How Much Astigmatism Do I Have?
Understanding the amount of astigmatism you have- or don't have- is important. If you have a lot, you might choose to invest in a potentially dramatically better result from your cataract surgery. If you don't have a lot, you can avoid spending a significant amount of money unnecessarily.
It all depends on how much astigmatism you expect to have postoperatively.
This more technical page shows you two ways to estimate the amount of astigmatism you can anticipate having after standard cataract surgery. If either method predicts that you might have 1.00 Diopter or more of astigmatism postoperatively, then it is likely you will experience noticeable benefit from astigmatism correction during cataract surgery.
1. Refraction Method:
If you have a recent refraction, the second number (shown in pink below) describes the amount of astigmatism in your glasses. Do not use your contact lens prescription for this, only your glasses prescription. Your refraction is a standard part of your medical record with your eye doctor and will be provided if you ask for it.
Here is an example of a refraction:
OD -1.00 + 2.00 x 085
OS -2.25 + 1.00 x 010
This patient has 2.00 Diopters of astigmatism in the right eye (OD), and 1.00 Diopters of astigmatism in the left eye (OS). Sometimes you will see a (+) before the astigmatism number, as in this case, and sometimes you will see a (-). You can ignore this for our current purposes. Only the size of the number matters for this analysis. The higher this number, the more likely you are to benefit from astigmatic management during cataract surgery.
The refraction method provides a rough estimate of the amount of astigmatism you can expect to have after cataract surgery if you do not choose to have it treated.
2. Corneal Curvature Method:
This gives you a better estimate than the refraction method. Your "K values" are the numbers that describe your anterior corneal curvature, which is typically the major source of astigmatism after cataract surgery. Your Ophthalmologist or Optometrist will provide these numbers for you if you ask.
For the above refraction, the K values might look like this.
OD K1 = 44.25 @ 085 K2 = 42.25 @ 175
OS K1 = 44.00 @ 010 K2 = 43.00 @ 100
The K values describe the steepest and flattest anterior corneal curvature, and the orientation of each curvature. These two values are always 90 degrees apart.
Reading the K values above:
The right eye (OD) has a steep curvature of 44.25 Diopters at 85 degrees and a flat curvature of 42.25 Diopters at 175 degrees. The left eye (OS) has a steep curvature of 44.00 Diopters at 010 degrees and a flat curvature of 43.00 Diopters at 100 degrees.
Below is a schematic drawing of these astigmatic values. This is drawn as if you are looking at yourself in the mirror, so you will notice that right and left are reversed. This is done because most technical drawings in ophthalmology are drawn from the observer's (doctor's) point of view. It's like you are looking at yourself in the mirror.
The thick line represents the steepest corneal curvature, and the thin line represents the least steep corneal curvature.
To obtain the actual astigmatic value, subtract the smaller flat value from the larger steep value. In the example above:
OD 44.25 - 42.25 = 2.00Diopters @ 085 degrees
OS 44.00 - 43.00 = 1.00 Diopters @ 010 degrees
This is drawn schematically below:
Next, we will step into your perspective. You will notice that right and left are now from your point of view. You are no longer looking in the mirror; now you are looking out of your own eyes. The images below approximate how things would look to you without glasses if you had the levels of astigmatism described above.
Correction of astigmatism using toric lenses could potentially achieve the following vision without glasses in each eye.
To estimate your postoperative astigmatism, call your Ophthalmologist or Optometrist and ask for your refraction and your K-values. Once you have your predicted astigmatism, use the astigmatism simulator to approximate how you would see without glasses after cataract surgery.
You might have noticed that the K values and refraction match one another in this example. This can occur if the anterior cornea is the only source of astigmatism. In real life the numbers are almost always a little different from one another. The reasons for this are explored here.