Which of the following BEST describes irregular astigmatism?

Irregular astigmatism is characterized by the principal meridians of the cornea not being perpendicular to each other. In a normal or regular astigmatism, the corneal curvature is such that one principal meridian is 90 degrees apart from the other; however, in irregular astigmatism, this alignment is disrupted.

This irregularity can arise from various conditions, such as keratoconus or corneal scarring, resulting in a situation where the curvature varies significantly in different meridians and does not conform to typical geometric rules. Such a configuration complicates vision correction, as standard cylindrical lenses may not adequately correct the visual distortion caused by this irregularity.

In contrast, attributing the condition to specific locations along the meridians or merely describing the axes as oblique does not capture the essential definition of irregular astigmatism, which is fundamentally rooted in the relationship (or lack thereof) between the principal meridians themselves.