Which type of astigmatism is represented by K's 42.50 @180 /40.50 @90?

The noted astigmatism measurement, expressed as K's 42.50 @180 /40.50 @90, indicates the curvature of the cornea in two principal meridians. The first number, which is 42.50, represents the power measured at 180 degrees (the horizontal meridian), while the second number, 40.50, is the power at 90 degrees (the vertical meridian).

In the context of astigmatism, the terms "with the rule" and "against the rule" refer to the orientation of the steeper corneal meridian. "With the rule" astigmatism occurs when the steepest meridian is vertical (90 degrees), meaning the vertical curvature is greater than the horizontal. Conversely, "against the rule" astigmatism occurs when the steepest meridian is horizontal (180 degrees), indicating that the horizontal curvature is greater than the vertical.

Given that in this case, the power at 180 degrees (42.50) is greater than that at 90 degrees (40.50), this configuration confirms the presence of against the rule astigmatism. Therefore, the answer provided accurately identifies the type of astigmatism represented by these