contact lenses. Common surgeries include various methods of remodeling of the cornea (keratomileusis), lens implantation, or lens replacement.
The most common refractive surgeries include:
LASIK (laser in-situ keratomileusis)
LASIK surgery starts with a small flap created by a microkeratome or femtosecond laser. Then the surgeon uses an excimer laser to reshape the cornea and correct myopia, hyperopia, astigmatism, or presbyopia.
PRK (photorefractive keratectomy)
PRK is a similar procedure to LASIK, but instead of creating a flap, the entire outer layer of the cornea (epithelium) is removed. This grows back in a few days following the surgery.
LRI (limbal relaxing incision)
Limbal Relaxing Incisions (LRI) correct minor astigmatisms by making incisions at the opposite edges of the cornea, following the curve of the iris, causing a slight flattening in that direction.
ALK (automated lamellar keratoplasty)
ALK is used to correct major myopia or hyperopia. The surgeon uses a device called a microkeratome to separate a thin layer of the cornea and create a flap. Then part of the cornea is removed to flatten it. The results of this surgery are slightly less predictable than LASIK and PRK.
LTK (laser thermal keratoplasty)
LTK uses a holmium laser to heat and shrink the cornea. It is used on patients who have mild hyperopia or astigmatism. The results may be temporary.
CK (conductive keratoplasty)
Conductive keratoplasty is very similar to laser thermal keratoplasty, except it uses radiofrequency energy instead of laser energy to heat and reshape the cornea. CK is more common in the United States because its effects are more consistent and last longer.
Intacs (intracorneal ring)
Intacs corneal inserts or implants are used to treat mild myopia. In this procedure, two small, translucent, and crescent-shaped pieces of a plastic polymer are inserted into the cornea to reshape the front surface of the eye
Lasik is the world’s most popular elective procedure, with more than 28 million LASIK procedures performed worldwide.—American Academy of Ophthalmology