Astigmatism, as well as most visual problems, are confusing and threatening to most pilots.
The International Standards specify a value for the astigmatism - if it is greater than 2.00 (diopters is the unit of measure)there may be a restriction, however, it should not eliminate the possibility of your acquiring a Class I medical.
In the U.S. there is no specification for refractive error - it is simply stated in terms of resultant visual acuity "20/20 with or without corrective lenses". (This would translate to 6/6 on international eye charts). Further, it is possible to pursue a waiver for monocular vision (loss of vision in one eye) and to obtain a First Class Medical.
A key difference that I see in my office is pilots and candidates with "Color Confusion" who cannot pass the color plates test. The FAA permits "alternative" tests to be administered and if you are able to pass such a test, it is possible to document the results in a way that makes the pilot eligible for a "letter of competency" - if it is awarded by the FAA the color vision testing is waived on all future medicals.
So, if there is astigmatism in your prescription (90% of the population has it, so you are in good company) the prescription will read something like +1.00(sphere power)-2.00 ("cylinder" or astigmatism correction) and an axis ("X" which places the astigmatism correction in the proper orientation). So it looks like this:-1.00 -2.00X90.
I hope these hints help.
Best Wishes,
Doc Monaco
Aviation Vision Specialist