Myopia – Astigmatism – Hypermetropia – Presbyopia
Myopia
Myopia is considered one of the most common refractive errors of the eye and causes difficulty in distant vision.
A person with myopia sees far away blurry, while at close distances he can maintain good vision. A nearsighted person often half-closes his eyes in an effort to see clearly far away and this can tire them (copiopia).
When he half-closes his eyes, he tries to keep the most central rays of light from passing through the opening of his pupil and exclude the more peripheral ones that undergo more aberrations and cause the image to blur.
In myopia, the rays of light entering the eye are not concentrated on the retina as is normal, but at some point in front of it, so a sharp image is not formed.
The reasons why the light rays are not concentrated on the retina, but further in front of it, are either refractive, i.e. the curvature and therefore the power of the cornea (the front lens of the eye) is greater than what the eye needs to focus the light on in the retina, either axial, i.e. the anterior-posterior axis of the eye is larger (the latter basically occurs in high myopia).
Myopia is treated with:
- The use of eyeglasses.
- Contact lenses.
- With laser refractive surgery.
- With surgery to replace the crystalline lens (in old age with coexistent cataract).
Astigmatism
Astigmatism is a refractive error of the eye, which affects both distance and near vision. It usually shows up as difficulty in focusing, blurred vision, blurred outline of objects, an image somewhere around the object we are looking at, difficulty reading small details with headache and eyestrain (copiopia).
Astigmatism is in fact an asymmetry in the curvature of the cornea (the front lens of the eye), and/or the crystalline lens (the second lens of the eye located inside the eye).
In other words, the curvature and therefore the strength of the cornea and/or the crystalline lens is not the same in all meridians, with the result that the rays entering the eye are not concentrated in one point of the retina, but in lines perpendicular to each other, in front or behind from the retina. For this reason in a prescription for glasses, the strength of the astigmatism is accompanied by its axis, i.e. in which degrees this asymmetry is located.
Astigmatism is treated with:
- The use of eyeglasses.
- The use of contact lenses.
- With laser refractive surgery.
- With surgery to replace the crystalline lens with a toric intraocular lens (in old age with coexistent cataract).
Hypermetropia
Hypermetropia is a refractive error of the eye that mainly causes difficulty in near vision and depending on its size and the age of the person can also affect distant vision.
It is important to be diagnosed from an early age, because a large hypermetropia can cause amblyopia (irreversible low vision) and strabismus. Children have an ability to hide some degrees of hypermetropia and be able to see clearly while farsighted, but this tires their eyes and can cause headaches and easy reading fatigue (copiopia).
A special technique is needed during the examination to relax the lens of the children's eye and reveal the degrees of hypermetropia they may be hiding. In children it usually appears with difficulty in reading, close focus and indifference to small objects around them.
In hypermetropia the rays of light entering the eye are not concentrated on the retina as is normal, but at some point behind it, so a sharp image is not formed on the retina.
The reasons why the light rays are not concentrated on the retina, but behind it are either refractive, i.e. the curvature and therefore the power of the cornea (the front lens of the eye) is less than what the eye needs to focus the light on the retina, whether axial, that is, the anteroposterior axis of the eye is smaller (e.g. in small children who have small eyes, and as the child grows and his eye grows, the hypermetropia may decrease).
Presbyopia
Presbyopia is the difficulty in near vision due to age. It is the most common vision disorder of middle age.
When we look at objects at close range a lens inside our eye (the crystalline lens) changes shape, swells, zooms and clears our vision at close distances.
Somewhere after the age of 40 this lens is no longer as flexible and has a hard time swelling and zooming to focus up close. This weakness of the lens to change its shape so that the image becomes clearer near is called presbyopia.
Almost all people experience presbyopia gradually in their lives after the age of 45 to 50, having an inability to focus on close points, difficulty in near vision, fatigue and headaches after reading for a while, especially if there is not good lighting.
At which age someone will need near glasses and what degree, depends on their age, their refractive error (myopia, hypermetropia, astigmatism) and on their daily needs for sharp near vision.
