EYESIGHT DEFECTS: MYOPIA, ASTIGMATISM, HYPEROPIA AND PRESBYOPIA:
Myopia (nearsigthtedness)
The most common worldwide visual impairment is myopia. 25-30% of the Italian population is affected with this condition.Myopia consists in reduced eyesight for far- vision and good eyesight for near- vision. Myopia is measured in diopters. Low level myopia ranges up to four diopters, medium myopia ranges from five to eight diopters, while anything more is considered to be a high level. Myopia is usually caused by the eye’s excessive anterior -posterior length. In fact, in myopia images are focused in front of the retina and not directly upon it and thus appear blurred.
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Hyperopia (farsigthtedness)
Hyperopia is the opposite of myopia. In hyperopia either the cornea is too flat or the eye is not long enough, so that images come into focus behind the retina and are therefore blurred.
10% of the Italian population is affected with hyperopia.
A person who is hyperopic has difficulty seeing objects at near distance and can only see well through ocular accommodation focusing. This focusing mechanism results from the action of an internal eye muscle. This mechanism is efficient only in young age and the capability decreases approximately at age 40-45 and progresses to its maximum value at age 65-70.
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Astigmatism
Astigmatism consists in reduced eyesight for far or near vision. The first and foremost lens of the eye is the cornea. Its normal shape is spherical but an astigmatic cornea is imperfectly spherical. 5% of the Italian population is affected by astigmatism. The astigmatic patient has difficulty seeing from far or near, depending on whether the problem is respectively myopic astigmatism or hyperopic astigmatism.
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Presbyopia
Our eyes are capable of detailed vision at various distances, this capability is provided by a complicated and marvelous focus mechanism. This physiological system requires the continuous action of a small muscle that is located in our eyes, the ciliary muscle. The contraction of the ciliary muscle modifies the power of the main intraocular lens, and thus, modifies the total dioptric power of the eye allowing variable focus of our vision.
With advancing age, the efficiency of the ciliary muscle diminishes because of progressive hardening of the crystalline lens, hence, reduction of lens flexibility combined with an actual weakening of the ciliary muscle reduces our capability of near vision. The process is gradual and starts in the fourth decade of life but individual variability exists.
The difficulty of near vision is initially greater for the smallest characters, for the nearest distances, and in dim lighted conditions, but sooner or later the necessity for an optical correction arises.
Reading spectacles must be prescribed by the ophthalmologist after a thorough examination that aims to ascertain the complete health and well being of the visual apparatus.
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