Standards for analyzing astigmatic outcomes: Part I: astigmatism basics.

Astigmatism is the difference between the orthogonal principal planes of minimal and maximal powers of a toric surface, lens, or refraction. Corneal astigmatism is a common condition, and studies indicate that over 60% of individuals have more than 0.50 diopters of astigmatism. Corneal astigmatism is measured in various ways, and measurements from different devices are usually not interchangeable because of variations in technology and the corneal regions that are imaged. The integrity of the ocular surface is critically important for accurately measuring corneal astigmatism. Optimizing the ocular surface before cataract surgery can significantly improve the accuracy of preoperative astigmatism measurements and refractive outcomes. It is advisable to obtain at least 2 or preferably 3 measurements when measuring corneal astigmatism. When a discrepancy in astigmatism meridian and magnitude occurs between the measurements, it is advisable to settle or at least minimize this discrepancy by repeating the measurements after optimizing the ocular surface. Posterior corneal astigmatism is an important component of total corneal astigmatism. Lens tilt can also induce visually significant refractive astigmatism and ideally should be accounted for in toric intraocular lens planning.