Astigmatism and Cataract Surgery
Astigmatism is a condition where the parts of the eye that focus images are imperfect, resulting in blurred vision. This is similar to nearsightedness or farsightedness, and can be corrected by glasses or contact lenses. Astigmatism is very common and can change over time.
There are two main parts of the eye that focus light. The cornea, which is the front surface of the eye (where one would place a contact lens) is the first part of the eye that light encounters. The natural lens, inside the eye, is the second part of the eye that focuses images. If either the cornea or the lens focus light imperfectly, this will cause nearsightedness, farsightedness or astigmatism, depending upon how the light rays are bent. All of these can generally be corrected with glasses or contacts.
Cataract surgery is the process of removing the natural lens in the eye and replacing it with a man-made intra-ocular lens implant (IOL). Before cataract surgery, measurements are taken of the eye. Because the lens is removed during surgery, the curvature of the cornea will determine how much astigmatism will be left after the lens (cataract) is removed. The remaining astigmatism after lens removal (cataract surgery) can be quite different from the preoperative astigmatism that the patient has in their eye.
Modern cataract surgery is an opportunity to become more independent of glasses and contact lenses. Lens implants that correct astigmatism (toric lens implants) can be used. For smaller amounts of astigmatism, the cornea can be reshaped using a technique called limbal relaxing incisions (LRIs).
Determining how much astigmatism needs to be corrected is based on measurements of the cornea. Some people have astigmatism that is caused mainly by imperfections in how the lens focuses light. They have very little corneal astigmatism. In this case, taking out the cloudy lens (cataract surgery) will address most of the astigmatism and no further treatment of astigmatism may be needed. In other cases, the astigmatism is mostly in the cornea, and taking out the lens will leave astigmatism that will need to be addressed with glasses, contacts, an astigmatism correcting lens implant, or LRIs to reshape the cornea.
Unmasking of astigmatism:
The amount of astigmatism that needs to be corrected at the time of cataract surgery can be more, less, or equal to the amount of astigmatism in their preoperative glasses prescription.
In some people, preoperative astigmatism in the cornea and the lens neutralize each other. If the astigmatism is oriented in one direction in the cornea, and in the opposite direction in the lens, these can cancel each other out. In this case the patient may never have needed astigmatism correction in glasses or contact lenses before cataract removal. But when the lens is removed during cataract surgery, the astigmatism in the cornea can become evident as the lens is no longer present to neutralize the corneal astigmatism. This is how corneal astigmatism in some patients becomes unmasked. In this case a patient who has never had astigmatism in their glasses or contact lens prescription may have corneal astigmatism after cataract removal. The astigmatism needs to be corrected for clear vision. This can be done in the eye itself (with an astigmatism correcting lens implant or with corneal reshaping LRIs, depending upon the amount of astigmatism). Alternatively, astigmatism can be corrected outside of the eye in glasses or contact lenses.
Before cataract surgery, special measurements of the eye are taken so that we know whether or not astigmatism will need to be addressed with cataract removal. The patient is given options before the cataract is removed as to how they would like to address expected astigmatism if it is likely to be significant.
