High astigmatism is a severe form of astigmatism, a medical condition that causes blurry vision. It is caused by an irregularly shaped cornea. There are no hard and fast rules for categorizing astigmatism, but astigmatism of over four diopters is generally considered high. Glasses, contact lenses and surgery are all potential treatments for high astigmatism, depending on the severity of the astigmatism and the irregularity of the shape of the cornea.

People who have normal vision have perfectly round corneas. Corneas are the transparent lenses that cover the pupil of each eye. Many people have an irregularly shaped lens in one or both eyes, which are elongated in some direction, like a balloon. This is called astigmatism, and the irregularity is measured in diopters. If the astigmatism is less than 0.6, then vision is still considered normal and does not need correction.

A high degree of astigmatism, of over four diopters, will cause significant vision problems. The high astigmatism will cause straight lines to appear wavy, making driving a vehicle or operating machinery not only difficult but dangerous. Having astigmatism is different from near or farsightedness because blurry vision occurs at all distances. Reading with a high astigmatism will usually cause eyestrain, leading to frequent headaches.

Moderate and high astigmatism can usually be corrected. Many brands of toric lenses, which correct astigmatism, are available for diopters 0.75 to 2.25. Specialty lenses, such as one-day, colored and multifocal lenses may not be available at these strengths. As the degree of astigmatism gets higher, the choices of soft contact lenses become slimmer, and rigid contact lenses may become the only available choice. This is also true for people whose astigmatism, or irregular shape, does not occur on or near the vertical or horizontal axis.

For extremely high astigmatism, over five diopters, surgery may be a suitable option. Astigmatic Keratotomy and laser-assisted in situ keratomileusis (LASIK) surgeries both attempt to permanently correct cornea abnormalities. Often high astigmatic patients will see improvement, but may still need to wear glasses or contact lenses to perfectly correct their vision. Other unlucky patients will need to have more surgery. New vision problems, such as trouble with night vision and sensitivity to light, may be side effects of corrective surgery.

Some sufferers of high astigmatism may need to stick with glasses. People with small eyeballs may have difficulty finding comfortable contact lenses, while people with small irises are not good candidates for surgery. The high astigmatism of people with keratoconus, or cone-shaped corneas, usually cannot be corrected by anything but glasses.