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Eye care information on diseases and treatments
Astigmatism is a result of refractive irregularities of the eye caused by a misshaped cornea. In astigmatism the vision of the eye is hazy and blurred. The condition can be corrected with either a prescription of glasses or contact lenses.
Comprehending the astigmatism 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 eye disorders
A refractive disorder occurs when the eye fails to bend light for focusing on 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. When light waves 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 density of the two mediums are hugely different.
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.
Astigmatism results when the cornea of the eye has an incorrect shape and does not refract light properly. This causes the retina to fail to produce a clear image.
Sometimes astigmatism is caused by the pressure of the top eyelid applying pressure on the cornea. Astigmatism is also the result if cataract surgery distorts the cornea. However more and more doctors are performing a procedure called phacoemuslification to reduce this side effect of cataract surgery.
Other causes of astigmatism are corneal scars, a conical cornea or keratoconus.
Correction of astigmatism
The usual treatment is the prescription of glasses that correct the refractive power of the person’s misshapen cornea. Soft contact lenses are not suitable for correcting astigmatism as they meld to the contours of the cornea and do not improve the vision.
The lenses prescribed for astigmatism boast a straight line through the lens that refracts light in different ways depending on how the lens manipulated.
Prevalence of astigmatism
Astigmatism usually develops very early in life even if it does not become a problem until we are older. Research has shown that an estimated fifty percent of infants suffer from astigmatism. The older you get the less likely you are to suffer from the effects of congenital astigmatism. Astigmatism can also be the right of intraocular surgery like corneal transplants or cataract surgery.