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Keratoconus (KC) is a disorder of the eye that results in progressive thinning of the cornea. [3] The protrusion of the cornea may result in blurry vision, double vision, nearsightedness, irregular astigmatism, [4] and light sensitivity leading to poor quality-of-life. [3] [5] [6] Usually both eyes are affected. [3]
Astigmatism may also occur following a cataract surgery or a corneal injury. [15] Contraction of the scar due to wound or cataract extraction causes astigmatism due to flattening of the cornea in one direction. [15] In keratoconus, progressive thinning and steepening of the cornea cause irregular astigmatism. [16]
Also, the alignment of the measurement can be difficult, especially with eyes that have keratoconus, a significant astigmatism, or sometimes after refractive surgery. Corneal topography instruments generate a measurement called simulated keratometry (SimK), which approximates the classic measurement of the widely used keratometer .
Pellucid marginal degeneration, a bilateral, noninflammatory disorder, characterized by a peripheral band of thinning of the inferior cornea. [4] Posterior keratoconus, a rare condition, usually congenital, which causes a nonprogressive thinning of the inner surface of the cornea, while the curvature of the anterior surface remains normal.
The distribution of the degeneration is crescent or arcuate shaped. The cornea just above the region of thinning is of normal thickness, and may protrude anteriorly, which creates an irregular astigmatism. [5] This is described as a "beer belly" appearance since the greatest protrusion occurs below the horizontal midline (unlike keratoconus).
Astigmatism can be often corrected by glasses with a lens that has different radii of curvature in different planes (a cylindrical lens), contact lenses, or refractive surgery. [5] Astigmatism is quite common. Studies have shown that about one in three people suffers from it. [6] [7] [8] The prevalence of astigmatism increases with age. [9]
Keratoconus, a progressive thinning of the cornea, is a common corneal disorder. Keratoconus occurring after refractive surgery is called Corneal Ectasia. It is believed that additional thinning of the cornea via refractive surgery may contribute to advancement of the disease [31] that may lead to the need for a corneal transplant.
In order to see a clear image, the eye must focus rays of light on to the light-sensing part of the eye – the retina, which is located in the back of the eye.This focusing – called refraction – is performed mainly by the cornea and the lens, which are located at the front of the eye, the anterior segment.