Eye astigmatism
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Refractive disorders involve focusing problems caused when the eye fails to bend light for focusing on the fovea of the retina. Technically refraction means the bending of light but in medical terms it can also refers to the method of measuring the power of the lens needed.

Now, what does bending of light mean? Light slows down when it travels from one medium to another of greater density. All the light particles slow down simultaneously when light hits a flat surface at right angles. Hence, it continues to travel in a straight line.
However, if light hits a surface at an angle, the particles that first enter the new medium slow down first, followed by those that follow. This phenomenon results in the bending or refracting of light. So, when light passes from the air into the cornea of the eye (the two mediums have a huge difference in density) it refracts and then focuses onto the retina with the help of the lens of the eye.

The eye needs to do a lot of accommodation in order to refract light from various objects, angles and distances. Hence, the lens of the eye must accordingly increase its power to ensure focusing adaptability.

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 focuses light inwards
  • A 'minus' lens is concave and turns light outwards

The more the dioptre count of a lens, the stronger its power for either plus or minus type.

Astigmatism

Astigmatism occurs when the cornea of the eye is incorrectly shaped and fails to refract light properly. Hence, the retina fails to form a clear image of objects.

A certain amount of astigmatism is present in most people, caused by the pressure of the top eyelid on the cornea. In mild cases, it is only a negligible difference of half a dioptre.

Astigmatism is also often caused if a cataract surgery distorts the cornea. The modern surgical method of phacoemulsification has reduced this risk, since it involves minimum invasion. Another cause of astigmatism is corneal scars and an even rarer cause is keratoconus or more commonly called the conical cornea.

Prevalence of presbyopia

Astigmatism develops very early in life. Statistics have shown that an estimated 50% of full-term infants suffer from astigmatism of over 1D in their initial phase of life. The frequency of astigmatism reduces with adulthood and may occur only due to some kind of intraocular surgery like corneal transplants, cataract surgery or treatment of corneal damages.

Correction of astigmatism

The conventional treatment is the use of spectacles that correct the refractive power of the lens of the eye. It has been advised that soft contact lenses are not suitable for correcting astigmatism as they fit into the contours of the cornea. Lenses for spectacles should also be properly aligned with the eye.

The spectacle prescribed for astigmatism have a straight line through the lens and rotates if the lens is moved both in clockwise and anti-clockwise directions.

 
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