Refractive Errors and Optics

What are Refractive Errors?

The cornea and lens of the eye refract the light rays from an object to focus them on retina. These rays create an image on the retina. The image is transmitted by optic nerve to the brain which identifies the object. 
When for some reasons light rays are not refracted on the retina, images appear blurred.

What are the types of Refractive Errors?


(1) Myopia (Minus number)

The eyes of  people with this condition are either longer than normal eyes or their corneas are of a shape that refracts light rays to focus them in front  of   the  retina.  Due  to   this  condition  distant objects appear blurred and near objects    appear sharp.
Such defect is normally hereditary. It is noticed in a child between 8 to 12 years of age. Minus numbers increase in a child between 12 and 20 years when the growth is rapid. There may be insignificant change between 20 and 40 years. The risk of retinal detachment is higher in people with high minus numbers. Such people should get their retina checked at regular intervals.

(2) Hyperopia (Plus numbers)

The eyes of   people with such defect are either smaller than normal eyes or their corneas are of a shape that refracts light rays to focus them behind the retina.  Due to  this  condition, objects at  short  distance  appear  blurred and those at long distance appear sharp. The quality of the natural lens in children helps them overcome such defect. However, with ageing this ability decreases, leading to vision defect. This defect is largely hereditary.
 
(3) Astigmatism(Cylindrical numbers)

The cornea in a normal eye is round like a basketball with even surface. However, the cornea of eyes with Astigmatism is oval-shaped, bent on one side, like an American football. Due to this condition, near as well as distant vision of 
a person becomes blurred and uneven. Such Defect can exist with either myopia or hyperopia.

(4) Presbyopia

For seeing near objects, it is   essential that the diopter of our natural lens      increases by +3. In young age the lens is flexible and is able to change shape as required. This automatically takes care of accommodation in vision. After 40 years of age, the lens becomes hard and can not change shape easily. This leads to problems in near vision. This problem escalates up to 60 years. This   defect is known as Presbyopia.

How can refractive error be corrected?

No scientific evidence exists of correction of refractive errors by either exercise or taking vitamins or medicines. Specs and contact lenses are used to  focus light rays on retina. However, LASIK treatment is a permanent solution.

Specs: 

This is a simple solution. Special lenses are available to protect against ultra-violet rays. Specs made of polycarbonate can be used in risk-prone activities like sports.
Plus numbers are prescribed for presbyopia. Those with hyperopia can use bifocal, progressive or separate specs for near and distant vision. Those who do not have number for distant vision should use appropriate specs for activities like reading, embroidery and picking grains. Presbyopic numbers increase every 2 to 5 years between ages 40 and 60. It is natural for presbyopic numbers to increase even though one uses specs. This is not a cause of worry.

Contact Lens: 

If one does not like to use specs or is worried about misplacing them, one can use lenses to be worn on the cornea. Various types of contact lenses are available. A suitable lens would depend on the number, measurement of the cornea and lifestyle of the person. This can be decided in consultation with the doctor.

LASIK: 

It is used to treat all types of refractive errors. The cornea is polished and its shape is changed using a micro-instrument called Microkeratome and laser. This enables light rays to be focussed on retina and gives clear vision. There is no pain and no patch is required on the eye. The person can resume routine activities from the next day. The person should be over 18 years and the numbers should be stable for six months prior to LASIK treatment.
  • The eye is anaesthetized and prepared for procedure
  • The uppermost layer is lifted after a cut with ‘Microkeratome’
  • Laser rays are given to reshape the cornea
  • Finally, the layer that was lifted is replaced