Keratoconus is a progressive degenerative disorder of the eye, which causes the cornea to become thin and take on a “cone” shape. A cone shaped cornea is an irregular cornea and causes blurred vision, double vision, and excessive glare. Many with keratoconus describe their vision as having a "tail" or"ghosting" of images.
Spectacles cannot correct for the irregular shape of the cornea. Special lenses for keratoconus masks the irregular corneal shape to dramatically improve vision beyond glasses. The lenses used are usually in a gas permeable material to allow maximum oxygen diffusion to the diseased eye. Additionally, there are soft lenses for keratoconus. Types of lenses include corneal gas permeable lenses, hybrid lenses, and scleral lenses.
Other than keratoconus, there are a myriad of conditions that could cause an irregular cornea: pellucid marginal degeneration, LASIK induced corneal ectasia, post-corneal transplant, primary or secondary dry eye. Any condition causing an irregular cornea will result in poor vision in spectacles and may require a gas permeable lens to achieve functional vision.