Irregular astigmatism is a relatively rare form of astigmatism in which the principal meridians of the cornea are not 90 degrees apart from each other. In some cases, irregular astigmatism is caused by keratoconus or scarring of the cornea from an injury.
The unusual shape of irregular astigmatism makes it difficult to correct with eyeglass lenses or soft contact lenses designed for astigmatism. In many cases, however, irregular astigmatism can be corrected with rigid gas permeable contact lenses.