Common types and categories of astigmatism
Astigmatism is a refractive error that comes in many types and categories. Your eye doctor determines what kind of astigmatism you have in order to properly correct your vision.
Reviewing the key types of astigmatism can help you understand its effect on vision and how you and your eye doctor can team up to correct it.
Astigmatism usually results from flaws in the cornea, a clear, window-like covering on the front surface of the eye. A normal cornea is rounded like the lens of a camera, and it performs much the same function. Working in unison with the eye’s internal (or intraocular) lens, the cornea uses refraction to bring objects into visual focus.
Though other parts of the eye, such as the intraocular lens, may contribute to the development of astigmatism, a diagnosis depends primarily on identifying three categories of refractive errors in the cornea: myopic, hyperopic and mixed astigmatism.
Eye doctors scan the cornea for signs of these imperfections. If they find them, they then use lines called meridians to divide the arc of the cornea into sections. These lines in the cornea function much like the north-to-south and east-to-west meridians on a map or globe.
When assessing astigmatism, eye doctors view meridians like the hands on a traditional clock. One meridian, for example, connects the 6 (bottom) to the 12 (top). Another meridian goes from 9 (left) to 3 (right).
A normal cornea has a consistent arc. With astigmatism, the arc is inconsistent: too steep in one meridian and too flat in another. Diagnosing astigmatism usually requires detecting principal meridians — the steepest and flattest parts of the cornea — and figuring out how they blur or distort vision.
An astigmatism diagnosis generally falls into one of three categories:
Myopic astigmatism. Myopia means nearsightedness — blurry distance vision and clear near vision. If the cornea has the proper arc, then flaws elsewhere in the eye typically produce shortsightedness. Myopic astigmatism, by contrast, happens when the principal meridians in the cornea generate nearsightedness and astigmatism.
Hyperopic astigmatism. Hyperopia means farsightedness — blurry near vision and clear distance vision. Thus, hyperopic astigmatism results when the cornea’s principal meridians produce farsightedness and astigmatism.
Mixed astigmatism. Some corneas produce both myopia and hyperopia. When one principal meridian of the cornea results in nearsightedness and the other in farsightedness, doctors call it mixed astigmatism.
Common astigmatism types
Your eye doctor might describe your astigmatism as regular or irregular. You might also hear terms like corneal and lenticular.
Regular astigmatism. Most of the time, the principal meridians (steepest and flattest areas of the cornea) are 90 degrees apart, or perpendicular to each other. Thus, eye doctors refer to this as regular astigmatism.
Irregular astigmatism. Principal meridians that aren’t perpendicular are considered irregular. Injuries or scarring also may produce irregular astigmatism in the cornea. Other causes include keratoconus, a long-term thinning of the cornea, and some forms of eye surgery.
Corneal and lenticular astigmatism. Though corneal astigmatism is most common, some people have the lenticular variety — astigmatism occurs in the intraocular lens as opposed to in the cornea.
Flaws in multiple structures of the eye beyond the lens and cornea may contribute to astigmatism, though they are comparatively rare.
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Page published in July 2020
Page updated in July 2021