You may encounter this problem so infrequently that you can't remember the proper procedure. So this is a reminder! I guarantee that you exam takers will encounter this on a certifying exam.
Corneal astigmatism above 4 diopters will cause a reading error due to distortion of the mires from the normal circular shape. It is not often that some of us encounter 4 diopters plus of corneal astigmatism, but is does happen, especially if you happen to work for a corneal surgeon. The problem is that high astigmatism distorts the shape, and particularly the relative size of the mires.
If you were to see the image below when performing applanation tonometry, you would notice that the upper mire is larger than the lower mire.
Ordinarily this would indicate to you that the tip is too high on the cornea and that you should adjust downward in order to make the the mires the same size for an accurate measurement. However, if this image is caused by high astigmatism, vertical adjustment would do you no good.
This error can be compensated for by aligning the red line on the applanator tip with the number on the mount that corresponds to the minus cylinder axis of the astigmatism. Suppose the eye has a glasses prescription as follows:
-2.00 + 4.50 x 30. Transpose this to +2.50 -4.50 x 120. The 120 axis number is then used to rotate the applanation tonometer tip in the mount, as follows.
Notice that the tip has been rotated so that the "120" on the scale has alignedwith the red mark on the tip holder. This is the proper position for measuring this cornea. You will now see an image that resembles the image below. You will have to measure using the image on the slant, but the mires will be the same size and the measurement will be more accurate.