Astigmatic Correction

What is astigmatism?

Astigmatism is a name given to describe the shape of the cornea. The cornea is the clear front layer of the eye responsible for two thirds of the eye's focusing power. When the cornea has a spherical shape, astigmatism is absent. When the cornea is not spherical in shape (aspheric) it is said to have astigmatism. Astigmatism is primarily found on the front surface of the cornea. One can think of astigmatism as the front corneal surface being shaped like a rugby ball with both steeper and flatter curved meridians. A cornea with astigmatism will create two zones of focus which is generally perceived as blurring of vision. Most individuals have some astigmatism and its significance is determined by the amount.

How is astigmatism measured?

Astigmatism is measured during preoperative assessments by instruments that evaluate the shape of your cornea known as keratometers and topographers. A number of measurements are taken with multiple instruments to achieve the best evaluation of your astigmatism. The amount of astigmatism is measured in units called diopters.

How is astigmatism treated?

There are several options for treating astigmatism and these fall into the categories of non-surgical and surgical. Glasses and contact lenses can be created with compensatory curves that match the shape of a cornea in order to give clear vision. Laser vision correction is a surgical option for treating astigmatism. Cataract surgery also provides an opportunity to reduce astigmatism.

Astigmatism treatment and cataract surgery:

How is astigmatism treated at the time of cataract surgery?

Cataract is a clouding of the eyes naturally clear lens. The lens is responsible for about one third of the focusing of the eye. During cataract surgery the cloudy lens is replaced with a clear lens implant which occupies the same position as the natural lens.

Astigmatism can be reduced at the time of cataract surgery by making extra incisions in the peripheral cornea called "relaxing incisions" or by using an astigmatism correcting lens implant called a "toric" implant.

How do surgical techniques for correcting astigmatism work?

Corneal relaxing incisions correct astigmatism by reducing the curvature of the steeper corneal meridian. Corneal relaxing incisions are placed at the limbus. (The front of the eye where the white part meets the clear cornea). Consequently, the term limbal relaxing incision is used. Preoperative measurements determine the orientation of steep corneal meridians and one or two incisions can be placed in the same orientation resulting in a local corneal flattening and overall reduction of astigmatism.

Toric lenses are implanted at the time of cataract surgery. They are, in all ways, identical to regular soft monofocal lens implants with the exception of a curve greater in one meridian to counter the effect of corneal astigmatism. The procedure for toric lens implants is exactly the same as for regular soft lens implantation with the exception of the final step which involves matching of the lens implant meridian to the individual's astigmatism by rotating the lens implant to the correct orientation.

Does all astigmatism need to be treated at the time of cataract surgery?

Astigmatism of the cornea results in multiple zones of focus created inside the eye, which is perceived as a blur by the retina. Smaller degrees of astigmatism (less than 1.25 diopters) may actually give people increased depth of focus which can help with their near vision after cataract surgery and may not significantly affect their distant vision. Consequently, less than 1.25 diopters of astigmatism may not need to be treated. Larger amounts of astigmatism usually require correction and most individuals in this range will already have been wearing glasses or contact lenses prior to cataract surgery.

How can my astigmatism be corrected?

Your astigmatism may be corrected at the time of cataract surgery by the techniques mentioned above or after cataract surgery with glasses or contact lenses. All these methods will improve your quality of vision providing you have no other limiting factors.

If you would like to be less dependent on glasses for clearer distant vision after cataract surgery and you have 1.25 diopters or more of astigmatism then you may want to consider one of the surgical options for decreasing your astigmatism at the time of cataract surgery.

If you are happy with the option of glasses or contact lenses to correct your astigmatism then that will be the best way for you to proceed.