Blurry vision caused by irregularly shaped corneas or lenses is called astigmatism. It is a common “refractive error” and, in most cases, easily corrected.
Shape is an essential component of optics. Incoming light must converge (focus) —from all angles of entry through our corneas at precisely the same point on the retina to produce clear vision.
That precise convergence happens naturally when the cornea shape is spherical, like a baseball or basketball. Then all the lines of light converge at the same point, and vision is clear and distinct.
But irregularly curved corneas, ones that are shaped more like an American football (prolate spheroid), cause astigmatism. Their irregular football-like shape means that the lines of curvature from the center to the vertical and horizontal edges are not uniform throughout the curve and so light traveling along this irregular curve will not ALL strike the retina uniformly. Vision is blurry because of these multiple focal points created by the irregular curve.
Although astigmatism is a common condition, estimated to be present in more than 30 percent of the U.S. population, its causes are not known. It can be present from birth or develop later, and it can decrease or increase over time. It can also be caused by an eye injury that scarred the cornea. Astigmatism is not a disease or eye health issue, it is simply a refractive error caused by eyeball shape.
Three Common Types of Astigmatism
- Myopic astigmatism is when one or both eyes are nearsighted because of an irregular cornea curvature.
- Hyperopic astigmatism is when one or both eyes are farsighted because of an irregular cornea curvature.
- Mixed astigmatism is when one eye is nearsighted and the other farsighted because of irregular cornea curvatures.
Because astigmatism usually occurs along with other refractive errors that cause near and farsightedness, the eyeglass prescription to correct a refractive error and an astigmatism are written with three numbers that indicate:
The spherical lens power is to correct the near or farsightedness and the “cylinder lens” is shaped to correct the refractive error caused by the astigmatic cornea curvature and the axis number is to indicate where the astigmatic curvature correction lies along the vertical or horizontal axis so when the glasses are being made the optician can correctly shape the slope of the corrective curve.
Toric soft contact lenses have different powers along the lenses to correct for the irregular cornea curvature causing the specific astigmatism. The lenses rotate to the proper orientation on the cornea, so the corrective power is properly aligned.
Gas permeable contact lenses are rigid and retain that spherical shape when worn and this rigid uniform surface replaces the irregularly shaped cornea as the primary refracting surface and thereby corrects for the astigmatism. Fitting gas permeable lenses generally takes more time and fitting expertise to find the optimal lenses.
Hybrid contact lenses have a central zone of rigid gas permeable lens material surrounded by a hydrogel material. These lenses also take additional fitting time.
Refractive surgery such as LASIK which reshapes the cornea can correct most types of astigmatism.
Find out which correction is right for you!
Give us a call or consult your eye doctor for a complete eye exam to find out which astigmatic corrective method is right for you.
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