Michael Cole, OD
Child and Family Eye Care Center
Of all the eye care related terms, “astigmatism” is possibly the most misunderstood. Patients who have been diagnosed with astigmatism in the past often enter our clinic worried that this is a serious medical condition that may threaten their vision. Rather than an alarming diagnosis, astigmatism is merely a refractive state akin to nearsightedness or farsightedness and is present in most people. The dictionary defines it as: “A refractive error of the eye in which parallel rays of light from an external source do not converge on a single point on the retina.” That may be a mouthful, but hopefully we can put some minds at ease about this commonplace finding.
The eye contains a series of optical interfaces that refracts or bends light to focus that light on the retina, where that light is formed into an image to be sent to the brain. The first and most impactful optical surface that light interacts with is the cornea. The cornea is a dome shaped structure located at the front of the eye. It is composed of collagen fibers arranged in such a way that it is transparent, allowing light to pass through. In a perfect situation, the central cornea is spherical in shape. A perfect sphere would allow for light from all directions to be focused in one singular point, achieving the clearest image possible.
In human eyes, the anatomical shape of the cornea is rarely spherical in shape without any anatomical imperfections. If any aspherical shape of the cornea is present, that is termed astigmatism. This sometimes sounds as though there is something terribly wrong with the eye, but it is, in fact, the norm rather than the exception. The vast majority of people across the world have astigmatic corneas to one degree or another. It is actually fairly rare to encounter perfectly spherical corneas, even after a surgical procedure like LASIK, which seeks to reshape this tissue.
For the most part, even eyes with astigmatism are symmetrical, having a predictable refractive pattern. These eyes have corneas that rather than being perfectly round like a ping pong ball, are shaped more like a football — having a steeper curvature in one direction and a flatter curvature in the opposite direction. The good news is that the glasses and contact lenses that we use can also be made in this fashion, and this correction becomes part of a normal glasses prescription. In many cases, the amount of astigmatism present is so low, that it is ignored and not included in glasses and contact lens prescriptions. Astigmatism is not typically something that worsens over time, but rather a result of the anatomy of the cornea, which is stable throughout life.
Sometimes, the shape of the cornea does not match a predictable pattern, which we term “irregular” astigmatism. If there is an irregular optical surface that light has to travel through, the image formed is very distorted, and is not well corrected with conventional glasses and contacts. In such circumstances, specialty contact lenses such as scleral lenses are needed to compensate for the poor optics of the eye itself. This type of corneal anomaly is uncommon, and usually occurs after trauma, surgery, or when a genetic anomaly is present.
So, the next time you hear an eye doctor mention astigmatism, don’t fret — it is normal and will be compensated for in a pair of glasses or contact lenses if necessary.
For more information, or to schedule an appointment in our clinic, please contact us at 435-363-2980.