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Eye care information on diseases and treatments
Presyopia is due to refractive irregularities of the eye caused by a misshapen cornea. In this condition the vision of the eye is hazy and blurry. Both conditions can also be corrected with either the prescription of glasses or contact lenses.
Comprehending this clinical condition means knowing a little more about the structure and components of the eye and the way light refracts when it hits a lens like the cornea.
Refractive disorders of the eye
A refractive disorder results when the eye fails to bend light for focusing the fovea of the retina. In scientific terms it means the bending of light but in medical terms it also refers to the power of the lens needed (plus or minus) to correct the condition.
When light bends it slows down. This happens when it must travel from one medium to another that is of greater density as a shorter or thicker lenses (think of how light can behave when it hits a thick piece of glass.) When light particles hit a flat surface at right angles they slow down and then continue to travel in a straight line. However if the light strikes a surface at an angle, the particles slow down which causes the bending of light rays. When the light rays are bent it is called refraction. When light passes from the atmosphere into the cornea of the eye it is refracted because the two mediums of light transfer are hugely different in density.
The lens of the eye must therefore flex and adjust itself to accommodate the amount of light that is refracted onto the retina from various objects. If it is compromised the eye must constantly adjust to try and ensure focus.
The lens of the eye and its types
The power of the lens is measured in dioptres (D). There are two types of lenses:
- A 'plus' lens is convex and turns light inwards
The more the dioptre count (D) of a lens, the stronger its power. This is true whether it is for either a plus or minus type.
In Greek Presbyopia means “Old Man’s Eye.” It is an age related condition that is characterized by an inability to focus on objects that are close up (such as that newspaper you are reading.) All humans develop it at some time during middle or old age.
The cause of presbyopia is weakened proteins in the eye lens that cause them to become less elastic. It also marks the onset of astigmatism and the need for prescription bifocals, trifocals or reading glasses. A temporary solution for readers is to hold the text as far away from you as possible.
Some other conditions like diabetes, severe dehydration and too much ultraviolet also cause protein in the eye lens to break down which of course, can result in astigmatism.
Prevalence of presbyopia
Although presbyopia triggered by aging there is no set age at which it occurs. It varies from individual to individual and is also more common in some places of the world than others. Research has shown that the closer a person lives to the equator the more likely they are to develop it. It is also more common in climates that do not have one consistent high temperature. As most well developed industrial countries tend to have the climate with varied temperatures there are more cases of presbyopia. The result is that over sixty five percent of all people who wear glasses do so because they suffer from presbyopia.