Refractive error correction methods (Part 1)

1399/10/16 - 17:45 / 39

 

Refractive Errors

 

Refractive error means that the shape of your eye does not bend light correctly, resulting in a blurred image. The main types of refractive errors are myopia (nearsightedness), hyperopia (farsightedness), presbyopia (loss of near vision with age), and astigmatism.

 

Symptoms

 

  • Blurred vision
  • Difficulty reading or seeing up close
  • Crossing of the eyes in children (esotropia)

 

 

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Causes

 

Overuse of the eyes does not cause or worsen refractive error. The causes of the main types of refractive error are described below:

  • Myopia (close objects are clear, and distant objects are blurry)
    Also known as nearsightedness, myopia is usually inherited and often discovered in childhood. Myopia often progresses throughout the teenage years when the body is growing rapidly. 
  • Hyperopia (close objects are more blurry than distant objects)
    Also known as farsightedness, hyperopia can also be inherited. Children often have hyperopia, which may lessen in adulthood. In mild hyperopia, distance vision is clear while near vision is blurry. In more advanced hyperopia, vision can be blurred at all distances. 
  • Presbyopia (aging of the lens in the eye)
    After age 40, the lens of the eye becomes more rigid and does not flex as easily. As a result, the eye loses its focusing ability and it becomes more difficult to read at close range. This normal aging process of the lens can also be combined with myopia, hyperopia or astigmatism. 
  • Astigmatism Astigmatism usually occurs when the front surface of the eye, the cornea, has an asymmetric curvature. Normally the cornea is smooth and equally curved in all directions, and light entering the cornea is focused equally on all planes, or in all directions. In astigmatism, the front surface of the cornea is curved more in one direction than in another. This abnormality may result in vision that is much like looking into a distorted, wavy mirror. Usually, astigmatism causes blurred vision at all distances. 

 

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Risk Factors

 

People with high degrees of myopia have a higher risk of retinal detachment which may require surgical repair.

Tests and Diagnosis

A refractive error can be diagnosed by an eye care professional during a routine eye examination.  Testing usually consists of asking the patient to read a vision chart while testing an assortment of lenses to maximize a patient’s vision. Special imaging or other testing is rarely necessary.

 

Treatment and Drugs

 

Refractive disorders are commonly treated using corrective lenses, such as eyeglasses or contact lenses. Refractive surgery (such as LASIK) can also be used to correct some refractive disorders.  Presbyopia, in the absence of any other refractive error, can sometimes be treated with over-the-counter reading glasses. There is no way to slow down or reverse presbyopia.

 

surgery

 

There are three primary types of refractive errors:

Myopia: persons with myopia, or nearsightedness, have more difficulty seeing distant objects as clearly as near objects.

Hyperopia: persons with hyperopia, or farsightedness, have more difficulty seeing near objects as clearly as distant objects.

Astigmatism: astigmatism is a distortion of the image on the retina caused by irregularities in the cornea or lens of the eye.

Combinations of myopia and astigmatism or hyperopia and astigmatism are common. Glasses or contact lenses are designed to compensate for the eye's imperfections. Surgical procedures aimed at improving the focusing power of the eye are called refractive surgery. In LASIK surgery, precise and controlled removal of corneal tissue by a special laser reshapes the cornea changing its focusing power.

 

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Other types of refractive surgery

 

 

Radial Keratotomy or RK and Photorefractive Keratectomy or PRK are other refractive surgeries used to reshape the cornea. In RK, a very sharp knife is used to cut slits in the cornea changing its shape. PRK was the first surgical procedure developed to reshape the cornea, by sculpting, using a laser. Later, LASIK was developed. The same type of laser is used for LASIK and PRK. Often the exact same laser is used for the two types of surgery. The major difference between the two surgeries is the way that the stroma, the middle layer of the cornea, is exposed before it is vaporized with the laser. In PRK, the top layer of the cornea, called the epithelium, is scraped away to expose the stromal layer underneath. In LASIK, a flap is cut in the stromal layer and the flap is folded back.

Another type of refractive surgery is thermokeratoplasty in which heat is used to reshape the cornea. The source of the heat can be a laser, but it is a different kind of laser than is used for LASIK and PRK. Other refractive devices include corneal ring segments that are inserted into the stroma and special contact lenses that temporarily reshape the cornea (orthokeratology).

 

 

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Prepared:
Neshat Khosravi - Microbiologist

Reference:

https://www.medicinenet.com

 



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