What is astigmatism?
In an astigmatic eye, the image is formed differently depending on the meridians.
There is a meridian of maximum power and a meridian of minimum power, between which the power varies continuously. Different combinations are possible: the astigmatic eye may be myopic or hyperopic in one meridian and without refractive error in the other (simple astigmatism); myopic or hyperopic in all meridians (compound astigmatism); or myopic in one meridian and hyperopic in the other (mixed astigmatism). The astigmatic eye often has an oval-shaped cornea rather than a round one. People with astigmatism often have imprecise vision at any distance and do not clearly perceive contrasts between horizontal, vertical, or oblique lines.
The principle of astigmatism correction is:
- Either placing a toric lens or contact lens in front of the eye, with power that varies by meridian in the opposite way to the eye’s astigmatism.
- Or modifying the curvature of the cornea using a laser to harmonize the eye’s optical power. This is the principle of the (excimer) laser used in PRK or LASIK procedures.
- Or implanting a lens inside the eye (intraocular lens, a technique mainly used for high corrections that do not allow laser reshaping of the cornea).
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