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Irregular astigmatism
Astigmatism occurs when light is bent by the cornea as it enters the eye and doesn't focus clearly on a single point on the retina (resulting in distorted vision).
A perfectly round cornea (shaped like a baseball) bends light into perfect focus and doesn't create astigmatism.
A cornea that is not perfectly round (but shaped more like a football) bends light that results in a distorted image being projected on the retina (think of the distorted reflection one sees when looking into an oval-shaped spoon). When the bending of the light is uniform it is called “regular astigmatism.” Regular astigmatism (if it is not too severe) can usually be reduced or eliminated with glasses and certain types of soft and rigid contact lenses.
A cornea can also be either perfectly round or football shaped, but have a bumpy surface. Light passing through each of these bumps bends light a different way, resulting in a distorted image being projected on the retina. If the bumps on the cornea are big enough, then multiple blurred images are projected upon the retina. This type of astigmatism is called “irregular astigmatism” (meaning that the way the light is distorted has no regular pattern). Bumps that are very large are usually called “central islands” since they appear as little islands in the middle of a blue lake on many topographers (machines that measure the surface of the eye). Visual effects range from image distortion that is mildly annoying to multiple images in each eye that is incapacitating.
Irregular astigmatism is also called “higher order aberrations,” (HOAs) from the mathematical term used to describe and categorize these bumps on the cornea. [The lower level aberrations are focus and sphere (with sphere being the way regular astigmatism is measured).]
Irregular astigmatism occurs during refractive surgery when the laser does not leave a smooth surface. The more uneven the surface created by the laser, the bigger the visual distortion.
Currently there is no reliable surgical fix for irregular astigmatism, but technology continues to advance.
Glasses and soft contact lenses usually cannot mask irregular astigmatism. Rigid gas permeable contact lenses (RGPs, aka “hard lenses”) have been used successfully to mask these aberrations, but they are often difficult to fit (due to the changed curvature of the cornea). Not everyone is able to wear these lenses successfully and those who can generally find relief from visual distortion only while they are wearing them.
To imagine a case of moderate irregular astigmatism, consider looking through glasses or sunglasses that have scratches across them (the scratches mimic a bumpy corneal surface).
To imagine a case of severe irregular astigmatism cross your eyes and, while keeping them crossed, try to walk across a room or read a book. The two images you see with two eyes are what some people with irregular astigmatism from refractive surgery see with one eye. The number of images an eye with irregular astigmatism sees is dependent upon the bumpiness of the corneal surface.
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