Vision Defects - by Essilor

Myopia 

What is myopia?

Myopia is an eyesight problem mainly caused by the eye being "too long", meaning the distance between the cornea and the retina is too great. In such cases, the image forms just in front of the retina, which means a myopic has trouble seeing things far away, but not close up. The more the person is nearsighted, the more he/she must approach an object to distinguish it clearly.

Hyperopia 

What is hyperopia?

Hyperopia is mainly caused by the eye being "too short", meaning the distance between the cornea and the retina is not great enough. In such cases, the image forms just behind the retina, which means a hyperopic sees things better far away than close up.

Clear vision can only be achieved using forced accommodation which can be tiring in the long term. This problem is therefore generally accompanied by a feeling of eye fatigue.

Astigmatism 

What is astigmatism?

Astigmatism is an eyesight problem mainly caused by "incorrect curvature of the cornea", i.e. the cornea is slightly oval in shape instead of being spherical.
Astigmatism is described in terms of :

module (difference in curvatures) 

axis (orientation of curvatures)

Astigmatics have imprecise near and far vision their peripheral vision is unclear and they cannot clearly distinguish certain shapes and details or see contrasts clearly between horizontal, vertical or oblique lines.

Astigmatism may be combined with other eyesight problems such as myopia, hypermetropia or presbyopia.

Astigmatism is very common among young children and can have a negative impact on academic success since it causes confusion between letters and numbers.

Presbyopia 
What is presbyopia?

Presbyopia is not a visual defect but a natural change in vision which affects everyone. Over time the crystalline lens loses some of its suppleness and therefore its ability to bulge out and focus. The effects of this change are generally felt around the age of 40. Like a badly adjusted camera, the eye no longer focuses the image correctly. For emmetropic presbyopes (those who do not have any trouble seeing things far away) or ametropic presbyopes corrected for distance vision, the difficulty is seeing things close up.



Strabismus 
What is strabismus?


Strabismus is an eyesight disorder related to a defect in the parallelism of the visual axes due to a muscular imbalance. This disrupts the sensory and motor correspondence between the two eyes.


Depending on the direction of distortion in the visual axes, the strabismus is either convergent, divergent or vertical.


Strabismus in children is mostly convergent strabismus – children who squint – which appears between birth and the age of five or six. Divergent strabismus often appears later, between six and 10 years old.


A permanent strabismus results in diplopia: the child sees double. To rectify this sensory problem, the brain ignores the image supplied by one of the two eyes. This results in the loss of binocular vision and 3D relief perception.
If not corrected, diplopia can cause amblyopia or “lazy eye”.


Amblyopia 
What is amblyopia?

Amblyopia, or "lazy eye", is a gradual decrease in visual acuity due to a problem with either or both eyes' development during childhood.
  
There are two types of amblyopia:


• passive amblyopia: this is linked to ametropia, blurred vision which requires optical correction.
• active amblyopia: this is due to conflicting interpretations by the brain between the images from the right eye and the left eye. Causes are strabismus or anisometropia.   


Low vision 
What is low vision?

Low vision, or visual impairment, is due to a significant decrease in visual acuity or a reduction in the field of vision. The term low vision is used when this degradation limits the fulfillment of daily tasks.
Anyone can suffer from low vision at any age, but elderly people are the most commonly affected.

Low vision can have several causes: Age-related macular degeneration (AMD), cataracts, glaucoma, diabetes, retinitis pigmentosa, etc.