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How much astigmatism do I have?
You can estimate how much astigmatism you will have after cataract surgery in a couple of ways, depending on what information you have about your eyes.
1. Refraction Method:
If you have a recent refraction, the second number describes the amount of astigmatism in your glasses. Your contact lens prescription does not provide this, only your glasses prescription.
Here is a sample of a glasses prescription:
OD -1.00 + 1.50 x 175
OS -2.25 + 0.50 x 034
This patient has 1.50 Diopters of astigmatism in the right eye (OD), and 0.50 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. Only the size of the number matters for our current purposes.
This provides a rough estimate of how much astigmatism you can expect to have after cataract surgery. In general, the higher this number, the more likely you are to benefit from astigmatic management during cataract surgery. However, it is possible that this number could be very low and your surgeon might still recommend a toric intraocular lens.
2. Corneal Curvature Method:
This gives you a much better estimate of what you can expect after surgery. Your "K values" are the numbers that describe your anterior corneal curvature. Obtaining these values is a standard part of any general eye exam, and your surgeon or Optometrist will be able to provide these for you. Your K values might look like this:
OD K1 = 44.25 @ 085 K2 = 42.00 @ 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 42.00 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.
Estimating the amount of astigmatism you can expect to have after cataract surgery requires two steps.
Step 1: Subtract the smaller flat value from the larger steep value. In the example above:
OD 44.25 - 42.00 = 2.25 Diopters @ 085
OS 44.00 - 43.00 = 1.00 Diopters @ 010
Step 2: Adjust for the axis according to the following table:
0-30 degrees: Increase by 0.25 Diopters
30-60 degrees: No change
60-120 degrees: Decrease by 0.25 Diopters
120-150 degrees: No change
150-180 degrees: Increase by 0.25 Diopters
Applying this adjustment to the example above: For the right eye, the steep axis is between 60-120 Degrees, so we subtract 0.25 Diopter. For the left eye, the steep axis is between 0-30 degrees, so we add a 0.25 Diopter:
OD: 2.25 - 0.25 = 2.00 Diopters
OS: 1.00 + 0.25 = 1.25 Diopters
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