With-The-Rule

When asked about their history of eye problems, many people tell me they have “a stigmatism in [their] eye.” What is that?

Actually, astigmatism is a single word that indicates that the corneal curvature is not perfectly symmetric and smooth. The cornea is the very front part of the eye. Sometimes, people describe an cornea without astigmatism as “looking like a basketball.” Instead, let’s consider eyes that do have astigmatism. One common pattern of astigmatism involves two axes with different degrees of curvature 90 degrees away from each other; in other words, this cornea “looks like a football.” A couple of common types of astigmatism are “with-the-rule” and “against-the-rule.” With-the-rule astigmatism has the steeper (or “curvier”) axis at 90 degrees. This is like a football placed with the pointy ends to the left and right sides. Against-the-rule astigmatism has the steeper axis at 180 degrees–like a football with the pointy ends up and down.

Over decades, human corneas tend to change from with-the-rule astigmatism to against-the-rule astigmatism.