Residual Refractive Errors

The most common complications from corneal refractive surgery are residual refractive errors.

Residual refractive errors are the differences between the intended refractive correction and the actual refractive correction. Refractive surgeons generally aim for 20/20 vision (good vision without glasses or contacts). But since sculpting human eye tissue is a bit like sculpting Jell-O, many people don’t wind up with perfect 20/20 vision.

There are numerous types of residual refractive errors.
Most people who do not achieve 20/20 vision after the first surgery have several options. They can wear glasses or contact lenses to correct the remaining refractive error, or they can choose to undergo additional surgery to correct the residual refractive error (provided their corneas have enough thickness to allow the removal of more tissue). Additional surgery usually comes with a risk level similar to the original surgery.

Some people can achieve 20/20 vision on a high contrast eye chart but still have horrible vision.
If the area of correction placed on the eye is smaller than the pupil, one may experience glare, arcing, starbursts and/or halos (commonly called “GASH”). Others are left with a “bumpy cornea” (“irregular astigmatism”) and/or “higher order aberrations” (HOAs) that cause the light to scatter as it enters the cornea. When this happens, the patient will see multiple images with each eye. There is no reliable surgical fix for these problems at this time, although technology continues to advance. Since glasses and soft contact lenses usually cannot mask these problems, rigid gas permeable prosthetic contact lenses (“RGPs”, aka “hard lenses”) are the primary rehab option for this group of patients. Fitting prosthetic RGPs after refractive surgery can be challenging (due to the changed curvature of the cornea) and not everyone is able to wear these lenses successfully.

Other types of residual error
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 are incapacitating.

Irregular astigmatism, or higher order aberrations (HOAs), from the mathematical term, is 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), and require the expertise of a specially trained optometrist. 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.