By David F. Chang MD
What is astigmatism?
People wear eyeglasses if their eyes are not naturally in good focus. Such eyes are said to have “refractive error”. Besides myopia (nearsightedness) and hyperopia (farsightedness), the third major category of refractive error is called astigmatism. This blur results from a cornea that is not perfectly spherical in shape.
The cornea is the optical window of the eye and should be perfectly spherical – just like a camera lens. The back of a spoon is oblong instead of spherical and illustrates a shape that has “astigmatism”. If the cornea has significant astigmatism, then details such as print or street signs will be blurred without glasses. Eyeglasses correct the blur by incorporating an oblong lens component that cancels out the oblong shape of the cornea.
Like the shape of our ears, we inherit the shape of our corneas. For this reason, people of all ages may have astigmatism and require spectacles to optimize their focus. The more astigmatism one has, the blurrier the vision is without eyeglasses. Astigmatism blur can also be corrected with hard contact lenses, or special “toric” soft contact lenses. Finally, refractive procedures, such as Lasik or Astigmatic Keratotomy, can surgically reduce astigmatism by re-shaping the oblong curvature of the cornea.
Does cataract surgery change the astigmatism?
Large incisions and sutures can cause unwanted changes to the natural corneal shape. Thus, large incision cataract surgery usually worsens corneal astigmatism, and represents another advantage of sutureless, small incision techniques. Unlike eyeglasses, standard intraocular lens implants do not correct astigmatism. Because they are placed inside the eye, they do not alter or improve the shape of the cornea.
Differences in one’s astigmatism can explain why some patients see better without glasses than others following cataract surgery. While no one can see optimally at all distances (far, intermediate, or close-up) without glasses after cataract surgery, most patients want decent distance vision without glasses. For this to occur, the amount of blur caused by astigmatism must be minimal.
What are the surgical options for reducing astigmatism?
In addition to Lasik, it is possible to reduce astigmatism by making small incisions in the outer surface of the cornea. This technique is called “astigmatic keratotomy”, and it is actually more effective the older the patient is. The incisions must be placed in a precise location in order to reduce the undesirable oblong shape of the cornea. Varying the length, depth, and location of the incisions determines how much of an effect they have. Unlike a cataract incision, these incisions are only partial thickness. Special, expensive surgical blades made from diamonds are used. Astigmatic keratotomy can be done at the same time as cataract surgery, because these incisions do not weaken the eye, or prolong the surgical or recovery time. Indeed, the patient is already taking antibiotic drops and having new glasses prescribed following surgery. The main risk would be improper incision depth. However, using specially designed “guarded”; blades can accurately prevent this.
Astigmatic keratotomy is optional, and has nothing to do with the cataract procedure itself. It does not at all affect the eye2s health. The goal is to reduce one2s dependence upon eyeglasses. Usually astigmatism will not be completely eliminated. Nevertheless, if it is significantly reduced, the vision without glasses will be improved and the eyeglass prescription will be less “strong”. Like Lasik and other refractive procedures, astigmatic keratotomy addresses the inconvenience of eyeglasses and is not covered by health insurance, such as Medicare. For those patients hoping to enhance their ability to see without glasses, this is an excellent option to combine with the cataract operation.